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UPPERLITTLE
MISSOURI
TONGUE
POWDER
NORTH PLATTE
LOWERYELLOWSTONE-
SUNDAY
LITTLEPOWDER
CHEYENNE
BIGHORN
BELLE FOURCHE
P l a t t eP l a t t e G o s h e nG o s h e n
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W a s h a k i eW a s h a k i e W e s t o nW e s t o n
P e n n i n g t o nP e n n i n g t o n
J o h n s o nJ o h n s o n
L a w r e n c eL a w r e n c e
S h e r i d a nS h e r i d a n
C a m p b e l lC a m p b e l l
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B i g H o r nB i g H o r nC a r t e rC a r t e r
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S l o p eS l o p e
F a l l o nF a l l o nM u s s e l s h e l lM u s s e l s h e l lR o s e b u dR o s e b u d
C u s t e rC u s t e r
P r a i r i eP r a i r i eB i l l i n g sB i l l i n g s
G o l d e nG o l d e nV a l l e yV a l l e y
W i b a u xW i b a u xP e t r o l e u mP e t r o l e u mD a w s o nD a w s o nG a r f i e l dG a r f i e l d
PowderRiverBasin
Wyoming
Montana
DAT
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PREPARED BY: ALL CONSULTING AND THE MONTANA BOARD OF OIL AND GAS CONSERVATION
MAY 2006
PREPARED FOR: U.S. DEPARTMENT OF ENERGY NATIONAL PETROLEUM TECHNOLOGY OFFICE
THIS PAGE INTENTIONALLY LEFT BLANK
i
ACKNOWLEDGEMENTS AND DISCLAIMER
This project was funded by a United States Department of Energy (DOE) office of Fossil Energy Grant
under the National Energy Technology Laboratory (NETL) program. This study was conducted by ALL
Consulting (ALL) and the Montana Board of Oil and Gas Conservation (MBOGC). The contractor’s effort
was overseen by NETL’s National Petroleum Technology Office located in Tulsa, Oklahoma. The National
Petroleum Technology Office project manager for this effort was Mr. John Ford. The Department also
wishes to extend its appreciation to those companies operating in the Powder River Basin, the staff of the
Bureau of Land Management in Miles City and Buffalo, and the Montana and Wyoming State agencies
that provided the technical input and assistance that enable DOE to improve the scope and quality of the
analysis.
This information is distributed solely for the purpose of technical peer review under applicable information
quality guidelines. It has not been formally disseminated by the DOE and should not be construed to
represent any Agency determination or policy.
The findings and conclusions in this report are those of the authors and do not necessarily represent the
views of the DOE.
This material was prepared as an account of work sponsored by an agency of the United States
Government. Neither the United States nor the United States Department of Energy, nor any of their
employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the
accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or
represents that its use would not infringe privately owned rights.
ii
ABSTRACT The use of surface impoundments as a practice for managing water produced from Coal Bed Natural Gas
(CBNG) development has become a popular choice for many operators in the Powder River Basin (PRB)
of Wyoming and Montana. According to the Wyoming State Engineer’s office, nearly 2,000 of these
impoundments exist in the Wyoming portion of the PRB alone. The potential impact from these
impoundments is widely unknown, and this has given cause for alarm from many environmental groups
claiming the impoundments will have negative impacts to the existing resources. Research has indicated,
however, that the negative impacts claimed by these groups are rather inflammatory, and given the
proper conditions and planning, the impacts can be minimal, and in several cases the existing conditions
improve as a result of the impoundments.
This Guidebook was developed to be used as a planning tool by operators and engineers working in the
Powder River Basin. By following the guidelines set forth in this Guidebook, operators and engineers can
hope to mitigate the environmental impacts associated with the construction and operation of
impoundments before they become a problem, and avoid several pitfalls that could drastically impact the
economic feasibility of a CBNG project. This Guidebook is structured to give the reader a step by step
planning process to follow by providing:
• background information on the several beneficial uses of the produced water,
• criteria that can be utilized in the planning and siting of future impoundments,
• recommendations for establishing a baseline of the existing conditions of impoundment sites,
• recommendations for establishing a monitoring program during operations,
• guidelines to aid in the design, construction, and reclamation of impoundments,
• guidelines for preparing and maintaining a water balance, and
• a discussion of the current regulatory requirements in both Wyoming and Montana.
Due to the complex nature of each impoundment site and the high number of variables that could affect
the success of an impoundment, it is not guaranteed that all issues will be resolved and/or avoided by
following the guidelines provided in this Guidebook. However, many pitfalls can be avoided if the process
is followed.
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
iii
TABLE OF CONTENTS ACKNOWLEDGEMENTS AND DISCLAIMER................................................................................................ I ABSTRACT ......................................................................................................................................................... II LIST OF FIGURES ...........................................................................................................................................VI LIST OF TABLES............................................................................................................................................ VII 1. INTRODUCTION ...................................................................................................................................... 1
1.1. OBJECTIVE .......................................................................................................................................... 1 1.2. SCOPE OF THE GUIDEBOOK ................................................................................................................. 1
2. BENEFICIAL USES................................................................................................................................... 3 2.1. BENEFICIAL USES FOR WILDLIFE AND LIVESTOCK ............................................................................. 3
2.1.1. Site Topography and Land Use................................................................................................ 3 2.1.2. Produced Water Quality ........................................................................................................... 3 2.1.3. Wildlife Distributions................................................................................................................. 4 2.1.4. Landowner Relations ................................................................................................................ 4
2.2. BENEFICIAL USES FOR FARMING AND RANCHING .............................................................................. 5 2.2.1. Water and Soil Quality Requirements for Irrigation ............................................................... 5 2.2.2. Existing and Salt-Tolerant Varieties of Plants ......................................................................... 7
2.3. BENEFICIAL USES FOR OPERATORS .................................................................................................... 9 2.3.1. Well Treatment ......................................................................................................................... 9 2.3.2. Operations ............................................................................................................................... 10 2.3.3. Enhanced Oil Recovery........................................................................................................... 10
3. GENERAL SITING CRITERIA .............................................................................................................. 13 3.1. ON-CHANNEL IMPOUNDMENTS......................................................................................................... 14
3.1.1. Existing Data ........................................................................................................................... 14 3.1.2. Pros and Cons of New On-Channel Impoundments............................................................. 16 3.1.3. Pros and Cons of Existing On-Channel Impoundments ....................................................... 16
3.2. OFF-CHANNEL IMPOUNDMENTS........................................................................................................ 17 3.2.1. Pros and Cons of New Off-Channel Impoundments............................................................. 19 3.2.2. Pros and Cons of Existing Off-Channel Impoundments ....................................................... 19
3.3. GEOMORPHOLOGY ............................................................................................................................. 20 3.3.1. Stream Classification .............................................................................................................. 20 3.3.2. Geomorphology Siting and Design Criteria ........................................................................... 23
3.4. TOPOGRAPHY .................................................................................................................................... 24 3.4.1. Topography Siting Criteria – On-Channel ............................................................................. 24 3.4.2. Topography Siting Criteria – Off-Channel ............................................................................. 24
3.5. GEOLOGY, SOILS, AND HYDROGEOLOGY........................................................................................... 26 3.5.1. Background Geology of the Powder River Basin .................................................................. 27 3.5.2. Background Hydrogeology of the Shallow Powder River Basin Formations ....................... 27 3.5.3. Alluvium................................................................................................................................... 30 3.5.4. Coal Clinker ............................................................................................................................. 33 3.5.5. Wasatch and Fort Union Formations ..................................................................................... 34 3.5.6. Geology and Hydrogeology Siting and Design Criteria ........................................................ 35 3.5.7. Groundwater Flow and Impoundments................................................................................. 36
3.6 SOILS OF THE POWDER RIVER BASIN............................................................................................... 37 3.7 VEGETATION...................................................................................................................................... 42
3.7.1 Vegetation Siting and Design Criteria ........................................................................................ 43 4. ESTABLISHING A BASELINE AND MONITORING PROGRAM .................................................... 45
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
iv
4.1. SOILS BASELINE................................................................................................................................ 45 4.1.1. Methods for Collecting Soil Samples and Mapping Shallow Geology .................................. 46 4.1.2. Geochemistry .......................................................................................................................... 48 4.1.3. Geotechnical............................................................................................................................ 48
4.2. REMOTE SENSING OF SITE GEOLOGY BASELINE ............................................................................... 48 4.2.1. Shallow Seismic....................................................................................................................... 49 4.2.2. Electromagnetic (EM) Mapping.............................................................................................. 49
4.3. GROUNDWATER BASELINE ................................................................................................................ 50 4.4. GROUNDWATER MONITORING .......................................................................................................... 50 4.5. SURFACE WATER BASELINE .............................................................................................................. 51
4.5.1. Existing Streams, Creeks, and Rivers .................................................................................... 51 4.5.2. CBNG Produced Water ........................................................................................................... 51
4.6. SURFACE WATER MONITORING ........................................................................................................ 52 4.6.1. Existing Streams, Creeks, and Rivers .................................................................................... 52 4.6.2. Water in the Impoundment ................................................................................................... 52
4.7. EROSION AND NON-POINT SOURCE INSPECTIONS .......................................................................... 53 4.8. IMPOUNDMENT INSPECTIONS ........................................................................................................... 53
5. DESIGN AND CONSTRUCTION GUIDELINES ................................................................................. 55 5.1. DESIGN – GENERAL CONSIDERATIONS ............................................................................................ 55
5.1.1. Estimating Volumes of Earth Cut/Fill ..................................................................................... 55 5.1.2. Sediment and Erosion Control ............................................................................................... 56 5.1.3. Surface Water Diversion......................................................................................................... 58 5.1.4. Spillway Design ....................................................................................................................... 61
5.2. DESIGN CONSIDERATIONS – ON-CHANNEL DAMS ........................................................................... 61 5.2.1. Foundation Design Considerations ........................................................................................ 61 5.2.2. Cutoff/Core Trench Design Considerations........................................................................... 62 5.2.3. Seepage Control Design Considerations ............................................................................... 63 5.2.4. Top Width and Alignment ...................................................................................................... 65 5.2.5. Side Slopes .............................................................................................................................. 66 5.2.6. Freeboard ................................................................................................................................ 66 5.2.7. Settlement Allowance ............................................................................................................. 67
5.3. DESIGN CONSIDERATIONS – OFF-CHANNEL DAMS.......................................................................... 67 5.3.1. Soils ......................................................................................................................................... 67 5.3.2. Dimensions .............................................................................................................................. 69 5.3.3. Excavated Material Management........................................................................................... 69
5.4. DESIGN DRAWINGS AND SPECIFICATIONS ....................................................................................... 70 5.5. CONSTRUCTION – GENERAL CONSIDERATIONS ................................................................................ 71
5.5.1. Erosion and Sediment Control Installation............................................................................ 71 5.5.2. Clearing and Grubbing............................................................................................................ 71 5.5.3. Spillway Installation................................................................................................................ 72 5.5.4. Vegetation ............................................................................................................................... 72
5.6. CONSTRUCTION – ON-CHANNEL DAMS ............................................................................................ 73 5.6.1. Preparing the Foundation....................................................................................................... 73 5.6.2. Building the Dam .................................................................................................................... 74
5.7. CONSTRUCTION – OFF-CHANNEL DAMS ........................................................................................... 74 5.8. RECLAMATION ................................................................................................................................... 75
6. PREPARING AND MAINTAINING A WATER BALANCE ................................................................ 77 6.1. IRRIGATION ...................................................................................................................................... 77 6.2. LIVESTOCK CONSUMPTION ............................................................................................................... 79 6.3. OTHER BENEFICIAL USES ................................................................................................................. 79 6.4. INFILTRATION ................................................................................................................................... 80
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
v
6.4.1. Initial Soil Conditions .............................................................................................................. 80 6.4.2. Potential Changes to Soil Structure During Operations ....................................................... 80 6.4.3. Hydraulic Conductivity............................................................................................................ 81 6.4.4. Calculating Infiltration Rates.................................................................................................. 81
6.5. EVAPORATION ................................................................................................................................... 82 6.6. DISCHARGE AND STORAGE................................................................................................................ 82 6.7. OTHER WATER MANAGEMENT SCHEMES........................................................................................... 83 6.8. EXAMPLE WATER BALANCE ............................................................................................................... 83
7. REGULATORY REQUIREMENTS ......................................................................................................... 87 7.1. PERMITTING IN WYOMING ............................................................................................................... 87
7.1.1. Impoundment Determinations ............................................................................................... 87 7.1.2. General or Individual WYPDES Permit .................................................................................. 89 7.1.3. WSEO Reservoir Permit Process ............................................................................................ 90 7.1.4. WOGCC Permit for Produced Water Storage ........................................................................ 91 7.1.5. BLM Permit for Impoundments.............................................................................................. 92 7.1.6. WSEO Safety of Dam Law...................................................................................................... 92 7.1.7. WDEQ Compliance Monitoring Plan....................................................................................... 93
7.2. PERMITTING IN MONTANA................................................................................................................ 93 7.2.1. Impoundment Determinations ............................................................................................... 93 7.2.2. MBOGC Permit for Storage Impoundments .......................................................................... 94 7.2.3. MDEQ Individual MPDES Permit ............................................................................................ 95 7.2.4. Status of MDEQ General MPDES Permit................................................................................ 95 7.2.5. MDNRC WRD Permitting Requirements ................................................................................ 95
8. REFERENCES .......................................................................................................................................... 99
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
vi
LIST OF FIGURES
Figure 2.1 Classification Index for Soil Salinity and Sodicity .................................... 7 Figure 2.2 Threshold Tolerances of Crops.................................................................. 8 Figure 2.3 Crop Sensitivity and Salinity ..................................................................... 9 Figure 3.1 Existing On-Channel Impoundments........................................................ 15 Figure 3.2 Existing Off-Channel Impoundments........................................................ 18 Figure 3.3 Chart of the Rosgen Classification of Natural Streams ............................ 21 Figure 3.4 Off-channel Impoundment Examples ....................................................... 26 Figure 3.5 Geologic Outcrop Map of the PRB............................................................. 28 Figure 3.6 Stratigraphic Column of Tertiary Coals in the Powder River Basin .......... 29 Figure 3.7 Stream Stage and Infiltration................................................................... 31 Figure 3.8 Average Cation Exchange Capacity for Surface Soils, by County ............. 40 Figure 3.9 Average Cation Exchange Capacity for Max Depth Soils, by County ....... 41 Figure 5.1 Vegetative Buffer Strip Example............................................................... 56 Figure 5.2 Silt Fence .................................................................................................. 57 Figure 5.3 Stray Bale Dike.......................................................................................... 57 Figure 5.4 Hydrologic Regions for Determining Peak-flow
Characteristics of Wyoming Streams........................................................ 59 Figure 5.5 Hydrologic Regions for Determining Peak-flow
Characteristics of Montana Streams......................................................... 60 Figure 5.6 Conceptual Cross-Section of a Cut-off Trench.......................................... 63 Figure 5.7 Sand Filter Pack ........................................................................................ 64 Figure 5.8 Anti-Seep Collar ........................................................................................ 65 Figure 6.1 Examples of Soil Structures ...................................................................... 80 Figure 6.2 Example Monthly Water Balance Year 1 and Year 2................................. 85 Figure 7.1 Decision Flow Chart for Permitting in Wyoming....................................... 88 Figure 7.2 Decision Flow Diagram for WSEO Reservoir Permitting Process.............. 91 Figure 7.3 Decision Flow Diagram for the Montana Permitting Process ................... 94
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
vii
LIST OF TABLES
Table 2.1 Analytical Parameters for Assessing Irrigation Suitability of Produced Water .................................................. 6 Table 2.2 Analytical Parameters for Assessing Irrigation Suitability of Soils .......... 6 Table 2.3 Salt-Tolerant Varieties of Plants ............................................................... 7 Table 3.1 On-Channel Impoundment Statistics by Watershed ................................ 14 Table 3.2 Off-Channel Impoundment Statistics by Watershed................................ 17 Table 3.3 Encroachment Ratio Descriptions............................................................. 21 Table 3.4 Stream Classifications for the Tongue and Powder Rivers ....................... 23 Table 3.5 Management Interpretations of Various Stream Types............................ 24 Table 3.6 Siting Considerations for Various On-Channel Site Characteristics ......... 25 Table 3.7 Hydrogeologic Properties of Formation Present at the Surface in the PRB .................................................................................... 30 Table 3.8 Pros and Cons of Constructing an
Impoundment Over Various Geological Formations................................. 35 Table 3.9 Geochemical Data for the Shallow Surface Soils of the PRB by County ... 38 Table 3.10 Geochemical Data for the Maximum Depth
Surface Soils of the PRB by County........................................................... 39 Table 3.11 Native Vegetation Tolerance Levels for Salt and CaCO3 .......................... 43 Table 4.1 List of Potential Constituents.................................................................... 51 Table 4.2 Impoundment Inspection Potential Mitigation Measures ........................ 54 Table 5.1 100 Year Regression Equations for Regions 1, 2, and 3 of Wyoming....... 59 Table 5.2 100 Year Regression Equations for Southeast Plains Region of Montana 60 Table 5.3 Various Foundation Conditions and Design Considerations ..................... 62 Table 5.4 Recommended Top Width of Dam for Various Heights............................. 65 Table 5.5 Side Slope Guidelines for Various Types of Materials............................... 66 Table 5.6 Freeboard Height Design Guidelines......................................................... 66 Table 6.1 Kc Values for the Different Growth Stages of Selected Crops ................. 78 Table 6.2 Irrigation requirement Example Monthly Calculation .............................. 79 Table 6.3 Daily Water Consumption by Adult Animals ............................................. 79
Table 6.4 Hydraulic Conductivity for Different Soil Types........................................ 81 Table 6.5 Evaporation Data for Sheridan, WY, in inches/month and inches/year... 82
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
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Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
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1. INTRODUCTION This Guidebook is part of a multi-phase research project that is focused on the potential impacts caused
by the construction and operation of coal bed natural gas (CBNG) produced water impoundments in the
Powder River Basin (PRB) of Montana and Wyoming. The information provided in this Guidebook is a
compilation of the research into the regional and local characteristics of the PRB and the degree to which
these characteristics restrain or augment the use of impoundments as a management option for the
handling of CBNG produced water. The research included the gathering and evaluating of site specific
and regional data within the PRB related to siting impoundments, as well as compiling information on
recommended steps to follow during the design and construction of impoundments.
1.1. Objective
The primary objective of this Guidebook is to provide a summary that enhances the understanding and
use of impoundments as a CBNG produced water management option, thus reducing the reliance on
more costly produced water management options such as deep injection and treatment of produced
water.
This Guidebook is intended to provide technical guidance to operators, engineers, and government
employees for the siting, construction, operation, and reclamation of impoundments in the PRB. In
addition to the technical aspects involved, a discussion of the current regulatory aspects, both in Montana
and in Wyoming, is included.
Various technical presentations and workshops will be made available to the public in conjuncture with
this Guidebook.
1.2. Scope of the Guidebook
This Guidebook sets forth technical guidance for siting, constructing, operating, and reclaiming
impoundments in the PRB of Montana and Wyoming. Section 2 lays the background information
necessary to understand the importance of impoundments in the PRB and the role they play in ensuring
beneficial uses can be realized from the produced water, where appropriate.
There are several variables that form a complicated web of how an impoundment will perform and impact
the surrounding environment. The variables are dynamic, and often times they rely on each other for
balance. When one variable is put off balance, the entire system can be impacted an order of magnitude
greater than the original impact. Section 3 focuses on these variables by looking at the local and regional
aspects of the PRB as they relate to the planning phase of siting an impoundment. The performance and
problems of existing impoundments are included in the discussions, where information was available, as
is what to look for in an impoundment site by considering topography, geomorphology, surface water,
groundwater, hydrogeology, outcrop geology, soil character, and vegetation.
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
2
Section 4 discusses how to establish a baseline study of a potential impoundment site, and the merits
behind investing in a baseline study to save time and money that are lost due to costly problems that
may arise during construction and operations. Section 4 also discusses how the investment made for the
baseline study can add value to the project by using wells in monitoring the site to remain in compliance
with regulatory requirements, while saving money in well installations. Defining baseline conditions is an
important step for gauging possible affects upon surface water and groundwater quantity and quality,
which may lead to regulatory closure of an impoundment if the surface water and/or groundwater is
impacted negatively. Additionally, the baseline conditions drive the regulatory need for a monitoring plan
for the impoundment based on the classification of the groundwater and the proximity to surface water.
Section 5 is a compilation of information gathered from various sources to enable operators, contractors,
and engineers to have a single point source of information to help in the design, construction, and
reclamation of impoundments. Guidelines for the design and construction of impoundments are
discussed in terms of how to take advantage of local conditions and how to minimize unexpected
problems during construction and operations due to proper planning.
Section 6 provides the framework for establishing a dynamic water balance that can be used as a
planning tool to ensure that the resources invested on an impoundment are well spent. If the
impoundment is over designed (i.e. more capacity than required), then the operator may have wasted a
significant amount of money in the design and construction of the impoundment. Conversely, if the
impoundment is under designed (i.e. capacity is exceeded at some point in time) it can result in shutting
wells in, which can cause loss of both time and money in developing the CBNG, and may result in having
to produce even more water to get the same amount of hydrostatic pressure drop in the wells once they
are put back online.
Section 7 considers the regulatory requirements for constructing impoundments in both Montana and
Wyoming. Regulatory requirements are important to consider when developing a Guidebook such as
this, as they may dictate how or where the operator constructs an impoundment. Regulations have been
developed specific to each state and some regulations are specific to federal rather than state or fee
minerals or surface ownership. Regulations are also specific to those whether the impoundment
constructed is on-channel versus off-channel, and whether or not the water in the impoundment will be
utilized for beneficial uses.
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
3
2. BENEFICIAL USES In order to create a mutually beneficial relationship between CBNG operators and landowners, previous
research efforts have identified several beneficial uses for CBNG produced water in the Powder River
Basin (PRB) (ALL, 2003). Beneficial uses have been identified and categorized for both lined surface
impoundments and unlined surface impoundments. This section provides: 1) a summary of background
information on how operators are incorporating these additional beneficial uses into existing
impoundments, and 2) recommendations on how to incorporate them into future designs for the
construction of the impoundment. The Handbook of Coal Bed Methane Produced Water Management
and Beneficial Use Alternatives (Beneficial Uses Handbook, ALL 2003) can be consulted for a more in-
depth look at the pros and cons for the various beneficial use alternatives of CBNG produced water
presented here.
2.1. Beneficial Uses for Wildlife and Livestock
In arid regions such as the PRB, additional habitat can be created for fish, waterfowl, and other animals
by providing an additional source of water where water resources at the surface are limited. As
discussed in the Beneficial Uses Handbook (ALL, 2003), site topography, land use, produced water
quality, wildlife distributions, and landowner wishes are all factors that can be considered by an operator
when siting and designing an impoundment to optimize the pond’s use as a water source for wildlife and
livestock.
2.1.1. Site Topography and Land Use
Where practical, the following siting and design considerations can be made to take advantage of
topography and land use and encourage the use of the impounded water by wildlife and livestock:
• Locate the impoundment near existing forage or near the location of planned new forage resulting from irrigation activities. Locating new irrigated forage near impoundments will increase the use of both elements by livestock and wildlife.
• Provide a gentle, stable slope at the water’s edge to:
o allow for a shallow area that wildlife and/or livestock can enter without creating a drowning hazard or other physical hazard; and
o improve water quality by reducing erosion and by preventing the introduction of suspended solids into the water (Rumble, 1989).
2.1.2. Produced Water Quality
Produced water quality determines the extent to which the impoundment can be used for wildlife and
livestock watering. There are national guidelines for livestock water quality which determine the extent
produced water can be used; therefore, several water quality parameters must be included in water
analysis prior to designing or constructing access for livestock to an impoundment (ALL, 2003). Quality
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
4
of CBNG produced water throughout the PRB is suitable for wildlife and livestock, but high rates of
evaporation can reduce water quality in the impoundment.
In addition to drinking water quality concerns for wildlife, water quality within the impoundment can be
evaluated prior to stocking the system with fish to determine the adequacy of the water for fish growth
and development throughout the year. For example, a CBNG industry leader in the PRB has performed a
study to demonstrate that the water quality was not detrimental to tilapia in a CBNG produced water
pond over the course of two years (DeLapp, 2005).
Seasonal temperature variations can be considered to ensure the health of either cold water (such as
trout) or warm water fish species (such as bass). Depending on which fish species are to be stocked,
these variations in temperature may be critical to species survival. According to the Wyoming Fish and
Game Department, trout are the best fish to stock in Wyoming (WGFD, 2002). Furthermore, there are
several cases where trout have been stocked in a CBNG produced water pond and survived for over four
years with no observable loss in fish life due to water quality or temperature variations. However, in
some shallower ponds (less than 15 feet deep), the temperature may rise and be sustained above
acceptable levels (above 70 degrees F for cold water fish), which can lead to severe loss of fish.
2.1.3. Wildlife Distributions
Determining the distribution of local wildlife is an important start of the planning process. Once the local
wildlife are identified, their habits and needs can be described and the impoundments designed and
located according to those habits. Migrational information can be reviewed in an effort to place resources
in areas which will not impact the migration paths of wildlife. Furthermore, consideration of the potential
for an increase in the herd density due to increased forage can be included in the planning process. Data
collected may include identifying seasonal habitat, breeding grounds, population density, and species
diversity (ALL, 2003).
2.1.4. Landowner Relations
Communication with the landowner and identifying landowner needs are also important in identifying the
size and location of impoundments in a CBNG project, especially for potential livestock/wildlife
impoundment locations. Many ranchers may be willing to accept as much water as their current herd can
consume, while others will use the additional supply of water to open new grazing land that was
previously unavailable (ALL, 2003). Landowners may want to locate impoundments in a manner that will
reduce the potential for overgrazing in areas near new impoundments. Some landowners may have
other needs or desires relative to where all types of impoundments are located on their surface. The
establishment of open communication lines between operators and landowners can reduce the number of
issues which may arise at a later time.
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
5
2.2. Beneficial Uses for Farming and Ranching
Some CBNG produced water is of sufficient quality to be used as irrigation water by farmers and ranchers
for their crops as well as the grazing pastures. CBNG produced water is considered potable and has been
utilized for human consumption and stock watering in the PRB of Wyoming, however, practices utilizing
CBNG produced water for irrigation have only recently been used within the region (ALL, 2003).
Considerable research has been performed in recent years to document the suitability of CBNG produced
water for managed irrigation projects including industry funded projects from CBNG operators and
publicly funded research performed at universities. Produced water quality is highly variable across the
basin and irrigation uses will likely be site-specific.
The irrigation of crops and grazing pastures using CBNG produced water has been performed with
various types of sprinkler irrigation; through the use of center pivots, lateral roll systems, and high
pressure, large flow impulse sprinklers (“big guns”). In addition, flood irrigation has been employed on a
limited basis by landowners who chose to divert CBNG produced water to their native grass fields (ALL,
2003). A discussion of each of these potential irrigation technologies, along with the effects of the
irrigation of CBNG produced water on plant growth and soil properties, is included in the Beneficial Uses
Handbook, (ALL, 2003). Additional research on the amendments that can be used when applying the
produced water has been done by DeJoia (2002). Based on the findings of his research, DeJoia
concluded that a source of calcium is needed to effectively amend CBNG produced water, and that
gypsum and sulfur were the most effective sources of amendments to be applied. DeJoia also noted that
amendment quantities vary from site to site as produced water and soil conditions vary (DeJoia, 2002).
The operator may elect to collect certain data prior to beginning siting and design activities if the water in
an impoundment is to be used for irrigation. Such data could include the quality of the produced water,
as well as the quality of the soil that will be irrigated, and the type of vegetation that is to be irrigated.
These issues are discussed further in this section.
2.2.1. Water and Soil Quality Requirements for Irrigation
The suitability of CBNG produced water for use in agricultural irrigation is largely governed by the quality
of the water (salinity and sodicity) and the physical (slope and structure) and chemical properties (soil
salinity and cation exchange capacity) of the irrigated soils. The quality of the CBNG produced water is
evaluated with respect to salinity (electrical conductivity, or EC) and sodicity (sodium adsorption ratio, or
SAR) to provide an initial indication of the general suitability for irrigation.
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
6
SAR is typically expressed as the ratio between sodium and calcium + magnesium as follows:
SAR = 2[Mg])-([Ca]
[Na]
Where [ ] indicates a molar value expressed in meq/l. In addition, other constituents in the water may also need to be evaluated to identify any potential
affects to irrigated soils and plants. These water quality factors need to be considered with respect to
additional characteristics inherent to the proposed irrigated soil, such as texture, drainage, and chemistry
(ALL, 2003). Tables 2.1 and 2.2 present some parameters that are typically analyzed in irrigation water
and soil, respectively, to assess irrigation suitability.
Table 2.1 Analytical Parameters for Assessing Irrigation Suitability of Produced Water
Table 2.2 Analytical Parameters for Assessing Irrigation Suitability of Soils
Only a limited number of studies and field experiments utilizing CBNG produced water for irrigation have
been conducted and the results of some of this research are discussed in the Beneficial Uses Handbook,
along with additional issues associated with water and soil chemistry. More recently, Fidelity published
their results from utilizing managed irrigation in the Powder River Basin (Harvey and Brown, 2005).
General Parameters Soluble Ions
(Saturated Paste Extract) Exchangeable Ions
(Ammonium Acetate Extraction) • pH • EC • CEC • Alkalinity • Hardness • CaCO3 content
• Calcium • Magnesium • Sodium • Potassium • Bicarbonate/ Carbonate • Sulfate • Chloride • Fluoride
• Calcium • Magnesium • Sodium • Potassium • Fluoride
Source: ALL, 2003
General Parameters Soluble Ions Trace Constituents • pH • EC • TDS • Alkalinity • Hardness
• Calcium • Magnesium • Sodium • Potassium • Bicarbonate/ Carbonate • Sulfate • Chloride • Fluoride
• Iron • Manganese • RCRA 8 (As, Ba, Cd, Cr,
Pb, Hg, Se, Ag)
Source: ALL, 2003
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
7
Figure 2.1 Classification Index for Soil Salinity and Sodicity
Source: ALL Consulting
Table 2.3 Salt-Tolerant Varieties of PlantsCommon Name Scientific Name Function
Amshot Grass Echinochloa stagninium ion accumulator
Suada vera Forsk Suaeda fruiticosa ion accumulator Rice Oryza sitiva ion accumulator
Sunflower Selianthus annuus ion accumulator Sharp-leaved rush Juncus acutus ion accumulator
Samaar morr Juncus rigious ion accumulator Salt Cedar Tamarix L. ion extractor Goosefoot Chenopodium spp. ion extractor
Summer Cypress Kochia spp. ion extractor Salt Wort Salicornia spp. ion extractor
Russian Thistle Salsola spp. ion extractor Seablite Suaeda spp. ion extractor
Sorghum-sudan grass
Sorghum- sudanese
pore size enhancer
Barley Hordium spp. limited ion accumulator
Wheat Triticum spp. limited ion accumulator
Cotton Gossypium spp. limited ion accumulator
Sugarbeet Heterodera spp. limited ion accumulator
Source: Phelps and Bauder, 2002.
Irrigation water with an EC > 3
dS/m is generally categorized as
saline, and irrigation water with an
SAR > 12 is generally categorized as
sodic (Hansen, et al, 1999). A
classification scheme for soil salinity
and sodicity is shown in Figure 2.1.
Additional ongoing research is being
performed in the Tongue River by the
Agronomic Monitoring and Protection
Program (AMPP). AMPP is a soil and
crop testing program developed to
better understand the potential effects
of CBNG water production on the soil and crops in the Tongue River drainage area of southeastern
Montana. Data collected through this program will create a baseline of information to determine what, if
any, impacts occur from the discharge of CBNG produced water. The most recent available data from the
AMPP can be found on their website at www.tongueriverampp.com.
2.2.2. Existing and Salt-Tolerant Varieties of Plants
Plant selection is an important consideration
when evaluating the potential for irrigation
with CBNG produced water. Some plant
species are less sensitive to impacts on plant
growth from salinity and sodicity, and some
plants can assist in mitigating long term
impacts to the soil. Table 2.3 presents a list
of plants that have shown bioremediation
benefits by minimizing the detrimental affects
of saline and/or sodic irrigation water on soils.
These plants can be used both in irrigated
fields and around impoundments.
It is also important to consider the salt
tolerance threshold of existing plants within
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
8
Source:Tanji, 1990; based on Rhoades, 1982.
the areas that may be irrigated with CBNG produced water. Figure 2.2 provides salt tolerance thresholds
of several crops common to the PRB along with comparison data for the EC of the irrigation water to the
average root zone EC of the soil.
Figure 2.2 Threshold Tolerances of Crops
Figure 2.3 compares the sensitivity of different crops to increases in soil salinity. These figures can be
used not only to determine the sensitivity of existing crops, but future crops can be selected and planted
according to the site specific soil and water chemistries.
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
9
Source: Miller and Donahue, 1995.
Figure 2.3 Crop Sensitivity and Salinity
2.3. Beneficial Uses for Operators
CBNG operators can add value to their CBNG projects by using CBNG produced water to support oil and
gas operations. Methods for obtaining this added value, such as using the produced water for well
treatments, during operations, dust control, and for enhanced oil recovery are discussed in more depth in
this section. These uses can be accommodated in the water budget portion of the Water Management
Plan for the CBNG development if the drilling and completing schedule for the project is known. Periodic
maintenance will add to the volume of use but can also be estimated and added to the management
budget.
2.3.1. Well Treatment
Well treatment is a stimulation method used to ensure the efficient flow of hydrocarbons (either oil or
natural gas) out of a formation. Most CBNG wells require only minimal well treatments due to the nature
of the cleating in the coal seam; however, well treatments have been used on conventional oil wells, as
well as on non-conventional gas wells (such as tight shale gases), quite effectively. Hydraulic fracturing
(or fracing) is a well treatment method that consists of injecting a fluid into the well to open up the
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
10
formation which allows the hydrocarbons to flow more easily toward the well bore. The injected fluid
(commonly referred to as frac) is typically water with a polymer additive, and a “propping agent” such as
silica sand, glass beads, or epoxy. Once injected, the fluid creates fractures in the formation, which are
left propped open by the “propping agents”, thus creating a higher permeability in the well and improving
hydrocarbon production (Blue Ridge Energy website, 2005).
CBNG produced water can be used as a source for frac water; however, if the quality of the CBNG water
is considered relatively high, other beneficial uses may be more appropriate. Furthermore, the distance
between the well that is to receive the well treatment and the CBNG producing area can affect the
applicability of this beneficial use; if the distance is minimized, the infrastructure required to transfer the
CBNG produced water from the CBNG well to the treatment well can be inexpensive. This beneficial use
could prove to be cost prohibitive if the treatment well is too far away.
2.3.2. Operations
CBNG produced water from existing CBNG wells can be used during drilling activities of future CBNG
wells. Uses of the water during drilling activities and maintenance operations include, but are not limited
to drilling mud, cement preparation, vehicle wash, and dust control. The amount of water required for
each well is dependant on site specific conditions including climate, depth of the well, and the producing
formation.
Drilling mud is a mixture of clay, water, and chemicals pumped into a well as it is being drilled to
lubricate the drill, remove debris, and prevent the escape of gas and/or oil. Once drilling is complete,
cement is required to complete a CBNG well to seal the borehole from the surface to the screened
interval. It is common practice for CBNG operators drilling in active fields to use CBNG produced water
for preparing drilling mud and cement for well completions.
The generation of fugitive dust emissions from construction activities can be an annoyance and
detrimental to short term air quality. Exploration and production of new CBNG wells requires
infrastructure such as pipelines to deliver the gas to market and roads to access the well sites. CBNG
produced water can be used to help with fugitive dust abatement on construction sites and on roads used
to access the sites by applying the produced water to bare soil and road surfaces.
2.3.3. Enhanced Oil Recovery
Another beneficial use of CBNG produced water is to inject the water into a secondary or enhanced
recovery well of conventional oil producing horizons. Primary recovery of oil is driven by the natural
energy within the reservoir and can be supplemented by pumping. When primary recovery ends,
secondary recovery begins and may be followed by enhanced recovery. Secondary and enhanced
recovery is the process of injecting a fluid into a reservoir creating a flood or sweep that displaces the oil
causing it to flow to the producing well (Collins and Carroll, 1987). Water is the fluid most commonly
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
11
used in secondary and enhanced recovery of oil in non-CBNG fields; CBNG produced water could,
therefore, be of beneficial use in secondary and enhanced oil recovery operations (ALL, 2003).
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Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
13
3. GENERAL SITING CRITERIA
The first step in determining a location for an impoundment is to resolve whether it is preferable to
construct an on-channel or an off-channel impoundment. The definition of on-channel and off-channel
impoundments is not always consistent; regulatory agencies within states and at the federal level have
different definitions of what constitutes an on-channel or off-channel impoundment. For instance, the
following definitions show how two agencies define on-channel and off-channel impoundments relative to
their regulations or guidance and the differences which can exist between physical and regulatory
definitions. On-channel impoundments are defined as any impoundment constructed by building an
embankment or dam across a stream, intermittent channel, or watercourse where the stream valley is
depressed enough to permit storing 5 feet or more of water (USDA NRCS, 1982a). The land slope may
range from gentle to steep. Off-channel impoundments are defined as any impoundment constructed by
digging a pit or dugout in a nearly level area outside of an existing stream channel or intermittent
watercourse (USDA NRCS, 1982b). Off-channel impoundments can be built in gently to moderately
sloping areas where their capacity is obtained both by excavating and by building a dam (USDA NRCS,
1982). The Wyoming Department of Environmental Quality has a regulatory definition for off-channel
impoundments which is “an impoundment that receives coalbed methane produced water will be
considered ‘off channel’ when the impoundment is not sited within a designated water feature as defined
on a United States Geologic Survey (USGS) 1:24,000 scale topographic map. These water features
include perennial and intermittent streams, dry washes, lakes, among others etc.” (WDEQ, 2005). An on-
channel impoundment is defined by the Wyoming Department of Environmental Quality (WDEQ) as “an
impoundment that receives coalbed methane produced water will be considered ‘on-channel’ when the
impoundment is sited within a designated water feature or within the floodplain or alluvium of a water
feature as defined above” (WDEQ, 2005).
One important difference between the two types of impoundments is the potential for infiltrated water to
discharge to surface waters. Due to the nature of the alluvium present beneath most on-channel
impoundments in the PRB, a hydrologic connection can be present between the impoundment and a
nearby surface water body which can allow water infiltrating into alluvium to communicate directly with
the water in the river or stream. This situation can lead to regulatory issues that are discussed in a later
section of this report. Sections 3.1 and 3.2 provide a discussion of the technical and logistical pros and
cons of each type of impoundment.
Once the determination has been made whether to construct an on-channel or off-channel impoundment,
additional siting criteria can be considered to optimize the performance of the impoundment over the life
of the project. General siting criteria can include geomorphology, soil type (both at the surface and at
depth), presence and nature of both shallow and deep groundwater, hydrogeology, regional geology, and
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
14
Table 3.1 On-Channel Impoundment Statistics by Watershed
Watershed On-Channel/ Total Percent On-Channel
Powder River 327/1206 27%
Little Powder River 65/239 27% Tongue River 46/173 27%
Belle Fourche River 119/227 52% Cheyenne River 83/223 37%
Total 620/1984 31%
vegetation. The remainder of this section will discuss the key technical issues related to each of these
criteria, followed by a summary discussion of the relative importance of each criterion.
3.1. On-Channel Impoundments
As previously mentioned, on-channel impoundments are defined as any impoundment established in an
existing surface water drainage path by the construction of a dam, or embankment, across the water
drainage path. This section discusses existing surface water and groundwater data from PRB on-channel
ponds. A discussion of the logistical and technical pros and cons for on-channel ponds is included here,
as well as for both newly-constructed ponds and converted stock ponds.
3.1.1. Existing Data
According to the Wyoming State Engineer’s database, there are nearly 2,000 surface impoundments in
the Wyoming portion of the Powder River Basin. Approximately 620 (roughly 31%) of those
impoundments lie within 500 feet of the alluvium outcrops and are classified as on-channel
impoundments by the Wyoming State Engineers office. This number may be low due to the fact that on-
channel impoundments in upland areas may be over 500 feet away from what is classified as alluvium
deposits, but they would still be considered on-channel nonetheless. Figure 3.1 depicts where the
existing on-channel impoundments exist in the Powder River Basin of Wyoming. Table 3.1 is a
breakdown of approximately how many on-channel impoundments were noted in each of the five
watersheds of the PRB by the Wyoming State Engineer’s classification of on-channel impoundments.
Research focused on understanding the geochemical changes that occur to the infiltrating water as it
passes through the vadose zone is currently being conducted at on-channel impoundments in Wyoming.
As of the production of this report, none of the data from this research has become available.
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
15
Figure 3.1 Existing On-Channel Impoundments
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Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
16
3.1.2. Pros and Cons of New On-Channel Impoundments
One option for an impoundment is to construct a dam or embankment in an existing wet-weather
drainage path to create an on-channel impoundment. The pros and cons of a newly-constructed on-
channel impoundment are discussed below.
Pros
• Planning and design allows for the pond to be centrally located so that transportation and infrastructure costs are minimized.
• Overall cost to manage produced water is lower than for alternative management options. • Beneficial uses such as livestock watering and wildlife habitat can be realized, opening up new
areas for grazing and recreational use. • Infiltration rates are generally higher in alluvium, thus enabling a greater potential for water
management. Alluvium has a tendency to have less geochemical impact on infiltrating water than shaley bedrock.
Cons • Planning, design, and construction of the system can be expensive. • Pond performance may not match the design life of the pond due to the tendency of the soil to seal
itself off with use. • Adverse groundwater impacts may be encountered due to presence of minerals, metals, and ions in
the soil mobilizing into the infiltrating water. • Potential for “direct” discharge to surface waters, if the alluvium layer is thin and an impermeable
layer, is encountered at a shallow depth. • Permitting process may be more extensive than for converted ponds or off-channel impoundments. • Water Rights Issues associated with the potential to reduce downstream water quantity and quality.
3.1.3. Pros and Cons of Existing On-Channel Impoundments
Another possible option for a CBNG impoundment is to convert an existing on-channel stock pond for use
as an on-channel impoundment. The pros and cons of using existing on-channel impoundment are
discussed below.
Pros
• Construction costs can be reduced compared to a newly-constructed impoundment. • Adverse groundwater impacts may not be a problem due to the possibility that the existing stock
pond has already “flushed” the soil, alluvium, and bedrock beneath the pond of soluble minerals, metals, and ions.
• Overall cost to manage produced water may be lower than for alternative management options. • Beneficial uses such as livestock watering and wildlife habitat can be continued and perhaps
increased. • Infiltration rates may already be known from historical use of the pit as a stock pond. Groundwater
quality under an existing pond may already be lower than other on-channel areas due to previous infiltration.
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
17
Table 3.2 Off-Channel Impoundment Statistics by Watershed Watershed Off-Channel/ Total Percent Off-Channel
Powder River 879/1206 73% Little Powder River 174/239 73%
Tongue River 127/173 73% Belle Fourche River 108/227 48%
Cheyenne River 140/223 63% Total 1364/1984 69%
Cons • Pond performance may not meet the design life of the pond due to the fact that the pond may
already have reduced permeability through the bottom. • Cost of infrastructure (pipe, roads, etc) may be higher due to the location of the existing pond and
the proximity to CBNG wells. • Potential for “direct” discharge to surface waters if the alluvium layer is thin and an impermeable
layer is encountered at a shallow depth. • Pond may have already developed a leak prior to conversion to CBNG water. • Pit locations within the drainage channel are more vulnerable to damage from storm water runoff
events.
3.2. Off-Channel Impoundments
As previously mentioned, there are nearly 2,000 existing surface impoundments in the Wyoming portion
of the Powder River Basin as identified in the Wyoming State Engineer’s Office (WSEO) database.
Approximately 1,364 (roughly 69%) of those impoundments are classified as off-channel impoundment in
the WSEO database. Figure 3.2 depicts where the existing off-channel impoundments are located. Table
3.2 is a breakdown of approximately how many off-channel impoundments were noted in each of the five
watersheds of the Powder River Basin in Wyoming.
Research focused on understanding the geochemical changes that occur to the infiltrating water as it
passes through the vadose zone is currently being conducted at off-channel impoundments in Wyoming.
As of the production of this report, none of the data from this research has become available.
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
18
Figure 3.2 Existing Off-Channel Impoundments
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UPPERLITTLE
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PowderRiverBasin
Wyoming
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LegendPRB Boundary
Major Watersheds
County Boundary
State Boundary
! Off-Channel Impoundment Locations
¬ Powder River BasinDistribution of Wyoming Off-Channel
Impoundment Locations
U.S. Dept. of Energy, NETL,Montana Board of Oil & Gas Conservation
Infiltration Study
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
19
3.2.1. Pros and Cons of New Off-Channel Impoundments
One option for an off-channel impoundment is to excavate a new pit and construct an off-channel
impoundment. The pros and cons of a newly-constructed off-channel impoundment are discussed below.
Pros • Planning and design allows for the pond to be ideally located so that transportation and
infrastructure costs can be minimized. • Overall cost to manage produced water may be lower than for alternative management options. • Beneficial uses such as livestock watering and wildlife habitat can be established, opening up new
areas for grazing and recreational use and helping the surface owner. • Planning and design allows for the impoundment to be located in geologically attractive areas
where infiltration rates are higher. • Reduced potential for infiltrated water to resurface via a hydrologic connection to nearby surface
water. Cons • Planning, design, and construction of the system can be expensive. • Pond performance may not meet the design life of the pond due to the nature of the soil (soil may
seal itself off after some time). • Adverse groundwater impacts may be encountered due to presence of minerals, metals, and ions in
the soil or bedrock mobilizing into the infiltrating water which eventually contacts existing groundwater.
• Infiltrating groundwater may encounter an impermeable layer which causes the water to migrate horizontally and discharge to the surface from a hillside.
3.2.2. Pros and Cons of Existing Off-Channel Impoundments Another option for an impoundment is to convert an existing off-channel stock pond for use as an off-
channel impoundment. The pros and cons of an existing off-channel impoundment are discussed below.
Pros • Construction costs are minimal as compared to a newly-constructed impoundment. • Adverse groundwater impacts may not be a problem due to the possibility that the existing stock
pond has already “flushed” the soil beneath the pond of soluble minerals, metals, and ions. • Overall cost to manage produced water may be lower than for alternative management options. • Beneficial uses such as livestock watering and wildlife habitat can be continued and perhaps
increased.
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
20
Cons • Pond performance may not meet the design life of the pond due to the nature of the soil (soil may
seal itself off after a relatively short time). • Cost of infrastructure (pipe, roads, etc) may be higher due to the location of the existing pond and
the proximity to CBNG wells. • Size of existing pond may not be sufficient to meet the operator’s water management needs.
3.3. Geomorphology
On-channel impoundments make use of natural channels that may be occupied by rivers or streams for
all or part of the year. As new features in these channels, the impoundments have the power to change
the flow of water. Rivers, streams, and drainage flow paths are dynamic systems that adapt as changes
in climate occur and other terrain developments alter the watershed. These adaptations occur as a result
of a stream’s natural tendency to meet a dynamic equilibrium (known as the most probable state)
between eight variables: discharge, width, depth, velocity, slope, channel roughness, sediment load, and
sediment size/volume (US Fish and Wildlife, 2000). The study of the inter-relationship between these
eight variables is known as geomorphology, or river/stream morphology.
Geomorphology can impact the decision making process of where to site an on-channel impoundment by
assessing the potential impact the impoundment could have on the stability of the existing waterway
where the impoundment is installed, or nearby waterways that may be altered as a result of the
impoundment construction.
3.3.1. Stream Classification
Stream classification can lead to a better understanding of the potential geomorphologic problems that
may occur as a result of the installation of an on-channel impoundment. David Rosgen created a stream
classification system for natural waterways according to Figure 3.3 (Rosgen, 1994). The proper use of
Figure 3.3 is to begin at the top (single thread or multiple channels) and work down the chart until a
classification has been made.
Single Thread versus Multiple Channels
The first distinction that can be made when classifying a stream is to determine if the stream has a single
thread (or a thalweg, the line following the lowest points of a valley) or does the stream have multiple
flow paths that intertwine with each other. Once this determination has been made, the entrenchment
ratio can be considered.
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
21
Table 3.3 Encroachment Ratio DescriptionsEncroachment
Ratio Description
Less Than 1.4 Entrenched 1.41 – 2.2 Moderately Entrenched
Greater Than 2.2 Slightly Entrenched Source: Compiled from Rosgen, 1994.
Entrenchment Ratio
Entrenchment is defined as the vertical containment of river and the degree to which it is incised in the
valley floor (Kellerhals, et al., 1972). The term “entrenchment ratio” was coined by Rosgen in an attempt
to provide a consistent method for various scientific professionals to determine the degree in which a
stream is entrenched in the alluvium. The entrenchment ratio is the ratio of the width of the flood-prone
area (~50 year return period) to the bank full surface width of the channel (~1.5 year return period).
The flood-prone area is defined as the width measured at an elevation which is determined at twice the
maximum bank full depth. Field observation shows this elevation to be a frequent flood (50 year return
period) or less, rather than a rare flood elevation (Rosgen, 1994). Table 3.7 depicts the various
encroachment ratio definitions that have been derived empirically from observation of hundreds of
streams.
A range of +/- 0.2 encroachment ratio units (known
as the continuum concept) is allowable for each of
Rosgen’s entrenchment ratio categories (shown in
Table 3.3) when determining stream type. In some
classification cases, it may be necessary to evaluate
all of the other attributes before assigning a stream
Figure 3.3 Chart of the Rosgen Classification of Natural Streams
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
22
type (Rosgen, 1994).
Width to Depth Ratio
The width/depth ratio refers to the ratio of bank full channel width to bank full channel average depth.
As mentioned previously, bank full flow is defined by the maximum water surface that results from a 1.5
year storm event.
According to Rosgen, values of low width/depth ratio are those less than 12 and values greater than 12
are considered moderate or high. As with the entrenchment ratio criteria, the continuum concept also
can be applied to the width/depth ratio. There are rare occasions (which can be allowed for) where
width/depth ratio values can vary by +/- 2 units outside Rosgen’s given ranges for stream types shown in
Figure 3.3. In some classification cases, it may be necessary to evaluate all of the other attributes before
assigning a stream type (Rosgen, 1994).
Sinuosity
Sinuosity can either be described as the ratio of stream length to valley length or as the ratio of valley
slope to channel slope. Mapping sinuosity from aerial photos is often possible, and interpretations can
often be made of slope, channel materials, and entrenchment once sinuosity is determined (Rosgen,
1994). Site specific issues such as vegetative type, bedrock control, presence of roads, channel
confinements, dams, etc., can impact/modify sinuosity values. It has been observed that as gradient and
particle size decrease, there is a corresponding increase in sinuosity (Rosgen, 1994). As with
entrenchment ratio and width/depth ratio mentioned earlier, an adjustment of +/- 0.2 units can be
applied to this criterion to maintain consistency with a “dominant” morphology. Once sinuosity is
determined, the stream type can be identified as discussed below.
Stream Type
As can be seen in Figure 3.3, once the sinuosity, entrenchment ratio and width/depth ratio are all
determined, a stream type can then be assigned (A, B, C, D, DA, E, F, or G). The stream classification
process is then completed by considering water surface slope and channel materials.
Slope
Water surface slope is determined by measuring the difference in water surface elevation per unit stream
length. As observed with the other criteria, slope values less or greater than the most frequently
observed ranges can occur. These can occur without a significant change in the other defining criteria for
that stream type. The most frequently observed slope categories are shown in Figure 3.3 (Rosgen, 1994).
In large scale delineations, slopes can often be estimated by measuring sinuosity from aerial photos and
measuring valley slope from topographic maps:
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
23
Channel slope = Valley slope/sinuosity
The basin and associated landform relief can also be used to estimate stream slope range, for example,
the terraces and slopes of alluvial fans (Rosgen, 1994).
Channel Material
The channel materials which make up the bed and bank of the channel influence sediment transport and
hydraulic loading. The type of channel material can also predict how the form, plan, and profile of the
stream will be modified over time to reach the desired state of stability. Mapped lithology, such as from
Natural Resource Conservation Service soil data, can be used to provide a reliable estimate of the
alluvium characteristics for broad delineation. Figure 3.3 provides six different classifications for channel
material (bedrock, boulders, cobble, gravel, sand, and silt/clay).
3.3.2. Geomorphology Siting and Design Criteria
Based on data taken from previous USGS studies, the two main rivers of the PRB (Powder River and
Tongue River) can be classified as shown in Table 3.4.
This same classification process can be used for all tributaries in the PRB (even smaller and intermittent
tributaries that are only wet during storm events). Once a site-specific stream classification has been
made according to the format discussed in Section 3.3.1, Table 3.5 can be used to determine the level of
impact an impoundment may have on the existing system, which in turn can be used as a guide for siting
criteria in the planning process. For example, the operator may want to avoid streams classified as G6
due to the very high sensitivity to disturbance, poor recovery potential, high sediment supply and erosion
potential, and the high dependence on vegetation to control stability. In the same vein, the operator
may want to be especially careful of local areas within generally F6 streams such as the Tongue River.
Even though the Tongue regionally is an F6, locally it could be much more sensitive to disturbance. The
operator may want to consider all five of the sensitivity factors when locating and planning an
impoundment. Other stream classifications that may be present in other rivers and tributaries of the PRB
are also included in Table 3.5. For a complete table that represents all stream classifications, see
Rosgen, 1994.
Table 3.4 Stream Classifications for the Tongue and Powder Rivers
River Single/ Multiple
Entrench Ratio W:D Ratio Sinuosity Slope Channel
Material Stream
Classification Powder1 Multiple 1.3 23.9 < 1.2 0.001 Silt/ Clay DA6 Tongue2 Single 1.1 21.8 >1.2 0.014 Silt/ Clay F6
Source: USGS, 1998. 1Powder River near Moorhead, MT, 2Tongue River at State Line.
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
24
3.4. Topography
Historically, topography has been perhaps the most influencial siting criteria utilized. Topography plays
such a key role due to the cost savings that can be realized by minimizing construction time by starting
with a site that already has a natural affinity to be an impoundment. This section will briefly discuss what
type of topography may be desirable for either on-channel or off-channel impoundments.
3.4.1. Topography Siting Criteria – On-Channel
The existing topography of a potential on-channel impoundment can impact the time to construct the
dam as well as the overall cost to construct the dam. Furthermore, the topography can impact the
potential capacity of an impoundment, which is also an important consideration. Why build a low cost
impoundment if it cannot hold the water that will be produced? Table 3.6 lists some site characteristics
to consider, relevant to topography, when siting an on-channel impoundment.
3.4.2. Topography Siting Criteria – Off-Channel
When siting an off-channel impoundment, it may be more desirable to utilize topographical features that
will minimize the likelihood of an unintended discharge to the surface through lateral infiltration. For
example, a site that is low and flat in elevation (relative to the surrounding topographical features), such
as Reservoir “A” in Figure 3.4 would be less likely to encounter problems due to horizontal infiltration
than a site that is on a mesa, or high in elevation, such as Reservoir “B” in Figure 3.4. In the latter
example, water may infiltrate vertically until it reaches a soil horizon that has a lower permeability. The
water will then follow the path of least resistance (horizontally along the top of the lower permeability soil
layer) until it outcrops, most likely on a hill side. This can lead to regulatory violations and fines,
depending on the nature of the discharge. Figure 3.4 depicts the two examples.
Table 3.5 Management Interpretations of Various Stream Types
Stream Type
Sensitivity to disturbance a
Recovery potential b
Sediment supply c
Streambank erosion
potential
Vegetation controlling influence d
A6 high poor high high negligible B6 moderate excellent moderate low moderate C6 very high good high high very high D6 high poor high high moderate
DA6 (Powder) moderate good very low very low very high E6 very high good low moderate very high
F6 (Tongue) very high fair high very high moderate G6 very high poor high high high
a Includes increases in streamflow magnitude and timing and/or sediment increases. b Assumes natural recovery once the cause of instability is corrected. c Includes suspended and bed load from channel derived sources and/or from stream adjacent slopes. d Vegetation that influences width/depth ratio-stability. Source: Rosgen, 1994
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
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Table 3.6 Siting Considerations for Various On-Channel Site Characteristics Site
Characteristic Example Cross Section
on On-Channel Site Siting
Consideration
Sideslopes 2:1 or steeper,
bottom width less than 10
feet
Cost to construct dam will be low, but it may be difficult to tie the dam abutments into the steep sideslopes. The capacity of the reservoir may be limited however, the drainage area is likely to be insignificant.
Sideslopes flatter than 2:1, bottom width less than 10
feet
Cost to construct the dam will be low. The capacity of the reservoir may be limited, and the drainage area may be significant
Sideslopes 2:1 or steeper,
bottom width more than 10
feet
Dam construction costs will be high. May be difficult to tie dam abutments into the sideslopes. The capacity of the reservoir may be improved, but the drainage area may be significant. Deposition of sediments during rain events may limit the life of the impoundment.
Sideslopes flatter than 2:1, bottom width more than 10
feet
Dam construction costs will be moderately high. The capacity of the reservoir may be improved, but the drainage area is likely to be significant. Deposition of sediments during rain events may limit the life of the impoundment.
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
26
Groundwater Table Infiltrating water
Infiltrating water
Surface Discharge
Reservoir “B”
Impervious Layer
Porous Layer
Porous Layer
Figure 3.4 Off-channel impoundment examples
Reservoir “A”
Impervious Layer
3.5. Geology, Soils, and Hydrogeology
An impoundment is designed with the intention of allowing water to pass through the materials at the
base of the impoundment through the underlying soils and bedrock. As the water moves horizontally and
laterally through the subsurface it will contact the various materials that are present in the soils and
bedrock, as well as interacting and mixing with any groundwater that is present. By understanding the
geology of an area, a CBNG operator can determine the extent to which an area is suitable for the use of
infiltration as a water management practice and changes that may need to be incorporated into the
impoundment design for a given site. The geologic conditions present across the PRB can vary enough
so that even within a single producing field variations can result in different design criteria for each
impoundment. An understanding of the mineralogy of the bedrock underlying the soils and the strata
that outcrop within the project area can help an operator predict geochemical changes which may occur
as infiltrating water comes into contact with these materials.
Furthermore, understanding bedrock and outcrop geology will help an operator determine if the water
that infiltrates through the strata of the PRB will either be contained within these formations as
groundwater or eventually discharged to surface water or springs, or withdrawn from the formation via a
well.
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
27
3.5.1. Background Geology of the Powder River Basin
The Powder River Basin is circumscribed by a geologic boundary which defines the area of major
Tertiary-age deposition. The formations within the PRB are the result of Late Cretaceous and Tertiary
sedimentary deposition and subsidence. The PRB basin is filled with several miles of accumulated
sediments that are predominantly sands, shales, coals, and limestones. The Tertiary sediments are of
particular interest for the potential coal and CBNG resources (ALL, 2001). Figure 3.5 presents a geologic
outcrop map of the Powder River Basin illustrating the broad geometry of the basin, while Figure 3.6 is a
stratigraphic column of these units with the major Tertiary coal units within the basin. Figure 3.5 shows
the outlines of the major rivers which dissect the PRB, an area where alluvium deposits dominate. The
alluvium present along these rivers is reworked sediments eroded from the other formations and re-
deposited by the actions of the rivers. Figure 3.5 also illustrates the presence of coal clinker where
outcrops of the Wasatch and Fort Union coals occur within the basin. These clinker outcrops represent
the recharge zones for the coal aquifers in the central part of the PRB.
The stratigraphy of the PRB reflects the continuous development of thousands of feet of sediments,
which include sands, coals and fluvial deposits, and other fine-grained sediments. The stratigraphic
column presented (Figure 3.6) shows the major formations with call-outs for the major Fort Union coal
seams which are present in discontinuous bands across most of the PRB. The coal seams of the Wasatch
and Fort Union Formations are of particular interest in the PRB because they are the source of CBNG
production both for natural gas and produced water, as well as being widespread aquifers which are
utilized for drinking and agricultural use water. All of the formations present in the Wasatch and Fort
Union Formations are important for this study in helping to understand areas where infiltration basins are
suitable as produced water management options.
3.5.2. Background Hydrogeology of the Shallow Powder River Basin Formations
Recharge of the groundwater in the PRB is derived from infiltrating precipitation, runoff that flows into
losing stream reaches, and other water bodies (ponds, pits, etc). The PRB has a semi-arid climate with
precipitation averaging less than 16 inches per year (NOAA, 2001). Currently, the PRB area is classified
as undergoing an extreme to severe drought by the U.S. Drought Monitor, which is the result of less than
average rates of precipitation over the last 7 years. It is important to understand the nature of the
climate and current drought when evaluating a location for the siting of an impoundment, because these
drought conditions have affected the current hydrogeologic conditions in the basin. The drought has
resulted in less than typical recharge to groundwater from infiltration of precipitation and from stream
flows, so water levels are reduced in many groundwater aquifers.
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
28
Figure 3.5 Geologic Outcrop Map of the PRB
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
29
Table 3.7 presents hydrogeologic data for the various shallow geologic formations which occur in the
PRB. The table presents characteristic ranges of values for aquifer thickness, water yields, water quality,
and depth for the different formations. Additionally, information related to the lithologic composition and
infiltration classification is presented. Infiltration classification includes the ability to infiltrate water and
the possible impact that water may have on CBNG development and on regulatory concerns such as
discharge to surface waters or seepage of the water at nearby outcrops.
Figure 3.6 Stratigraphic Column of Tertiary Coals in the Powder River Basin
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
30
Table 3.7 Hydrogeologic Properties of Formation Present at the Surface in the PRB Outcrop
Unit (Age) Thickness
(feet) Lithologies Water Availability
Water Quality
TDS (mg/L)
K Value (ft/s)
Infiltration Classification
Depth (feet)
Alluvium (Quaternary/
Recent)
10 to 40, as high as
90
Unconsolidated silt, sand, and
gravels
Yields to 1,000+ gpm
Good to poor
400 to 9,000
1 x 10-7 to 4.1 x 10-3
High infiltration rates, may impact area streams, may breakout to surface
Shallow
Ft. Union Coals
(Paleocene) 10 to 50 Coal with clay Yields to 30
gpm Good to
poor 400 to 2,800
4.6 x10-7 to 8.6 x 10-4
High infiltration rates, may impact CBNG prod., may breakout to surface
Up to 2500
Ft. Union Clinker (Recent)
5 to 50 Baked coal and clay Very high Good 2 x 10-5 to
6 x 10-6 Very high, may
breakout to surface Shallow
Ft. Union Sands
(Paleocene) 5 to 50 Clayey sand
Yields less than 10 gpm, average much
less
Poor 2,100 to 3,000
4.1 x10-6 to 4.1 x 10-4
Rates medium to low. Some sands
appear to be persistent, most do
not
Up to 4,000
Ft. Union Claystones (Paleocene)
10 to 50 Claystone, shale Aquitards None N/A 8.1 x 10-8 Poor Up to 4,000
Wasatch Sands 5 to 50
Lenticular sandstones, interbedded with fines
Yield 10 to 500 gpm,
average 10 to 50 gpm
Good to poor
600 to 4,000
2.3 x10-7 to 2.3 x 10-4
Medium to low. Sands are
discontinuous, lenticular, with fines present.
Up to 800
Outcrop geology is vitally important to the design of CBNG impoundments. The following section details
the structural configuration, lithology, mineralogy, and hydrology of the important outcropping units in
the Basin. Furthermore, these geological considerations are discussed as they influence the design of
CBNG impoundments.
3.5.3. Alluvium
Alluvium consists of unconsolidated sand, silt, and gravel that make up the floodplains and stream
terraces of creek valleys in the PRB (BLM, 1999). Alluvium is present in the PRB along the major rivers as
shown on the surface geology map in Figure 3.5. Thickness of the alluvium varies widely across the
basin with total thickness not expected to exceed 90 ft. Near the Powder River, thickness ranges from 4
ft. to 45 ft. thick but are commonly in the 10 to 30 ft. range (Ringen and Daddow, 1989). Similar
thicknesses of sediment accumulation may be expected along the other major rivers within the Powder
River Geologic Basin, while lesser thicknesses of accumulations would be expected along the tributaries
to these rivers. Lithology is somewhat dependent on bedrock outcrop. Alluvium derived from and
overlying Tertiary strata are mostly fine-grained to medium-grained sands and silts (Hodson, et al. 1973).
Coarser-grained alluvium has been associated with some of the larger rivers which are sourced from hard
rocks outside the PRB (Hodson, et al., 1973). Finer-grained alluvial material will likely have a lower
hydraulic conductivity than the coarser material. Mineralogy is also likely to be different with more
soluble material present in alluvium derived from Tertiary sediments.
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
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Understanding the location and thickness of the alluvium nurtures development of theoretical
expectations of the fate of water that infiltrates through an impoundment constructed over alluvial
materials as well as development of an understanding for the quality of the existing and resulting alluvial
waters. Previous research by Ringen and Daddow (1989) has demonstrated that the water in the
alluvium around the Powder River is primarily derived from seepage near losing stream reaches when
streamflow is high; the groundwater then discharges back into the river during periods of low flow.
Because of this interaction between the groundwater in the alluvium and surface water in the rivers, any
water that is allowed to infiltrate into the alluvium has the potential to be discharged directly to surface
streams during periods of low flow. The intimate connection between alluvial groundwater and surface
water in PRB streams makes the alluvial environment vulnerable to changes from in-channel
impoundments.
Design Considerations for Impoundments Constructed on Alluvium
There are potential advantages and disadvantages to constructing an impoundment over alluvial
materials in the PRB. An impoundment constructed over alluvium has the potential for greater infiltration
than some of the other geologic materials in the PRB, especially those materials with high clay content.
Because the groundwater present in the alluvium is in hydrologic connection to the surface water, the
groundwater quality can be very similar to the
surface water quality (high quality water would
be expected in alluvium near the Tongue
River, while low quality water is typical of the
Powder River alluvium). The nature of the
relationship between alluvial groundwater and
surface water also affects the infiltration
capacity as less pore space is available for
infiltration into the alluvium during high
surface water flow periods when streams and
rivers are loosing water to the alluvium.
Figure 3.7 illustrates how the stage of the
stream affects infiltration capacity in the
alluvium. Under the high flow conditions,
water is lost from the stream to the alluvium
raising the groundwater level in the alluvium
resulting is less available capacity for
infiltration.
Impoundments constructed over alluvium have
Figure 3.7 Stream Stage and Infiltration
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32
the potential to infiltrate large volumes of water; Ringen and Daddow (1989) approximated alluvial
transmissivity from aquifer diffusivity (ratio of transmissivity to storage coefficient) for Powder River
alluvium at 2,040 ft2/day. Aquifer diffusivity is used to assess alluvial responses to pumping and changes
in nearby surface water stages, but is not a common term for groundwater analysis in confined or
unconfined aquifers, and, therefore, the transmissivity value is discussed here. Because the
transmissivity values are high for the Powder River alluvium, it is anticipated that large volumes of water
could be infiltrated into the system and, therefore, impoundments of considerable capacity could be
constructed over alluvium. Considerations for how large an impoundment to construct would include
distance and elevation of the nearest surface water body, total thickness of the alluvium, aerial extent of
the alluvium, and depth to groundwater below the bottom of the impoundment.
The hydraulic head that is present beneath the impoundment and its elevation relative to the nearest
surface water body will help determine the extent to which alluvial groundwater will be discharged to the
surface water. Vertical and horizontal hydraulic conductivity of the alluvium will combine with head
differences to determine infiltration and migration. The distance and elevation of the nearest surface
water is one part in understanding the hydraulic gradient that will be present under the impoundment
after infiltrating water has saturated the space beneath the impoundment. A larger hydraulic gradient
between the impoundment and the surface water can lead to increased discharges of alluvial water to the
river depending upon horizontal and vertical conductivity.
The geometry of alluvial litho-facies are often complex and intricate relationships including both the aerial
extent and total thickness of alluvial materials and can affect how much water can be infiltrated. An
impoundment site with lateral continuity of alluvium and thick deposits of porous and permeable,
unsaturated alluvium has the potential to receive and distribute large volumes of infiltrating water. By
defining the aerial extent of the alluvium, finding internal flow-units, and estimating the porosity and
permeability of the alluvial compartment, calculations of the receiving capacity of the local alluvium can
be performed to help define capacity for an impoundment.
There are some other considerations that can be included in the design of impoundments over alluvium
within the PRB. Water rights issues can be considered when constructing dams on-channel in both
Wyoming and Montana. One option for dealing with the water rights issues is to create a diversion
system for water coming from up-channel of the impoundment to bypass the impoundment and be
returned to the channel downstream. A diversion channel around the impoundment could allow surface
water coming down the channel to continue past the impoundment without change to quantity and
quality for use by downstream water rights holders.
Other issues which may need to be addressed with state regulatory officials are the changes to
groundwater discharges to the rivers from the elevated groundwater levels near the impoundment.
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33
Elevated groundwater levels could result in a pressure head that acts to push existing alluvial
groundwater out into the channel of the river. During low flow periods or droughts, such as what the
PRB is currently experiencing, this phenomenon could provide additional water resources to downstream
users. In addition, in watersheds where the existing alluvial groundwater is lower quality than CBNG
produced water, there could be water quality benefits over the lifetime of the impoundment. These
benefits could include the dilution of lower quality alluvial groundwater with CBNG produced water,
unless that produced water picks up added minerals while infiltrating. Adding higher quality water to the
alluvium can result in future discharges into the surface water being of higher quality than current alluvial
groundwater discharges. Water from infiltration could also flush out salts that have built up in the alluvial
system over time, resulting in improved water quality within the alluvial aquifer system. In these cases,
regulatory agencies will need to be included in the planning and development to ensure that any
impoundment which has the potential to result in discharges to surface waters is properly permitted and
meets state regulatory requirements.
3.5.4. Coal Clinker
Coal clinker deposits are formed by the natural burning of coal beds and the resultant baking or fusing of
clayey strata overlying the burning coal. Coal clinker deposits are present throughout much of the PRB
and can be more than 125 feet thick (Tudor, 1975). Stratigraphically, the clinker bodies are part of the
Fort Union but the clinker is a separate lithological unit composed of baked and fused siltstone, clay, and
sandstone units that have undergone diagenetic changes during the combustion of the coal within the
past 3.0 million years (Heffern, 1999). As a result of the baking, the clinker deposits are resistant to
erosion by water and wind; so many of the hilltops within the PRB are capped by clinker (Heffern and
Coates, 1997). Baking also greatly increases porosity as well as horizontal and vertical permeability of
the clinker, allowing clinker beds to accept large volumes of infiltrate. Figure 3.5 illustrates the
distribution of the coal clinker along the eastern side of the PRB in Wyoming and the south central region
of the PRB in Montana.
Research on coal seam aquifers report that the coal clinker deposits are highly permeable and act as
recharge zones for coal seams and other underlying aquifers of the Wasatch and Fort Union Formation
(Moreland, 1987; Heffern and Coates, 1999; and Bartos and Ogle, 2002). In areas where Wyodak-
Anderson coal bed aquifers outcrop, coal clinkers are unconfined and the beds dip generally toward the
center of the basin (Bartos and Ogle, 2002). In these regions, impoundments constructed over coal
clinker may be used to recharge coal seams.
Design Considerations for Impoundments Constructed on Clinker
The design considerations for constructing impoundments on clinker deposits are related to identifying
and understanding the location, structural attitude, and stratigraphic distribution of the clinker. The
erosion-resistant nature of coal clinker can result in its presence on isolated topographic highs in addition
Siting, Design, Construction, and Reclamation Guidebook for CBNG Impoundments
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to lowland outcrops which are likely still hydrologically connected to deeper coal seam aquifers in other
parts of the basin (Heffern and Coates, 1997). In order to facilitate infiltration into coal clinker and
facilitate recharge to coal seam aquifers, an investigation of the surface environment surrounding the
proposed impoundment site can identify clinker outcrops, which could result in the surface discharges or
springs. Ideally, potential impoundment locations will also be identified within the project area where the
clinker exists in depressed or lowland areas where clinkers and coal beds dip toward the center of the
basin. Impoundments constructed in these areas could facilitate greater rates of infiltration into the
subsurface and prevent discharge back to the surface at nearby outcrop areas. Impoundment designs on
top of clinker deposits may also require alternative construction designs to account for the irregular
outcrop patterns of the clinker and maximize the area of interface between the bottom of the
impoundment and the clinker. On the other hand, large rates of infiltration may impact down-dip coal
seams that are productive to CBNG; structural analyses will predict those areas that may impact CBNG
production. The Wyoming Department of Environmental Quality has siting guidelines which set
limitations to the siting of impoundments over clinker zones; it is important for operators to understand
local regulations which may influence the siting of impoundment structures which are intended to
facilitate infiltration of water.
3.5.5. Wasatch and Fort Union Formations
The Wasatch Formation and underlying Tongue River Member of the Fort Union Formation are
geologically similar and consist of irregular and discontinuous sandstones with interbedded finer-grained
siltstones, claystones, mudstones and some coals (Bartos and Ogle, 2002). Figure 3.5 illustrates the
surface geology of the PRB, showing that the outcrop of the Wasatch Formation is primarily limited to the
central and western portions of the PRB in Wyoming. The Fort Union Formation’s Lebo/Tullock Members
outcrop along the eastern side of the basin in Wyoming and continue northward into Montana along the
northern boundary (Figure 3.5). The Tongue River Member outcrops along the central portion of the
basin in Montana (Figure 3.5). The Wasatch Formation is present at the surface throughout the central
part of the Wyoming portion of the PRB where current expansion of the CBNG producing area is
occurring. Similarities in the lithological composition and depositional history make differentiation of the
Wasatch Formation from the Tongue River Member of the Fort Union Formation difficult (Ellis, et. al.
1999). Localized differences such as changes in sand and silt content, clay mineralogy, gypsum and
other mineral content, and bedding characteristics can all be important when designing impoundments
over these formations. Finely divided mineral content in these Tertiary beds has great power to degrade
infiltrate as it moves through subsurface strata.
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Design Considerations for Impoundments Constructed Over the Wasatch and Fort Union Formation
Impoundment design considerations for the Wasatch and Fort Union Formation consist of identifying sites
where infiltration can be maintained with minimal change to the chemical character of the infiltrating
water. Sandstones occur throughout both formations and may be favorable receiving zones for
infiltrating water because of the higher transmissivity of the sandstones when compared to the finer
grained claystones, mudstones, and siltstones. In addition, because these sandstones are capable of
moving large quantities of groundwater, the amount of salts which may have accumulated in the
unsaturated zones of these lenses may be less than in the finer grained sediments which are likely to
retain water longer. Design considerations for impoundments may include excavation of overlying finer-
grained sediments to ensure the foundation or base of the impoundment is directly over the sandstone
units.
3.5.6. Geology and Hydrogeology Siting and Design Criteria
The design considerations for constructing an impoundment over various types of geological formations
have been discussed in this section. A summary of this section is provided in Table 3.8 to be used as a
tool in the decision-making process regarding the siting, design, and construction of impoundments
relative to the geological formations present in the PRB. This table can lead to better informed decision
making throughout the siting process as well as mitigation of potential risks during the design and
construction of an impoundment.
Table 3.8 Pros and Cons of Constructing an Impoundment Over Various Geological Formations
Outcrop Unit (Age)
Pros Cons
Alluvium (Quaternary/
Recent)
• High infiltration rates can lead to managing greater volumes of water in less time
• Increased discharge rates of alluvial water to the river may be experienced, providing more water to surface users
• Infiltration rates may be impacted during high surface flows due to less available pore space
• Mineral content in vadose zone may cause infiltrating water quality to degrade
• Faults and/or fissures in the claystones underlying the alluvium may provide a pathway for water to infiltrate to deeper aquifers
• Surface breakouts are possible, leading to regulatory violations
Ft. Union Coals
(Paleocene)
• The Fort Union unit makes up a large percentage of the outcrops in the PRB
• High infiltration rates can lead to managing greater volumes of water in less time
• Recharge of coal seam may impact CBNG production
• Nearby coal outcrops may serve as a source for water to discharge to the surface, leading to regulatory violations
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Outcrop Unit (Age)
Pros Cons
Ft. Union Clinker
(Recent)
• High infiltration rates can lead to managing greater volumes of water in less time
• Water quality in the clinker is good
• Recharge of coal seam may impact CBNG production
• Nearby clinker outcrops may serve as a source for water to discharge to the surface, leading to regulatory violations
Ft. Union Sands
(Paleocene)
• The Fort Union unit makes up a large percentage of the outcrops in the PRB
• May provide higher infiltration rates than finer grained claystones, mudstones, and siltstones
• Mineral content in vadose zone may cause infiltrating water quality to degrade
• Identifying ideal soil chemistry can be costly
• Sandstones may not be present at the required design depth
• Faults and/or fissures in the claystones underlying the sandstones may provide a pathway for water to infiltrate to deeper aquifers
Ft. Union Claystones (Paleocene)
• Not conducive for infiltration through impoundments
• Low infiltration rates unless faults/fissures exist
• If present, faults and/or fissures may provide a pathway for water to infiltrate to deeper aquifers
• Mineral content in vadose zone may be much higher due to the nature of the claystone to hold water use infiltrating water quality to degrade
Wasatch Sands
• The Wasatch unit makes up a large percentage of the outcrops in the PRB
• Sandstones present may provide medium to low infiltration rates
• Sandstone may also lessen impacts on infiltrating water quality (as compared to claystones, mudstones, and siltstones)
• Mineral content in vadose zone may cause infiltrating water quality to degrade
• Identifying ideal soil chemistry can be costly
• Sandstones may not be present at the required design depth
• Faults and/or fissures in the claystones underlying the sandstones may provide a pathway for water to infiltrate to deeper aquifers
3.5.7. Groundwater Flow and Impoundments
The following section presents some introductory discussion of groundwater theory for impoundments, as
well as discussion of the data on the hydrogeologic properties of the materials present in the shallow
subsurface (<200 ft) of the PRB, as presented in Table 3.7. The intent of the groundwater discussion is
to present conceptual analysis tools for infiltration conditions which can then be used to develop criteria
for identifying siting and design conditions on a site specific level, as well as relating general siting
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criteria. Hydrogeologic data for the geologic formations present in the shallow subsurface of the PRB are
presented in Table 3.7.
Groundwater flow through any porous media is generally considered to have two principal components,
vertical and horizontal, thus as water infiltrates through the bottom of an impoundment the water will
move both downward and outward from the base of the impoundment. In the initial period after the
start up of an impoundment the groundwater flow beneath the impoundment generally occurs under
unsaturated flow conditions. The soils beneath the impoundment are typically not saturated with respect
to groundwater for some depth. The depth to groundwater saturation will vary depending on site specific
conditions; for on-channel systems this depth will generally be less than for off-channel systems. For
instance, in an intermittent surface water drainage channel this depth may be only a few feet during
much of the year, while on a hilltop or elevated plateau this depth may be several hundred feet deep.
Generally, low lying areas that are exposed to greater volumes of surface flow will have shallower depths
to groundwater than areas which are only exposed to precipitation events.
Depth to groundwater, or thickness of the unsaturated zone, determines the volume of space available
for infiltrating water as well as the area with which the infiltrating water has to interact with the soluble
minerals present in the soils and bedrock. Unsaturated pore space in soils and bedrock are areas
available to water flowing from the bottom of the impoundment. Once these pore spaces are filled,
additional infiltration volume must be created beneath the impoundment, either by moving the existing
groundwater horizontally or vertically. The greater the distance or depth to groundwater the more
available free pore space there is for infiltrating water to encompass. However, the greater the pore
space available in the soils and bedrock beneath an impoundment the greater the area for contact
between the infiltrating water and soluble minerals present in the soils and bedrock. If the percentage of
soluble minerals present in the unsaturated zone is large, and the infiltrating water is undersaturated with
respect to these minerals, then the greater the dissolution of these minerals and the resulting greater
increase in the TDS of the infiltrating water. Thus, the depth to groundwater below an impoundment can
have both beneficial and detrimental assessment conditions for impoundments. In siting an
impoundment, consideration must be given to both sides when evaluating the depth to groundwater
under an impoundment.
3.6 Soils of the Powder River Basin
Surface and subsurface soils can present another element where changes can occur to infiltrating water
chemistry and therefore play a role in the siting of on-channel impoundments. With the construction off-
channel impoundments, soils are typically removed during the construction process, as excavation of
these soils is necessary to develop the storage capacity. Thus the applicability of any analysis of soils can
be predicated by the design of the impoundment, i.e. collecting shallow soil borings and running
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Table 3.9 Geochemical Data for the Shallow Surface Soils of the PRB by County Cation Exchange Capacity Percent Calcium Carbonate Percent Gypsum Wyoming
Counties Avg. Min Max Std Dev Avg. Min Max Std Dev Avg. Min Max Std DevCrook 16.5 0 42.5 7.91 1.15 0 22.5 2.28 0.20 0 37.5 2.31
Niobrara 13.4 4 35 6.15 1.03 0 12.5 1.89 0.01 0 2.5 0.17 Weston 15.2 3 37.5 7.01 1.88 0 12.5 2.77 0.05 0 2.5 0.28 Natrona 14.3 0 32.5 6.00 3.02 0 35 4.15 0.25 0 10 1.22 Sheridan 16.8 4.5 45 5.78 1.96 0 10 3.20 0.01 0 2.5 0.12 Campbell 15.1 3 49 6.42 0.76 0 5 1.07 0.02 0 2.5 0.21 Converse 12.6 3.5 32.5 5.65 1.85 0 20 2.80 0.01 0 2.5 0.15 Johnson 14.6 1.5 49 7.16 1.55 0 20 2.41 0.12 0 30 1.22
Cation Exchange Capacity Percent Calcium Carbonate Percent Gypsum Montana Counties Avg. Min Max Std Dev Avg. Min Max Std Dev Avg. Min Max Std Dev
Carter 18.9 7.5 35 7.04 3.11 0 7.5 2.75 -- -- -- -- Custer 17.7 7.5 35 6.02 4.60 0 7.5 2.80 0.5 0 2.5 1.12 Prairie 16.5 3 35 5.96 2.38 0 7.5 3.25 0 0 0 --
Treasure 19.9 3 40 7.25 2.74 0 7.5 1.70 -- -- -- -- Bighorn 20.4 7.5 37.5 6.38 3.54 0 10 2.19 -- -- -- -- Powder River 18.0 7.5 32.5 5.63 4.51 0 7.5 2.72 -- -- -- --
Rosebud 19.2 7.5 37.5 6.48 1.89 0 10 2.67 0 0 0 0 Source data: SSURGO county data.
subsequent data analysis at an off-channel impoundment site may not provide relevant information on
the infiltration potential at that site or in changes to infiltrating water chemistry. In cases where the
surface soils are removed during the construction of the impoundment, this data would not provide
information relative to the post construction conditions at the impoundment. However, understanding
the types of soils present near on-channel impoundments can lead to information relative to the clay
content, cation-exchange capacity, and the percentage of certain soluble mineral assemblages in the soils
each of which can cause changes to the infiltrating water chemistry.
Soil chemistry and physical soil property data can be obtained at general levels from the State Soil
Geographic Database (STATSGO) and the U.S. Department of Agriculture’s Natural Resource
Conservation Services Soil Survey Geographic database (SSURGO). While data from these resources is
not uniformly available at the site specific level for the siting of an impoundment, the data available
through SSURGO is more detailed at a 1:24,000 scale than the STATSGO data set which is 1:250,000
scale. The SSURGO dataset can be used for a preliminary assessment of impoundments, to evaluate soil
taxonomy and potential conditions such as clay percentage, percentage of soluble mineral assemblages,
and cation exchange capacities.
SSURGO data was gathered for seven counties in the Montana portion of the PRB and eight counties
within the Wyoming portion of the PRB and summarized in Tables 3.9 and 3.10. The data presented in
Tables 3.9 and 3.10 are statistical summaries of the cation exchange capacity, percent calcium
carbonate, and percent gypsum for surface soils and soils of maximum depth in the PRB, identified by
county. The soil data presented here is based on statistical summaries of the 1:24,000 scale data for
each county, and grouped by county to show generalized trends in the PRB. Within each of these
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Table 3.10 Geochemical Data for the Maximum Depth Surface Soils of the PRB by County Cation Exchange Capacity Percent Calcium Carbonate Percent Gypsum Wyoming
Counties Avg. Min Max Std Dev Avg. Min Max Std Dev Avg. Min Max Std Dev Crook 16.0 0 41 8.18 6.06 0 27.5 5.03 0.34 0 12.5 1.17
Niobrara 9.93 2 25 6.06 5.96 0 20 4.52 0.05 0 3 0.36 Weston 15.3 0 32.5 7.46 6.81 0 20 3.68 0.13 0 3 0.49 Natrona 13.8 0 30 6.96 9.63 0 55 7.38 0.88 0 15 2.19 Sheridan 16.8 0 35 7.58 8.83 0 45 5.90 0.15 0 3 0.42 Campbell 14.0 0 42.5 7.97 7.03 0 10.5 3.64 0.24 0 3 0.43 Converse 12.0 0 34 7.78 6.85 0 35 4.79 0.21 0 7.5 0.84 Johnson 15.0 0 45.5 8.29 8.22 0 27.5 4.70 0.53 0 10 1.39
Cation Exchange Capacity Percent Calcium Carbonate Percent Gypsum Montana Counties Avg. Min Max Std
Dev Avg. Min Max Std Dev Avg. Min Max Std Dev
Carter 16.8 0.5 35 8.18 8.35 0 22.5 4.00 2.36 0.5 10 1.64 Custer 14.1 0 35 7.32 8.68 3 22.5 3.61 2.00 0 4 1.18 Prairie 11.5 0 27.5 7.19 5.80 0 12.5 4.49 2.27 0 10.5 1.86
Treasure 14.6 0.5 37.5 7.90 7.41 2.5 20 3.44 3.00 3 3 -- Bighorn 16.5 0 35 7.91 12.9 0 50 12.8 1.00 1 1 -- Powder River 15.1 1 27.5 6.19 9.16 2.5 20 4.37 -- -- -- --
Rosebud 14.1 0 40 6.90 5.37 0 30 5.52 2.92 0 15 2.15 Source data: SSURGO county data.
counties there are approximately 100 to 600 different soil taxonomies which were statistically analyzed
over two depth intervals to provide the data in Tables 3.9 and 3.10. The shallow surface soil sample
interval depth varies, and is represented by the first horizon of soil. The maximum depth soil samples
represent the data from the deepest data point presented in the SSURGO data, up to 182 cm in depth.
Because the depth intervals for soil zones vary between different soil types, both the surface soil and
maximum depth range are presented to show the variation that occurs across the soil profile.
Cation exchange capacity (CEC) is a soil property attributed to the type and quantity of clay minerals and
organic matter present in a soil. The cation exchange capacity of a soil is the degree to which the soil
particles are capable of attracting and holding positively charged (cation) ions on their surface. Cation
exchange capacity of a soil can vary laterally as well as vertically within the soil profile. Data on CEC of a
soil can be found in SSURGO data, and site specific data can be acquired from a soil laboratory testing
facility. The CEC data presented in Tables 3-9 and 3-10 illustrate the variation that can be seen in soils in
the PRB[ generally the shallow surface soils have higher average CECs than the maximum depth soils.
There are three Wyoming counties, Weston, Sheridan, and Johnson, where the soils at depth have a
slightly higher average CEC. The data also shows a difference in the average CEC of the shallow surface
soils between the two states with the Montana soils having a higher average CEC than the Wyoming
soils. The Montana soil’s average CEC decrease with depth is 3.99 meq of cations/100g of soil compared
to the Wyoming soil’s average CEC decrease of 0.71 meq/100g. Figures 3.8 and 3.9 show plots of the
average CEC values for the shallow soils and maximum depth soil samples, respectively, within the
counties of the PRB. The maps illustrate the higher average CEC in the shallow soils in the northern part
of the basin with values decreasing toward the southeast of the basin.
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Figure 3.8 Average Cation Exchange Capacity for Surface Soils, by County
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Figure 3.9 Average Cation Exchange Capacity for Max Depth Soils, by County
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Based on the average CEC of shallow subsurface soils as shown in Figure 3.8, a preliminary assessment
of potential impoundment locations on a county wide basis would indicate areas in the southeastern
Wyoming portion of the basin have lower CEC potential which would result in greater geochemical
changes to infiltrating water. However, the CEC of the individual soil taxonomies within a county may
indicate zones where high CEC soils are present in Converse County, Wyoming, or areas where low CEC
soils are present within the counties to the north in the PRB.
Tables 3-9 and 3-10 also present statistical data for two soluble minerals commonly present in soils which
can affect the quality of infiltrating water, CaCO3 (calcite) and gypsum. The tables show that these two
soluble minerals are present in greater quantities at depth in the soil column. Within the PRB, these two
soluble minerals are generally present in greater quantities in the Montana counties than in the Wyoming
counties. Calcite in the shallow surface soils of the Montana counties averages between 1.9% and 4.6%,
while in the maximum depth soil samples the average percent calcite is 5.4% to 12.9%. In the Wyoming
counties, the average percent calcite in the shallow soils range from 0.76% and 3.0%, while the
maximum depth soil samples average between 6.0% and 9.6%. Although there is only limited gypsum
data present in the SSURGO database for shallow soils in the Montana counties, the maximum depth
samples appear to indicate that similar trends exist between the states for gypsum and calcite; there
appears to be more gypsum present on average in the Montana soils at depth than Wyoming.
In additional to soil chemical properties, clay mineralogy can affect impoundment design considerations.
Within the PRB, the clays that compose the surface soils are predominantly smectite clays
(montmorillonite family), a clay mineral commonly referred to as a “swelling” clay (Flores, et al. 1990).
The swelling nature of smectite is a result of its ability to take water into the clay’s internal structure
resulting in the expansion or swelling of the clay mineral. This swelling can result in decreased porosity
and permeability of the soils which could cause infiltration rates under impoundments to decrease
considerably. SSURGO data provides information relative to the percent clay and type of clay present in
the soil taxonomies within an area, which can be used to screen for areas of high swelling clay content.
3.7 Vegetation
The natural vegetation of most of the PRB is a mixture of grasses and sagebrush. Common plants of the
plains grasslands include prairie sand reed grass, needle and thread grass, western wheatgrass, blue
grama grass, little bluestem grass, big sagebrush, and greasewood. Plains cottonwood trees commonly
grow along stream bottoms in the plains, whereas breaks and upland areas may support thin stands of
ponderosa pine and juniper. The vegetation of the western edge of the PRB reflects greater precipitation
on the mountains than on the plains. Trees, such as Douglas firs, alpine fir, and aspen, are the
predominant vegetation in the mountains, while pines predominate in the foothills. Bluebunch
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Table 3.11 Native Vegetation Tolerance Levels for Salt and CaCO3 Adapted to Soil Texture: Plant
(common name) Tolerance to
Salt Tolerance to
CaCO3 Moisture Use
Coarse Medium Fine Grasses and Sagebrush
Prairie sandreed grass Low High Low Yes Yes No Needle and thread grass None Medium Low Yes Yes No Western wheatgrass High High Medium No Yes Yes
Blue grama grass Medium High Medium Yes Yes Yes Little bluestem grass None High Low Yes Yes Yes
Big sagebrush Low High Medium No Yes No Greasewood High High Low No Yes Yes
Bluebunch wheatgrass Low High Low Yes Yes Yes Columbia needlegrass Medium None Low No Yes No
Trees Alpine fir None Low Medium Yes Yes No
Aspen None High High Yes Yes Yes Ponderosa Pine None Low Medium Yes Yes No
Plains Cottonwood None Medium High Yes Yes Yes Source: Compiled from the USDA PLANTS database, 2004.
wheatgrass, Columbia needlegrass, spike fescue, and big sagebrush are common plants in the clearings
between the trees (USGS, 1986).
3.7.1 Vegetation Siting and Design Criteria
The presence of existing vegetation may indicate areas of higher permeability of the soil, and the type of
vegetation may indicate the texture and geochemistry of the soil. For example, sideoats grama is
indicative of porous, well-drained soils while switch grass is commonly found on wet, heavy soils (Ohio
DNR, 1999). Furthermore, vegetation can be used both during and after construction to protect slopes,
uptake water from the soil, and establish a suitable and durable groundcover at the site.
Table 3.11 has been compiled from the USDA PLANTS database to show native species tolerance to salt
and CaCO3, as well as the potential for uptake of water and adaptability to various soil textures. The
tolerance to CaCO3 refers to the plant’s relative tolerance of calcium carbonate in the soil, while the
tolerance to salt refers to the plant’s relative tolerance to saline soil conditions, and the moisture use
refers to the plant’s relative moisture requirements for growth (USDA, 2004). During the investigation of
the feasibility of a site, this table can be used to predict how well the native vegetation will sustain itself
once the impoundment is in place, and identify areas where additional focus may be required to ensure
proper vegetation of the site (once construction is complete). For example, a site with western
wheatgrass may be preferable to a site with needle and thread grass due to the fact that the western
wheatgrass has a high tolerance to both salt and CaCO3 and has the capacity for a medium uptake of
water, while the needle and thread grass has no tolerance to salt, a medium tolerance to CaCO3, and a
capacity for a low uptake of water. Therefore, a site with western wheatgrass could add value to the
project because it has the potential for increased vegetative survival rates once the construction is
complete.
In addition to considering native grass types for siting purposes, native trees can also be looked at during
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the investigation of a site. Plains cottonwoods are common along stream banks in Montana and
Wyoming and have the capacity for a high uptake of water and a medium tolerance to calcium carbonate
in the soil. Cottonwoods do not have a tolerance to salinity, however. Therefore, areas where there is a
significant amount of plains cottonwoods are more desirable to areas that have no trees, so long as the
produced water does not have high levels of salinity that could potentially damage the cottonwoods. The
operator may want to avoid areas with a lot of cottonwoods, however, if the produced water has high
levels of salinity that may cause damage to the existing trees and soil.
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4. ESTABLISHING A BASELINE AND MONITORING PROGRAM The operator may choose to establish a baseline study on the existing conditions of a site to identify any
site-specific problems that can be mitigated prior to construction and operation of the impoundment by
making the appropriate design adjustments. Furthermore, baseline studies can also be used to save
money during the operations phase as many of the activities performed in a baseline study can be
utilized in monitoring the site during operations.
As mentioned above, the operator may choose to establish a monitoring program at the impoundment
once construction is complete to help manage liability and protect natural resources. Similar to a
baseline study, monitoring activities can include surface and groundwater monitoring as well as
monitoring the site for erosion, non-point source discharges, and/or evidence of potential dam failure.
This section will focus on the aspects of a baseline study that can be used in the site planning and design
of proposed impoundments. The parameters that will be discussed here include soils (both from a
mineralization and textural/structural standpoint), bedrock, groundwater, surface water, and the use of
shallow remote-sensing techniques to further delineate the geology of the site. Furthermore, this section
will discuss how to plan for groundwater monitoring during the baseline study, so as to reduce the overall
costs of a monitoring program. Monitoring the nearby surface water at the site, along with suggestions
for inspecting the site for erosion and non-point source discharges will also be discussed.
Although several of these monitoring and inspection activities may be required for various types of
regulatory compliance monitoring, this is not meant to be a discussion of regulatory compliance
monitoring. Regulatory compliance monitoring requirements can change with the political and judicial
climate, as well as with the specific site conditions from site to site, and therefore the requirements are
not necessarily set at any time or place. A summary of the monitoring requirements, as they exist at the
time of this publication, and a discussion of where the most recent monitoring requirements can be
obtained is included in Section 7.
For further discussion on the benefits of a project area baseline study (versus a site-specific baseline
study, as discussed here), and what parameters that can be included, refer to the Coal Bed Natural Gas
Handbook (ALL, 2004).
4.1. Soils Baseline
As mentioned in Section 3.5.3, the SSURGO soil data can be used for a preliminary assessment of the
baseline for soils at a site. Properties such as clay percentage, percentage of soluble mineral
assemblages, and cation exchange capacities can be reviewed for broad areas, but this data may not be
uniformly available at the site specific level. Therefore, in addition to reviewing the most current
SSURGO soil data, a site-specific soil investigation can be conducted for each impoundment. The
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operator can install soil borings in and around the proposed impoundment location to establish a better
understanding of the existing soil conditions. By installing the soil borings in locations that can be
completed into groundwater monitoring wells, the operator can save time and money during the planning
phase of the project. A discussion of determining where groundwater wells can be strategically located is
included in Section 4.3.
Understanding the existing soil conditions, both at the surface and at depth, may aid operators during
impoundment siting and design. Soils analysis that include soil salinity, soil K- factors, textures, slope,
soil classification, Atterberg limits, location and extent of rock strata, and permeability can assist
operators to determine the areas most suited for construction of an impoundment. Section 4.1.1 includes
a discussion of the various methods that can be utilized to collect soil samples, map the shallow geology,
and potentially map the surface of the bedrock for a proposed impoundment. Section 4.1.2 includes a
discussion of the mineralization of the soil and how it can impact siting and design of an impoundment.
Section 4.1.3 includes a discussion of the geotechnical properties of the soil and how it can impact siting
and design of an impoundment.
4.1.1. Methods for Collecting Soil Samples and Mapping Shallow Geology
As mentioned in Section 3.5, the capacity of the impoundment to manage produced water is dependant
on the ability of the shallow aquifer to accept the infiltrating water. Thus, knowledge of the nature and
extent of the shallow geology and the first confining bedrock layer can benefit the operator by providing
insight into the long term performance of an impoundment. Three widely used and accepted methods
for collecting soil samples and mapping shallow geology: 1) drilling, 2) direct push technology (DPT), and
3) trenching, are discussed here. These methods are generally considered intrusive, as they require
disturbance of the surface and collection of visual and/or physical samples of the soils. Drilling and DPT
involve the creation of a soil boring and logging the stratigraphy of the soil column noted either in soil
cuttings or in a thin sampling tube. Trenching generally involves visual observation and documentation
of the stratigraphy of the trench walls.
Drilling
There are various types of drilling techniques and models of drill rigs available. One of the most common
techniques used in the Powder River is air rotary drilling. Air rotary drilling uses a rotary-driven drill bit
along with a high-pressure, high-flow air that flows through the drill rod, out of the drill bit, and up the
annulus of the bore hole. This cools the drill bit, removes soil cuttings from the bore hole to the surface,
and stabilizes the borehole during drilling operations. Air rotary drilling can drill through competent
bedrock, making it the most capable technique for drilling deeper soil borings.
Hollow stem auger (HSA) drilling is another drilling technique that is available in the Powder River Basin.
HSA drilling consists of a bit at the bottom of the auger that cuts into the subsurface material and a spiral
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auger on the outside to convey the soil cuttings to the surface during drilling activities. The center of the
auger is hollow and during drilling or formation cutting is filled with rods connected to a plug at the
bottom bit. Once the desired drilling depth is reached, the center plug and rods can be pulled out while
the hollow augers stabilize the borehole for sampling and/or well installation. HSA may be able to drill
through weathered bedrock, but will typically reach “refusal” when faced with competent bedrock, which
means that the range for HSA drilling is not as deep as for air rotary.
A log of the stratigraphy of the soil boring can be created by observing the soil cuttings as they come to
the surface, and then documenting at what depth the drill bit is for those particular cuttings.
Direct Push Technology
Direct Push Technology (DPT) units are hydraulically powered, percussion/probing machines designed to
drive small diameter sampling tools directly into the subsurface without the production of Investigation-
Derived Wastes (IDW). The tools, commonly referred to as geoprobes, are advanced by static hydraulic
force coupled with a percussion hammer.
DPT units are typically capable of pushing sampling tools at a penetration rate of 5 feet or more per
minute, and typically to depths between 30 to 60 feet, depending on site conditions. With DPT units
mounted on various types of vehicles, some sites can be accessed where other drilling rigs would not be
able to access. Furthermore, soil boring logs can be created by observing the soil collected in the thin
sampling tubes, and documenting at what depth the soil occurred. Small diameter wells and piezometers
can also be installed for sampling groundwater.
If bedrock mapping is the primary goal, the DPT unit can push to “refusal” at various locations in a short
amount of time. Coupled with a site survey to determine surface elevations, the surface of the bedrock
can be mapped with relative ease.
Trenching
If construction equipment is available, trenching can be a quick and economical way to visually determine
the stratigraphy of the soil beneath the proposed impoundment. A backhoe or a track hoe can be used
to cut trenches in the proposed area of construction and a visual inspection can be conducted on the
walls of the trench. Soil samples can be collected from the bucket of the excavation equipment for
laboratory analysis. Safety precautions, such as putting up an orange safety fence around the
excavation, can help reduce potential injuries or accidents from occurring if the trench is left open.
Although trenching can be quick and economical, if can also be limiting in the amount of information that
can be collected from it. The trench can only be as deep as the length of the arm of the excavator
digging the trench. Typically, this is between 20 – 25 feet for a tracked excavator, and can be as little as
8 – 10 feet for a rubber tire back hoe.
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4.1.2. Geochemistry
Through the use of soil sampling and geochemical analysis, operators can determine the potential of the
infiltrating water to leach minerals, salts, and metals out of subsoil. Samples from the soil borings can be
collected from beneath the bed of the impoundment and analyzed for calcium, magnesium, sodium,
gypsum, manganese, carbonate, and bicarbonate. The potential for the infiltrating water to leach these
constituents from the soil is dependant not only on the presence of the minerals in the soil, but also the
quality of the produced water in the impoundment as well. The higher the quality of the produced water,
the more susceptible it is to leaching minerals from the soil as it infiltrates and saturates the existing
vadose zone.
4.1.3. Geotechnical
In addition to geochemical analysis, operators can gain insight for the design of the impoundment by
submitting soil samples for geotechnical analyses. Geotechnical analysis can help determine the soils
suitability for the foundation and abutments, any required foundation treatment, excavation slopes, and
availability and characteristics of embankment materials (USACE, 1994). By understating the soil
classification, physical properties, location and extent of rock strata, and groundwater piezometric levels,
the operator can design the impoundment in such away as to mitigate potential sources of trouble during
operations (such as dam failure due to an inadequate foundation treatment). These subsurface
investigations can be included both at the proposed impoundment site, as well as at proposed borrow
source sites (as necessary).
4.2. Remote Sensing of Site Geology Baseline
The methods discussed in Section 4.1.3 will provide the operator with an understanding of the shallow
geology at the site, but a more complete understanding of the shallow geology of the site may be found
through remote sensing technologies.
A 1:100,000 scale map of the PRB (see Figure 3.5) shows the extent of alluvium and terrace deposits as
well as important types of bedrock and surface faults. Various technologies, such as shallow seismic and
electromagnetic mapping, exist that allow for remote sensing of the geological properties of a site.
Remote sensing can be an effective tool to use in areas on the map that are in the vicinity of faulting.
These technologies can be especially powerful in being able to map surface faults and fractures,
distribution of clinkers, distribution of aquitards such as coal seams, and thickness of soils. This allows
for a more complete coverage of the site versus extrapolating between data points that may miss faults,
fractures, or lenses that can impact how the produced water will infiltrate through the soil matrix. This
section will discuss how these technologies work, and how they can be applied in a baseline study at the
site. It is important to understand that these technologies provide relative readings, so that changes in
subsurface geology can be noted as they relate to one another. Therefore, sample results from direct
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methods can be used to “calibrate” the results and allow for a full evaluation of the site. Another
approach is to use the results of the remote sensing technology to dictate where the intrusive sampling
should occur, thus enabling the operator the ability to explore all anomalies at the site without mobilizing
the intrusive sampling team on two separate occasions.
4.2.1. Shallow Seismic
Seismic methods of subsurface evaluation involve the generation of pulses of energy, usually at the
ground surface, consisting of compression, shear and surface waves that propagate through the ground
and either dissipate, are “reflected” back toward the surface, or are “refracted” at and travel along
lithologic boundaries. The wave energy which returns to the surface is picked up by receivers
(geophones) placed in a line on the ground surface. The geophones convert the wave energy to an
electronic signal which is recorded on a seismograph (DOE, 2001). “Reflected” seismic is typically
associated with deep seismic surveys related to the exploration for oil and gas, and “refracted” seismic is
typically associated with shallow seismic surveys (less than 100 feet deep). In general, reflection surveys
can be applied to map deeper targets, can produce higher resolution images, and require greater effort in
data collection and processing, resulting in higher cost than with refraction surveys (DOE, 2001).
A shallow seismic survey can be performed at the site to obtain additional soil data, allowing for the
shallow geology at the site to be economically characterized. Possible benefits to the design and siting of
an impoundment include, but are not limited to, detecting shallow subsurface changes that might be
related to sinkholes, tunnels, construction from mines, and mapping faults or joints and bedrock surfaces.
If a subsurface feature, such a fault or fissure in the bedrock, is identified during seismic testing at the
site, there may be hydrologic connectivity between the shallow aquifer and deeper aquifers which could
lead to produced water infiltrating to more than one aquifer. Depending on the quality of the CBNG
produced water and the quality of the groundwater aquifers, such a scenario may deter the operator
from constructing an impoundment at that particular site to mitigate the potential impact to more than
one groundwater aquifer.
4.2.2. Electromagnetic (EM) Mapping
Various EM mapping techniques exist and are currently being applied in similar context to how an
operator could apply them in a baseline study on an impoundment. Techniques include, but are not
limited to, the following:
• Ground magnets, sensitive to bedrock material, can measure magnetism at the surface and create bedrock maps.
• Transient Electromagnetic (TEM) Surveying is sensitive to horizontal and vertical variations in conductivity (such as water content and salinity) of the shallow soil and bedrock. A conductivity map can be created showing where anomalies exist (such as fractures and relatively high salinity content in soil).
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• Direct-current resistivity is sensitive to salinity and lithology variations, as it measures the bulk resistivity of soils and bedrock with depth. This technology is often able to obtain data at greater depths than TEM surveys.
As with shallow seismic, an EM survey can be performed at the site to obtain additional soil and
geological data, allowing for the shallow geology at the site to be economically characterized. Possible
benefits to the design and siting of an impoundment include, but are not limited to, detecting shallow
subsurface changes that might be related to sinkholes, tunnels, construction from mines, and mapping
faults or joints and bedrock surfaces.
4.3. Groundwater Baseline
The CBNG produced water that is discharged to an impoundment has the potential to interact with
existing shallow groundwater aquifers. Due to the fact that CBNG water quality may differ from the
water quality of the shallow aquifer, it can benefit the operator to establish the existing shallow aquifer
water quality prior to construction. Prior to beginning any soil or groundwater investigation activities, the
operator can strategically locate the soil borings so they can be used not only for soil samples, but so
they can be completed as groundwater monitoring wells.
The operator may want to install groundwater wells, lysimeters, or piezometers in such a way so that the
potentiometric surface of the groundwater can be mapped and the direction of groundwater flow can be
understood. Some possible locations to consider installing groundwater monitoring structures include: 1)
“up gradient” of the impoundment to better illustrate the “background” quality of the groundwater during
operations; 2) in/around the proposed dam location to identify foundation suitability as well as monitor
the stability of the dam during construction; and 3) further “down gradient” from the impoundment, but
upstream from a potential surface water discharge point (such as a river or stream) to identify the
existing water quality and monitor the impacts the infiltrating water has on the groundwater over time.
The operator may want to consider the configuration of the monitoring locations if multiple monitoring
structures are to be installed. For example, a more reliable potentiometric map can be created if the
monitoring structures are configured in a triangle versus in direct line with each other. By understanding
the quality of the existing groundwater and the direction in which the groundwater is flowing, the
operator can realize benefits during the design and planning of an impoundment by including mitigation
measures for potential problems that may become apparent.
4.4. Groundwater Monitoring
Groundwater is an important resource and an important part of the water cycle in the Powder River Basin
as the groundwater in most of the PRB watersheds provides the base flow to streams, creeks, and rivers.
A groundwater monitoring program can be utilized to observe changes to the groundwater system and
minimize the potential for negative impacts to groundwater from impoundment operations. The three
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Table 4.1 List of Potential Constituents Constituents
Arsenic Fluoride Barium Copper
Cadmium TDS Chromium Calcium
Lead Sodium Selenium Magnesium
Iron SAR Potassium Manganese
Zinc Sulfate pH Chlorides
Boron Temperature Specific Conductance
primary components of a groundwater monitoring program are:
1) the number and location of monitoring wells, as established in
the baseline study; 2) the identification of constituents that may
need to be monitored (see Table 4.1); and 3) the frequency that
monitoring is conducted, which is typically driven by regulatory
requirements.
4.5. Surface Water Baseline
A baseline of the surface water quality can be coupled with the
baseline groundwater quality to provide the operator with a
better understanding of the potential interaction between
surface water and the local groundwater system. Furthermore,
the operator may want to determine the baseline produced water quality in order to have a better
understanding of how the produced water will interact with the soil, groundwater, and the surface water
in the event that mixing occurs.
4.5.1. Existing Streams, Creeks, and Rivers
The operator may choose to establish a baseline for the surface water quality of existing streams, creeks,
and rivers that are located near the impoundment. Due to the seasonal variations of the surface water
quality throughout the year, the operator may choose to collect monthly water quality samples of these
surface water bodies over an extended period (as much as a year) prior to beginning operations. The
baseline study can consist of a wide variety of activities that can include, but is not limited to grab
samples, continuous EC monitoring, flow rate monitoring, and ecological sampling. By establishing a
baseline that spans the entire year, the operator can evaluate the natural changes (or lack of changes) to
the surface water quality and assess later changes that may be attributed to CBNG operations. It may
not be feasible for the operator to collect surface water samples for a full year prior to operations, but an
instantaneous grab sample prior to operations is still valuable in determining the effects of CBNG
production. In this instance, regional water quality sample data (such as that provided by USGS research
projects) can be used to show how the water quality changes throughout the year. While the sample
stations maintained by the USGS may not be close by the impoundment location, the seasonal trends
displayed at the USGS gauging stations will supply valuable information about surface water flow and
quality.
4.5.2. CBNG Produced Water
Various benefits can be realized by establishing a baseline of the produced water quality. For example,
the quality of the water may dictate whether the water can be used for irrigation, how it might interact
with the soil as it infiltrates through the vadose zone, and how it might react when mixed with surface
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and groundwater in a shallow aquifer. An example list of constituents that the operator may benefit in
analyzing is shown in Table 4.1.
4.6. Surface Water Monitoring
The operator may elect to include surface water monitoring with groundwater monitoring if there is
evidence in the groundwater monitoring results that surface waters may be impacted. If evidence of
groundwater impacts is seen in a nearby monitoring well, then the operator may choose to sample the
surface water down-gradient from the monitoring well. Furthermore, some discharges may be allowed
over the spillway for on-channel impoundments during storm events, and the operator may want to
monitor surface water downstream from the discharge point. By monitoring the surface water and
comparing results to the baseline study performed on the surface water, the operator may be able to
reduce liability by showing that the CBNG produced water has had little to no impact on the surface water
quality. This section will discuss monitoring the water in nearby streams, creeks, and rivers and the
water in the impoundment.
4.6.1. Existing Streams, Creeks, and Rivers
The operator may choose to monitor the surface water quality of existing streams, creeks, and rivers that
are located near the impoundment. Once operations begin, monitoring existing streams, creeks, and
rivers can be coupled with the groundwater monitoring program as discussed above. If there is no
evidence of migration in the groundwater monitoring program and there are no direct discharges during
storm events, or otherwise, then the monitoring of nearby streams, creeks, and rivers may not be
justifiable. Once the groundwater monitoring program shows evidence of the infiltration water migrating
into the groundwater (i.e. through a rise in the water table, or through analytical results indicating CBNG
produced water signatures), then surface monitoring can be considered more closely. Monitoring
frequency of the existing streams, creeks, and rivers can coincide with the frequency of the groundwater
and impoundment monitoring.
4.6.2. Water in the Impoundment
Monitoring can be performed at the impoundment to determine the extent to which the water chemistry
changes due to exposure to the atmosphere and the materials of the impoundment. Research performed
at 23 sites across five watersheds indicates that the exposure to the atmosphere and the oxygenation of
the produced water alters the chemical composition of the water between a discharge point and the
impoundment (Jackson, et al, 2003). Namely, the conclusion of the research indicated that pH and
arsenic tended to increase from the time the water passes the discharge point to the time it spends in the
impoundment, while barium tended to decrease over the same interval. The same list of constituents
found in Table 4.1 can be used as a basis for determining what analyses to run on the water sample
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collected at the discharge point; however, it would seem logical to analyze the impoundment water for
the same constituents that the discharge point was analyzed for in the baseline.
4.7. Erosion and Non-Point Source Inspections
Non-point source (NPS) pollution arises from many everyday activities that take place in residential,
commercial, and rural areas and is carried by storm water runoff to streams or through infiltration to
groundwater and groundwater discharge to streams. The operator can improve surface water quality by
monitoring NPS pollution caused by erosion during the construction of the impoundment as well as during
CBNG operations. During construction, the operator can utilize one of several industry accepted erosion
control Best Management Practices (BMPs), such as silt fences or hay berms, to prevent erosion of soils
during storm events. The effectiveness of the erosion control devices used can be inspected after rainfall
events larger than 0.5” to ensure they are structurally sound and working properly. Any areas that are
not working properly can be fixed to prevent future problems. These erosion controls can either be left
in place or removed once construction is complete, depending on the nature of the site and the controls
used.
4.8. Impoundment Inspections
The operator can also realize a savings in overall project cost by monitoring the impoundment (or series
of impoundments) for minor problems caused by various events, such as erosion from rainfall or growth
of unwanted vegetation in the dam, and correcting these minor problems through routine maintenance
versus correcting the major problems that arise when a structure experiences an untimely breach. Table
4.2 provides a summary of what can be done to mitigate minor problems that are observed during a site
inspection before they become major problems. If during the course of dam inspections water borne
larvae and/or adult mosquitoes appear to be prevalent in the area, the operator may want to takes steps
to minimize the threat of West Nile Virus spreading by referencing the Guidelines for Surveillance,
Prevention, and Control of the West Nile Virus (CDCP, 2003).
The frequency of the dam inspection can be dependant on the nature of the site (i.e. dams that are in
series with each other can be inspected more frequently due to their proximity and their impact on one
another), the operator portfolio (i.e. an operator with thousands of dams runs a higher likelihood of
experiencing dam failure than one with only a handful of dams, thus a more rigid dam inspection
program may be appealing), and the size of the structure. Annual inspections may be deemed sufficient;
however, the results of annual inspections may be reviewed and deemed that semi-annual (twice a year)
or bi-annual (once every 2 years) is more appropriate.
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Table 4.2 Impoundment Inspection Potential Mitigation Measures Earthen Dam Inspection
• Remove trees and woody vegetation • Remove/trap burrowing animals • Re-seed and repair bare areas or gullies • Repair holes, depressions, and/or cracks • Repair seepage, leakage, and/or “piping”
Principle Spillway Inspection • Remove trash and/or debris from trash rack • Clear obstructed water quality orifice(s) • Repair leaking and/or damaged riser/barrel • Repair leaking and/or damaged concrete spillway • Repair eroded or blocked outlet pipe • Replace or unclog filter gravel around riser.
Emergency Spillway Inspection • Remove trees and woody vegetation • Re-seed and repair bare areas or gullies • Replace or repair displaced rip-rap • Remove obstructions from spillway
General Maintenance Inspection • Repair eroded inlet channel • Re-seed and/or repair bare areas or gullies • Replace or repair rip-rap at discharge pipe(s) • Remove trash and/or debris from pond area
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5. DESIGN AND CONSTRUCTION GUIDELINES The design and construction of an impoundment will vary depending on the type of impoundment being
considered (either on-channel or off-channel). This section provides guidance for the design and
construction of both types in an attempt to provide structure and guidance to operators and their
contractors throughout the design and construction process. This guidebook is intended to be used for
informational purposes only, and the operator may want to consult with a professional engineer,
registered to perform engineering services in the state where the work is to be completed, prior to
performing any design and/or construction activities.
5.1. Design – General Considerations
There are several general design considerations that are common to both on-channel and off-channel
impoundments such as estimating the cut/fill volumes, protecting the site from erosion through
engineered design, and calculating runoff estimates to determine flow rates and volumes of the design
storm event.
5.1.1. Estimating Volumes of Earth Cut/Fill
Estimating cut and fill allows for a better understanding of how the design can be altered to balance the
cut volumes with fill volumes. This can help to minimize either the amount of excess fill material that will
need to be managed, or the amount of fill required to be hauled from an offsite borrow area. It is also a
good way to understand project costs and develop a preliminary cost estimate for planning purposes.
End-Mean Method
The end-mean method for estimating volumes is an effective way to estimate both cut and fill volumes of
irregular shapes without having to use advanced computer aided design (CAD) software. The end-mean
method is most appropriate when the volume to be estimated generally has a typical cross section.
Several cross sections can be cut, beginning at one side of the volume to be estimated and continuing to
the other side of the volume to be estimated. The area can then be measured for each cross section, an
average area can be calculated between two adjacent cross sections, and then multiplied by the distance
between the cross sections to get a volume for the segment between the two cross sections. All segment
volumes can then be summarized to determine the total volume estimate.
Prismoidal Formula
The prismoidal formula is an effective way to estimate cut and fill volumes for excavations that are
generally symmetrical in shape such as a rectangular excavation for an off-channel pond.
The prismoidal formula is as follows:
V = (A+4B+C)/6 * D/27
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Where: V = Volume (cubic yards) A = Area at ground level (sq feet) B = Area at ½ D (sq feet) C = Area at D (sq feet) D = Height/Depth of Fill/Cut (feet)
5.1.2. Sediment and Erosion Control
Construction site soil erosion has the power to impact local and even regional water quality and has
regional planning implications (USEPA, 1990). Therefore, prior to construction, sediment and erosion
controls can be incorporated within the plan to minimize the potential for widespread damage from
sedimentation and erosion during construction. Measures can also be taken that will minimize long term
impacts once construction is complete.
The EPA publication Sediment and Erosion Control: An Inventory of Current Practices breaks erosion and
sediment controls into three basic categories, or “practices”: 1) vegetation practices, 2) structural
practices, and 3) special practices. A brief discussion of each practice, and the techniques that apply to
the PRB, is included here. A more in-depth presentation of each erosion and sediment control technique,
along with construction specifications, can be found in the above referenced EPA publication.
Furthermore, design drawings and standard specifications are available through the Department of
Transportation of the state where work is being performed.
Vegetation Practices
The establishment and maintenance of vegetation are the two most important factors in minimizing
erosion during and after construction. Vegetative cover reduces the potential for erosion in three ways:
• Absorbing and dissipating the kinetic energy of raindrops, • Intercepting runoff water so it can infiltrate into the ground rather than flow overland and carry
off soil, • Slowing the velocity and duration of
runoff to promote deposition of sediment rather than erosion (USEPA, 1990).
Vegetative practices that are relevant to CBNG
construction activities in the PRB include, but
are not limited to, temporary and permanent
seeding of appropriate groundcover grasses,
hydro-mulching on slopes, providing a
vegetative buffer strip (Figure 5.1), protecting
native trees, and dust control by minimizing
Figure 5.1 Vegetative Buffer Strip Example
Construction Area
Vegetated Buffer with Plantings
Creek Area
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exposed soil to wind erosion. A vegetated buffer is a transitional vegetated area located between areas
of development (such as CBNG drilling and operations) and aquatic habitats (such as a nearby wet
weather stream) that is used to reduce erosion and sediment loads that may be discharged to the
waterway from the development area during storm events. (Fischer and Fischenich, 2000)
Site-specific vegetation plans can be developed for post-construction conditions that include protecting
the auxiliary spillway, side slopes of the dam, and any other areas that are disturbed during construction.
The soil type and seasonal weather conditions can also be considered when determining how the
disturbed areas will be re-vegetated. A discussion of how re-vegetation can be implemented is included
later in Section 5.5.4.
Structural Practices
Structural practices involve the
construction of devices to divert flow,
trap flow, or limit runoff, and are
classified as either temporary or
permanent (USEPA, 1990). These
physical barriers can be used to slow
the flow of water and prevent erosion
of the soils during construction.
These measures also protect surface
water quality by minimizing solids
loading to surface water bodies such
as streams and creeks. Structural practices that are relevant to CBNG construction activities in the PRB
include, but are not limited to silt fences (Figure 5.2), earth and straw bale dikes (Figure 5.3), brush
barriers, drainage swales, level spreaders, pipe slope drains, temporary storm drain diversion, storm drain
inlet protection, rock outlet protection, sediment traps, temporary sediment basins, stabilized construction
entrances, temporary water crossings, geotextile
fabric/barrier, gabions, and wind breaks.
Special Practices
There are various types of products in the
marketplace that have been specifically designed for
use in special circumstances. These products were
not thoroughly investigated as a part of this report,
but they typically fall into three categories: chemical
solution mulch and tack coatings, natural fiber
Figure 5.2 Silt Fence
Figure 5.3 Stray Bale Dike
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58
matting, and synthetic fiber matting (USEPA, 1990). These commercial products can be of use in
specialized conditions where slopes are steep and the climate is especially dry.
5.1.3. Surface Water Diversion
The operator may choose to design a diversion for run-off, as this is both a water rights concern as well
as a constructability concern. If the operator elects to not design/construct a diversion for storm water
run-off, then the operator may need to establish an agreement with downstream water rights owners to
reduce liability associated with water rights issues.
Diverting surface water drainage flow paths away from the impoundment will minimize the potential for
costly problems with run-off entering the construction area prior to completion of the pond, while
maximizing the amount of produced water that can be discharged to the pond during operations.
Furthermore, the surface water rights of downstream landowners may be infringed upon by collecting
surface water that has historically been available downstream of the impoundment.
The first step in designing a surface water diversion is to size the diversion to accommodate the peak
runoff flow rate of the appropriate frequency-duration design storm. The design storm can be selected
based on the needs of the operator, but the 100 year, 1 hour storm event will be used as a reference for
this document. Once the peak runoff flow rate has been calculated, the site topography can be
evaluated to determine what type of diversion structure would be most effective (channel, berm, coffer
dam, etc) and the design of the diversion structure can proceed.
Peak Runoff Flow Rate
Regression equations have been derived for both Wyoming and Montana for determining flow rates for
various storm events at ungauged rivers and streams. The operator may choose to utilize these
regression equations to calculate the estimated peak flow rate to design the surface water diversion for
an impoundment.
Wyoming
Figure 5.4 depicts the regions into which the state of Wyoming has been divided into. Regression
equations have been developed for each region in order to calculate the estimated flow rate for the
various storm events (USGS, 2003). The operator may choose to design the diversion structure for the
100 year storm event, as the 100 year storm event is commonly used for the design of flood protection
structures.
Table 5.1 depicts the regression equation for the 100 year event for the various regions in Wyoming that
make up the PRB (Regions 1, 2, and 3). The USGS Report Peak Flow Characteristics of Wyoming
Streams can be referenced for further information in regards to how the regression equations were
developed, and for the regression equations for storm events other than the 100 year (USGS, 2003).
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Table 5.1 100 Year Regression Equations for Regions 1, 2, and 3 of Wyoming Region
Number Description 100-Year Regression Equations
1 Rocky Mountains
2 Central Basins and Northern Plains
3 Eastern Basins and Eastern Plains
Source: USGS, 2003
Figure 5.4 Hydrologic regions for determining peak-flow characteristics of Wyoming streams
Source: USGS, 2003
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Figure 5.5 Hydrologic Regions for Determining Peak-flow Characteristics of Montana Streams
Source: USGS, 1998
Table 5.2 100 Year Regression Equations for Southeast Plains Region of Montana
Description 100-Year Regression Equations
Basin and Climatic Active Channel Width
Bankful Channel Width Source: USGS, 1998
In all regions, the Q100 variable is defined as the peak flow rate for the 100 year recurrence interval in
cubic feet per second, and the AREA variable is defined as the basin area in square miles (USGS, 2003).
In Region 1, the ELEV variable is defined as the mean elevation of the basin, in feet, and the LNG
variable is defined as the longitude of the basin outlet, in decimal degrees (USGS, 2003). In Region 3,
the SOIL variable is defined as the unitless mean soils hydrologic index, which is defined for the entire
state in Plate 2 of the USGS report (USGS, 2003).
Montana
Regression equations have also been defined by the USGS for Montana in the USGS report Methods for
Estimating Flood Frequency in Montana Based on Data Through Year 1998 (USGS, 1998). The report
divides Montana into eight regions. The report also provides three regression equations for each region
based on the following: 1) basin and climatic characteristics, 2) active channel width, and 3) bankful
channel width. The Montana portion of the PRB falls in the Southeast Plains region (see Figure 5.5), and
the three regression equations established by the USGS are shown in Table 5.2. Depending on site
conditions and availability of information,
these regression equations can be used
independent of each other, or any
combination of the three can be estimated
and an average taken to calculate the 100
year estimated peak flow, as described in
the report (USGS, 1998). Methods for
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determining the standard error of prediction and confidence limits for any estimate, along with limitations
of the various equations, are also described in the report (USGS, 1998). Users of the report are
cautioned that they are responsible for all interpretations and assessments of calculated flood-frequency
data (USGS, 1998). In all equations, the Q100 variable is defined as the peak flow rate for the 100 year
recurrence interval in cubic feet per second (USGS, 1998). In the Basin and Climatic equation, the A
variable is defined as the basin area in square miles, and the F variable is defined as the percentage of
the basin covered in forest (USGS, 1998). In the Active Channel Width equation, the Wac variable is
defined as the mean width of the active channel in feet (USGS, 1998). In the Bankful Channel Width
equation, the Wbf variable is defined as the mean width of the bankful channel in feet (USGS, 1998).
Methods for making the measurements required for these regression equations, along with the regression
equations for various other storm recurrence intervals, can be found in the report (USGS, 1998).
5.1.4. Spillway Design
There are two types of spillways (pipe or earthen) that can be incorporated into the design of the dam.
Due to the likelihood of the pipe spillway getting blocked or clogged, it is recommended that an earthen
spillway work in conjuncture with the pipe spillway during peak storm events. Therefore, the pipe
spillway is considered the primary spillway and may be utilized to relieve flows through the earthen, or
emergency, spillway to minimize erosion, cutting, and/or dam failure. The operator can design the pipe
inlet to be 1 to 3 feet lower in elevation than the emergency spillway elevation to ensure the spillways
work together during peak storm events. The operator can design the emergency spillway to be either
natural or cut. Furthermore, the spillway design can also call for armoring of the spillway through one, or
a combination, of the following: vegetation, geotextile fabric, rock rip rap, concrete, or other hard surface
material on site.
Both spillways can be designed to pass the design storm event according to the federal, state, and local
regulations using the same approach described in Section 5.1.3 to calculate the design flow rates.
5.2. Design Considerations – On-channel Dams
The operator may choose to consider several issues prior to and during the design of an on-channel
impoundment. The following sub-sections will discuss some of these issues in an attempt to provide
structure and guidance to operators and their consulting engineers throughout the design process. As
previously mentioned, this is not a substitute for an engineered design, and it is recommended that all
work done be site specific and sealed by a professional engineer duly registered in the state where the
work is to be completed.
5.2.1. Foundation Design Considerations
A safe earth dam can be built on almost any foundation if the foundation is investigated and the design
and construction is adapted to the site conditions. Some foundation conditions require expensive
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construction measures that may make an impoundment uneconomical. These conditions can be avoided
if a proper foundation investigation is performed and the dam is located on a suitable substrate. Table
5.3 presents various foundation conditions and considerations for foundation design if that condition is
encountered.
Pre-construction geotechnical investigations and corrective site relocation can alleviate needless
construction expense. Geotechnical investigation procedures are discussed in the Baseline and
Monitoring Section.
5.2.2. Cutoff/Core Trench Design Considerations
Once the foundation investigation has been completed, it may be determined that the dam’s foundation
is either unconsolidated or is overlain by alluvial deposits of permeable sands and/or gravels at or near
the surface and rock or clay at a greater depth. If either of these conditions is the case, then the
creation of a shear line through the dam (also known as piping, or seepage through the unconsolidated
layer or permeable stratum) is a concern that can be addressed to prevent dam failure. To prevent
excessive piping that may lead to dam failure, a cutoff trench, also known as a core trench or key way,
can be designed and constructed to join the impervious stratum beneath the foundation with the base of
the dam (USDA NRCS, 1982).
A cutoff trench is typically an 8 to 10 feet wide trench (measured at the base of the trench) with 1.5:1 to
2:1 side slopes (depending on the depth and condition of the trench walls) down the entire centerline
length of the fill. The depth of the trench varies based on the existing site conditions and the type of
impoundment being constructed. Trenches are constructed from 1 foot (in off-channel impoundments)
to 3 feet (in on-channel impoundments) deep in impervious soil conditions (if a trench is deemed
necessary). If pervious soil conditions are observed (such as alluvium in on-channel impoundments), the
trench can be constructed as deep as necessary to reach the required depth into the impervious soils.
The trench can be alternately filled and compacted with 6-8 inch lifts of a clay/sand mix to ensure proper
consolidation. Once completed, the trench serves to anchor the dam and to prevent/slow water that may
flow beneath the dam.
Table 5.3 Various Foundation Conditions and Design Considerations Foundation Conditions Considerations for Design
Impervious consolidated clay, or sandy clay
Remove the topsoil/organic material and scarify the impervious layer to provide a bond with the dam material.
Sand or gravel with no impervious clay layer near the surface
Corrective measures may be required to prevent excessive seepage under/through the dam, which could result in possible failure. See Section 5.2.2. The operator may want a licensed professional engineer to design the dam.
Highly plastic clay, or unconsolidated material
Stability may be difficult to obtain. The operator may want a licensed professional engineer to design the dam.
Bedrock Investigate the nature of the rock to determine if seams, crevices, and/or fissures are present.
Source: Consolidated from USDA NRCS, 1982
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Prepared Foundation
Proposed Core Fill Alluvial Deposits
Impervious Stratum Alluvial Deposits
Proposed Pool Depth Proposed Dam
Cut-off Trench Min 8 ft Wide at bottom 1.5:1 – 2:1 V:H Slopes
Dam Centerline
Ground Surface
Impervious Stratum
Proposed Freeboard
Figure 5.6 Conceptual Cross-Section of a Cut-off Trench
Figure 5.6 provides an example of a typical cross section for a cut-off trench. The trench can extend into
and up the abutments to the start and end of the dam (to the extent where unconsolidated or permeable
material might allow piping).
The operator can design the cut-off trench in such a way to prevent dam failure as a result of piping
through the dam. The design can include requirements for the bottom of the trench to be no less than 8
feet wide (or the bulldozer blade width, whichever is greater), and the sides no steeper than 1.5:1 – 2:1,
depending on the existing site conditions. The operator can specify in the design documents to have the
trench filled with successive thin layers (6 – 8 inches) with a clay/sand mix, and for each layer to be
compacted thoroughly at near-optimum moisture conditions for high hazard fills, or wheel compact (semi-
compacted) for low hazard fills, before placing the next layer. According the Agricultural Handbook, the
moisture content is adequate for compaction when the material can be formed into a firm ball that sticks
together and remains intact when the hand is vibrated violently and no free water appears (USDA NRCS,
1982).
5.2.3. Seepage Control Design Considerations
An analysis can be made of anticipated seepage rates and pressures through the dam embankment,
foundation, and abutments. Seepage controls, such as anti-seep collars around pipes that extend through
the dam, can be designed to (1) accomplish the intended reservoir function, (2) provide a safely
operating dam, and (3) prevent damage to downstream water quality and property by preventing dam
failure (USDA NRCS, 1982). This section includes a discussion of two types of seepage control devices
(anti-seep collars and sand-gravel filters) that can be considered in the design.
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Figure 5.7 Sand Filter Pack
Source: USDA NRCS, 1982
Sand Filter Packs
An option for seepage control is to install a sand-gravel filter with a diaphragm. One filter and drainage
diaphragm can be used around any structure that extends through the embankment to the downstream
slope. The diaphragm should be located downstream of the centerline of a homogeneous embankment
core or downstream of the cutoff trench.
According to the Agricultural Handbook, the diaphragm should be a minimum of 3 feet thick and extend
around the pipe surface a minimum of 2 times the outside diameter of the pipe (2Do) (USDA NRCS,
1982). When a cradle or bedding is used under the pipe, the vertical downward 2Do is measured from
the bottom of the cradle or bedding. If bedrock is encountered within the 2Do measurement, the
diaphragm should terminate at the bedrock surface (USDA NRCS, 1982). The location of the diaphragm
should never result in a minimum soil cover over a portion of the diaphragm measured normal to the
nearest embankment surface of less than 2 feet. If this requirement is exceeded, the filter and drainage
diaphragm should be moved upstream until the 2-foot minimum is reached. The outlet for the filter and
drainage diaphragm should extend around the pipe surface a minimum of 1.5 times the outside diameter
of the pipe (1.5Do) that has 1 foot around the pipe being a minimum (USDA NRCS, 1982).
In most cases where the embankment core consists of fine-grained materials such as sandy or gravely
silts and sandy or gravely clay (15 to 85 percent passing the No. 200 sieve), an aggregate conforming to
ASTM C-33 fine concrete
aggregate is suitable for the
filter and drainage diaphragm
material. A fat clay or elastic
silt (more than 85 percent
passing No. 200 sieve) core
requires special design
considerations, and an
engineer experienced in filter
design should be consulted
(USDA NRCS, 1982). Figure
5.7 depicts a sand gravel filter
with a diaphragm.
Using a filter and drainage diaphragm has many advantages. Some are as follows:
• They provide positive seepage control along structures that extend through the fill. • Unlike concrete anti-seep collars, they do not require curing time. • Installation is easy with little opportunity for constructed failure. The construction can consist
mostly of excavation and backfilling with the filter material at appropriate locations.
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Figure 5.8 Anti-Seep Collar
Source: USDA NRCS, 1982
Table 5.4 Recommended Top Width of Dam for
Various Heights Height of dam Minimum top width
(ft) (ft) Under 10 6 11 to 14 8 15 to 19 10 20 to 24 12 25 to 34 14
Source: USDA NRCS, 1982
Anti-Seep Collars
An anti-seep collar can be used around pipes that extend through the dam to control seepage and
prevent dam failure. Anti-seep collars require more fabricated materials than sand gravel filters. If an
anti-seep collar is used, it should extend into the fill a minimum of 24 inches perpendicular to the pipe in
all directions (USDA NRCS, 1982). If the dam is less than 15 feet high, one anti-seep collar at the
centerline of the fill is enough.
For higher dams, use two or
more collars equally spaced
between the fill centerline and
the upstream end of the
conduit when a hood-inlet pipe
is used (USDA NRCS, 1982). If
a drop-inlet pipe is used, the
anti-seep collars should be
equally spaced between the
riser and centerline of the fill
(USDA NRCS, 1982). Figure 5.8
depicts an anti-seep collar.
5.2.4. Top Width and Alignment
A general guide that can be followed when designing the top width of the dam is included in Table 5.4.
For dams less than 10 feet high, a conservative minimum top width is 6 feet. As the height of the dam
increases, increase the top width (USDA NRCS, 1982).
If the top of the embankment is to be used for any kind
of vehicular traffic, a shoulder on each side of the
roadway can be included to prevent raveling. In this
instance, the top width should be at least 16 feet
regardless of the height of the dam (USDA NRCS, 1982).
In areas where dams are subject to heavy wave action
and erosion, the operator may elect to design a top
width wider than the minimum recommendations in
Table 5.4 to prevent failure/breach of the dam. A wider top may also prove to be cost effective by
improving ease of construction with heavy equipment.
Two types of alignment alternatives exist for on-channel dams: straight or curved. Curved fills may
require more effort and work to construct; however, in some situations a curved dam alignment is more
desirable than a straight alignment. Curvature can be used to retain existing landscape elements, reduce
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Table 5.5 Side Slope Guidelines for Various Types of Materials
Material Description Upstream Side Slope
Downstream Side Slope
Clayey sand, clayey gravel, sandy clay, silty sand, silty gravel
3:1 2:1
Silty clay, clayey silt 3:1 3:1 Source: USDA NRCS, 1982
Table 5.6 Freeboard Height Design GuidelinesPond Length Freeboard Height
Less than 660 feet long 1 foot Between 660 feet and
1,320 feet long 1.5 feet
Between 1,320 feet and 2,640 feet long
2 feet
Over 2,640 feet long Consult a PE Source: Compiled from USDA NRCS, 1982
the apparent size of the dam, blend the dam into surrounding natural landforms, and provide a natural-
appearing shoreline (USDA NRCS, 1982). The alignment can be considered during the design of the dam
to ensure that all interested parties are consistent with a cost effective and functional dam design.
5.2.5. Side Slopes
During the design of the dam, the
strength and stability of the
foundation and fill material can be
considered to determine what side
slopes can be safely applied to the
embankment of the dam. The more
stable the fill material, the steeper the side slopes; unstable materials require flatter side slopes. Table
5.5 depicts recommended slopes for the upstream and downstream face of a dam built with various
materials. A side slope of 4:1 or flatter may be desirable for livestock access, wildlife, and for ease of
construction with heavy machinery. If severe wave action is anticipated from prevailing winds, a 4:1 side
slope (or flatter) may also be desirable. Figure 5.6 depicts a dam cross section and shows where the
upstream and downstream side slopes are. End slopes, for off-channel impoundments, can be designed
with a slope of 4:1 or flatter.
For slope stability, the slopes should not be steeper than those shown in Table 5.5, but they can be
flatter as long as they provide adequate surface drainage. The side slopes need not be uniform, but can
be shaped to blend with the surrounding landforms. Finish-grading techniques used to achieve a smooth
landform transition include slope rounding and slope warping. Slope rounding is used at the top and
bottom of cuts or fills and on side slope intersections. Slope warping is used to create variety in the
horizontal and vertical pitch of finished slopes. Additional fill can be placed on the back slope and
abutments of the dam, if needed, to achieve this landform transition. (USDA NRCS, 1982).
5.2.6. Freeboard
Freeboard is defined as the vertical distance between the
elevation of the water surface in the pond when the
spillway is discharging at designed depth and the elevation
of the top of the dam after all settlement has occurred
(USDA NRCS, 1982). The additional height of the dam
provides a safety factor to prevent overtopping dam
failure. Some general guidelines for freeboard height, as it
relates to length of the pond, are included in Table 5.6.
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5.2.7. Settlement Allowance
When designing an earthen dam, allowances can be made to account for settlement, also known as
shrinkage or consolidation, of the dam materials over time by designing the dam dimensions higher than
the final desired dimensions. The amount of shrinkage depends on the height of the dam, the method of
construction, and the foundation material. For example, if the dam is constructed by placing and
compacting thin layers (6 – 8 inch lifts) under good to ideal moisture conditions, the dam itself should
experience little settlement or consolidation; however, some consolidation of the foundation may occur
unless the foundation is unyielding. Most foundations in the on-channel setting are yielding and
settlement may range from 1 to 6 percent of the height of the dam, mainly during construction. The
settlement for a dam where the fill is rolled (compacted to 95% with optimum soil moisture) can be
estimated to be about 5 percent. In other words, the dam should be designed to be constructed 5
percent higher than the final desired elevation. It is not uncommon for a dam that is less than 25 feet
high to be wheel, or tire, compacted (semi-compacted), and so 10 percent settlement can be allowed for
in this case (USDA NRCS, 1982).
5.3. Design Considerations – Off-Channel Dams
When considering the design of an off-channel impoundment, there are several issues that can maximize
the opportunity for success. The following sub-sections will discuss some of these issues in an attempt to
provide structure and guidance to operators and their consulting engineers throughout the design
process. As previously mentioned, this guidance is not a substitute for an engineered design, and it is
recommended that all work done be site specific and sealed by a professional engineer registered in the
state where the work is to be completed.
5.3.1. Soils
A geotechnical investigation can be performed to aid in the design of an off-channel pond. The results of
the geotechnical investigation can be used to determine suitability of the soil for infiltration, the natural
angle of repose of the material, and the nature of the bedrock. Once the geotechnical investigation is
complete, the soil and bedrock can be categorized according to the OSHA Technical Manual classification
system as: stable rock, Type A, Type B, or Type C. This classification can be used to determine
appropriate safety measures to be taken during the excavation of the impoundment. This section
includes a brief definition of each soil type, along with the OSHA recommended side slopes not to be
exceeded during construction. It should be noted that these are not the recommended side slopes for
final pond design; rather, these slopes are for safety purposes during construction. Recommendations
are also included for layered geological strata that have various types of soil in different layers.
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Stable Rock
Stable rock is natural solid mineral matter that can be excavated with vertical sides and remain intact
while exposed. It is usually identified by a rock name such as granite or sandstone. Determining
whether a deposit is of this type may be difficult unless it is known whether cracks exist and whether or
not the cracks run into or away from the excavation. Vertical cuts can be made into stable rock without
shoring or support during excavation activities to a maximum depth of 12 feet (USDL, 1999). Vertical
cuts are not recommended for final design at the edge of a pond due to the drowning hazard for wildlife
and livestock.
Type A Soil
Type A soils are cohesive soils with an unconfined compressive strength of 1.5 tons per square foot (tsf)
(144 kPa) or greater. Examples of Type A cohesive soils are clay, silty clay, sandy clay, clay loam and, in
some cases, silty clay loam and sandy clay loam. (No soil is Type A if it is fissured; is subject to vibration
of any type; has previously been disturbed; is part of a sloped, layered system where the layers dip into
the excavation on a slope of 4 horizontal to 1 vertical (4H:1V) or greater; or has seeping water). Type A
soils can be excavated with a ½:1 (V:H) slope for temporary excavations, and a ¾:1 slope for more
permanent excavations up to 12 feet in depth (USDL, 1999). Final grading for Type A soils should
provide for a 2:1 slope or flatter to maintain side slope stability at the edge of the pond.
Type B Soil
Type B soils are cohesive soils with an unconfined compressive strength greater than 0.5 tsf (48 kPa) but
less than 1.5 tsf (144 kPa). Examples of other Type B soils are angular gravel; silt; silt loam; previously
disturbed soils unless otherwise classified as Type C; soils that meet the unconfined compressive strength
or cementation requirements of Type A soils but are fissured or subject to vibration; dry unstable rock;
and layered systems sloping into the trench at a slope less than 4H:1V (only if the material would be
classified as a Type B soil). Type B soils can be excavated with a 1:1 (V:H) slope for excavations up to 12
feet in depth (USDL, 1999). Final grading for Type B soils should provide for a 3:1 slope or flatter to
maintain side slope stability at the edge of the pond.
Type C Soil
Type C soils are cohesive soils with an unconfined compressive strength of 0.5 tsf (48 kPa) or less. Type
C soils include granular material such as gravel; sand and loamy sand; submerged soil; soil from which
water is freely seeping; and submerged rock that is not stable. Also included in this classification is
material in a sloped, layered system where the layers dip into the excavation or have a slope of four
horizontal to one vertical (4H:1V) or greater. Type C soils can be excavated with a 1-½:1 (V:H) slope for
excavations up to 12 feet in depth (USDL, 1999). Final grading for Type C soils should provide for a 4:1
slope or flatter to maintain side slope stability at the edge of the pond.
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Layered Geological Strata
Where soils are configured in layers, i.e., where a layered geologic sequence exists, excavation of the soil
must be classified on the basis of the weakest soil layer. Layered sequences cover the majority of the
PRB where sands, clays, coals, and clinker can all be interlayered. Each layer may be classified
individually if a more stable layer lies below a less stable layer, i.e., where a Type C soil rests on top of
stable rock (USDL, 1999). Final grading for multiple layers of soil should follow the weakest soil layer’s
recommended grading.
5.3.2. Dimensions
The dimensions selected for an off-channel impoundment depend on the volume of water to be managed
as dictated by the rate of produced water and the estimated rate of infiltration and beneficial uses of the
water. In general, larger ponds can be constructed at a lower cost per acre-foot of capacity, but this will
depend upon site-specific conditions.
The type and size of the excavating equipment can limit the width of an excavated pond. For example, if
a dragline excavator is used, the length of the boom usually determines the maximum width of
excavation that can be made with proper placement of the excavated material. The minimum length and
depth of the pond is determined by the required pond capacity. To prevent sloughing, the side slopes of
the pond are generally no steeper than the natural angle of repose of the material being excavated. This
angle varies with different soils, but for most ponds the side slopes are 1:1 or flatter (USDA NRCS, 1982).
Various side slope recommendations are included in section 5.3.1.
If the pond is to be used for watering livestock, provide a ramp with a flat slope (4:1 or flatter) for safe
animal access. Regardless of the intended use of the water, these flat slopes are necessary if certain
types of excavating equipment are used. Tractor-pulled wheeled scrapers and bulldozers require a flat
slope to move material from the bottom of the excavation (USDA NRCS, 1982).
5.3.3. Excavated Material Management
The placement of the material excavated can be planned prior to construction activities to reduce the
amount of double and triple handling, and thus reducing overall project cost. Adequate placement
prolongs the useful life of the pond, improves its appearance, and facilitates maintenance and
establishment of vegetation. The excavated material can be stacked, spread, or removed from the site as
conditions, nature of the material, and other circumstances warrant.
Issues that can be considered when planning on where to place excavated material include:
• The weight of the excavated material cannot endanger the stability of the pond side slopes. The toe of the fill should be at least 12 feet away from the edge of the pond.
• The excavated material should be located where rainfall does not wash the material back into the pond.
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• The stacked excavated material side slopes should be no steeper than the natural angle of repose of the soil.
• Excavated material can be shaped and spread to blend in with the natural landforms versus stacked in geometric mounds.
• Avoid interrupting the existing horizon line with the top of the waste mound.
• Communicate with the landowner to identify how to locate the excavated material in a way that it can provide additional function to the site, such as screening the view of the CBNG wellhead, buffering noise from the compressor, provide a buffer from wind, or improve the site’s recreational appeal.
Perhaps the most satisfactory method of handling waste material is to remove it from the site. Complete
removal, however, is expensive and can seldom be justified unless the material is needed nearby. Waste
material can sometimes be used advantageously for filling nearby low areas in a field, in building roads to
access the impoundment or nearby well sites, in constructing diversion dikes, creating wildlife habitat,
and repairing existing structures, to name a few of the possible uses. Furthermore, if state or county
highway maintenance crews need such material, they may be able to remove it free of charge.
5.4. Design Drawings and Specifications
Design drawings and specifications can be prepared to include all planning information and design criteria
to ensure proper construction of the dam. These drawings and specifications can show all elevations and
dimensions of the dam; the dimensions and extent of the cutoff trench (as necessary) and other areas
requiring backfill; the location and dimensions of the principal spillway and other planned appurtenances;
and any other pertinent information (USDA NRCS, 1982).
If the construction of the dam is to be bid competitively, the drawings can also include quantity takeoffs
and a list of required materials. The construction and material specifications can state the extent and
type of work, site specific details, material qualities, and requirements for prefabricated materials.
The construction documents can also include requirements that the contractor shall observe all land
disturbance laws by including temporary protective measures during construction to minimize soil erosion
and sedimentation. By including this type of language in the construction documents, the liability is
transferred from the operator to the contractor, thus minimizing exposure to costly fines and project
delays.
A well defined set of construction documents provides a basis for contractors to bid on the proposed
work, allows fair competition among bidders, and states the conditions under which the work is to be
done. The specifications should:
• give all the information not shown on the drawings that is necessary to define what is to be done,
• prescribe how the work is to be done if such direction is required,
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• specify the quality of material and workmanship required, • describe the required quality control during construction, and • define the method of measurement and the unit of payment for the various items of work that
constitute the whole job.
Construction work of the quality and standards desired will not result unless there is a clear
understanding of these requirements between the operator and the contractor (USDA NRCS, 1982).
5.5. Construction – General Considerations
There are several general construction considerations that are common to both on-channel and off-
channel impoundments such as erosion and sedimentation control installation, clearing and grubbing,
spillway installation, and re-vegetation of the site once construction is complete.
5.5.1. Erosion and Sediment Control Installation
Prior to engaging in any construction activities, the appropriate erosion and sediment control devices can
be installed to prevent erosion problems from occurring once construction has begun. Erosion and
sedimentation control devices should be installed according to the design drawings, construction
specifications, and the manufacturer’s recommendations.
5.5.2. Clearing and Grubbing
Once the appropriate erosion and sediment control devices have been installed, the area where
excavation activities will occur can be cleared of trees and brush. The trees and brush can be cut as
nearly flush with the ground as practicable and then removed, along with any other debris from the dam
site.
All material cleared and grubbed from the site should be properly managed. This can be done by
stacking the debris nearby to encourage wildlife. Burning of the debris should be avoided due to the arid
conditions in the PRB.
Minimal clearing of brush and trees conserves site character and minimizes the difficulty and expense of
re-establishing vegetation. Confine clearing limits to the immediate construction areas to avoid
unnecessary disturbance.
Removing all vegetation within the construction limits is not always necessary. Selected groupings of
desirable plants can be kept. Trees and shrubs can often survive a 1- to 2-foot layer of graded fill over
their root systems or they can be root-pruned in excavated areas. Tree wells and raised beds can also be
used to retain vegetation. Clearing limits should be irregular to create a natural appearing edge and
open area. Further transition with vegetated surroundings can be accomplished by feathering clearing
edges. Density and height of vegetation can be increased progressively from the water’s edge to the
undisturbed vegetation. Feathering can be accomplished by selective clearing, installation of new plants,
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or both (USDA NRCS, 1982). Brush and trees can be left standing within the planned impoundment to
serve as wildlife shelter.
5.5.3. Spillway Installation
As mentioned in Section 5.1.4, there are two types of spillways (pipe and earthen) that can be installed,
depending on the design of the dam. As noted in Section 5.1.4, a pipe spillway may not provide
adequate capacity during peak storm events to prevent dam failure; therefore, the operator may elect to
install an earthen spillway on every dam.
Pipe Spillway Installation
Installation of the pipe spillway can include the black clad pipe, a riser (if applicable), seepage control
device (either a filter and drainage diaphragm or an anti-seep collar), trash rack, and other mechanical
components of the dam to the lines and grades shown on the drawings and staked at the site. All pipes
and conduits can be placed on a firm foundation, thus minimizing the danger of cracks or openings at the
joints caused by unequal settlement of the foundation. The backfill material around all pipes and
conduits can be compacted prior to placing the dam fill material over them to minimize consolidation and
settlement of the dam (USDA NRCS, 1982).
Earthen Spillway
The completed earthen spillway excavation should conform as closely as possible to the lines, grades,
bottom width, and side slopes shown on the drawings and staked at the site. The channel bottom can be
left transversely level to prevent meandering and the resultant scour within the channel during periods of
low flow. If it becomes necessary to fill low places or depressions in the channel bottom caused by
undercutting the established grade, fill them to the established grade by placing suitable material in 8-
inch layers and compacting each layer under the same moisture conditions regardless of the placement in
or under the embankment (USDA NRCS, 1982).
5.5.4. Vegetation
Establishing good plant cover can be a very effective way to enhance site aesthetics while also protecting
from wind and runoff erosion. As soon after construction as practicable (or during construction), prepare
a seedbed for the disturbed areas (the exposed surface of the dam, the auxiliary spillway, and the borrow
areas as well as other disturbed surfaces). This is generally done by placing topsoil back on all disturbed
areas and disking or harrowing.
The planted vegetation should be able to survive under prevailing conditions with minimum maintenance;
arid conditions prevalent in the PRB will, however, delay establishment. Native varieties are preferred for
new plantings (as recommended by the local NRCS office); however, a list of salt tolerant species of
plants is provided in Table 2.3. Seeding can be done by hand, by broadcast spreader behind a small
ATV, by drill if the area is large, or by sprayer when mixed with hydro-mulch. Protect the seedlings to
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establish a good stand by mulching. Mulching is necessary to protect the seeds from the wind as well as
minimize the need for irrigation. Amendments such as fertilizer and/or manure can be applied with the
mulch to aid in the growth process. In the arid PRB, irrigation is often needed to ensure good
germination and growth. If produced water is used for irrigation, the proper amendments can be added
with the application of the water to minimize the dispersive effects the water has on the soil.
5.6. Construction – On-Channel Dams
When constructing an on-channel impoundment in the PRB, several issues can be considered to maximize
the opportunity for success. The following sub-sections will discuss some of these issues in an attempt to
provide structure and guidance to operators and their contractors during construction.
5.6.1. Preparing the Foundation
To ensure the dam is built on a good foundation, the following steps can be taken to prepare the
foundation:
• treat the surface o using a scraper, or equivalent equipment, remove sod, boulders, and topsoil from the
entire area where embankment material is to be placed o stockpile the topsoil for later use at the site o fill all holes in the foundation area with the same method of placement and compaction
as for the dam o thoroughly break and turn the ground surface to a depth of 6 inches o roughly level the surface with a disk harrow and compact by rolling with heavy
equipment • excavate and backfill the cutoff trench (as necessary)
o if the foundation layer is impervious or is blanketed with an impervious layer, this step is not necessary,
o if a trench is necessary, ensure that the bottom of the trench penetrates the impervious layer at least 12 inches through the entire length of the trench
o reserve material excavated for the trench for use elsewhere on the site, such as the downstream toe of the dam as it is excavated from the trench, as appropriate
o pump all free water from the trench prior to backfilling o backfill the trench in the same method used for placing and compacting the dam by
placing a consolidated mixture of sands, silts, and clays to achieve a material with good binding properties
• excavate and backfill existing stream channels o remove all objectionable material from the channel where embankment material is to be
placed o deepen, slope back, and widen stream channels that cross the embankment foundation
(no steeper than 3:1 for channels that cross perpendicular to the dam, and 1:1 for channels parallel to the dam)
o backfill and compact these channels in the same method as the cutoff trench and dam
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5.6.2. Building the Dam
Once the foundation has been properly prepared, the dam can be built. Geotechnical analyses can be
run on the dam material to ensure the suitability of the material to be used for the dam. The material
should be free of sod, roots, stones more than 6 inches in diameter, and any material that could prevent
the desired degree of compaction. Do not use frozen material or place fill material on frozen foundations.
Once the dam material has been deemed suitable, placement and compaction in thin lifts (6 inches) can
begin.
Selected backfill material should be placed in the core trench and around pipes and anti-seep collars,
when used. To ensure proper compaction, the material can be compacted by hand tamping or manually
directed power tampers around pipes and anti-seep collars. To ensure proper placement and compaction
of the dam material, fill material can be placed at the lowest point first and brought up in horizontal
layers, longitudinal to the centerline of dam, approximately 6 inches thick. For fill placement around
risers, pipes and filter, and drainage diaphragms, the horizontal layers should be approximately 4 inches
thick. Fill should not be placed in standing water.
To ensure proper compaction, the moisture content can be deemed adequate through sand cone tests,
nuclear gauge tests, or by simple field observation (whichever method best suits the needs of the dam).
A simple field observation technique that can be used is to form the material into a firm ball that sticks
together and remains intact when the hand is vibrated violently and no free water appears. If the
material can be formed into a firm ball that sticks together, the moisture content is adequate for
compaction.
Laboratory tests (such as a sand cone, or moisture content) of the fill material and field testing (such as
in-place nuclear gauge testing) of the soil for moisture and compaction adequacy may be necessary for
large ponds or special conditions.
If the fill material varies in texture and gradation, the more impervious (clay) material can be used in the
cutoff trench, core of the dam, and upstream parts of the dam. Construction equipment can be used to
compact earth fill in an ordinary pond dam. Equipment that has rubber tires can be routed so each layer
is sufficiently covered by tire tracks. For dams over 20 feet high, special equipment, such as sheep foot
rollers, can be used (USDA NRCS, 1982).
5.7. Construction – Off-Channel Dams
Off-Channel pond construction is quite simple, and therefore not a lot of discussion is required. As
discussed in Section 5.5.2, the excavation area should be cleared of all undesired vegetation. The
outside limits of the excavation can be marked with stakes that indicate the depth of cut from the ground
surface to the pond bottom at that location. As mentioned in Section 5.3.3, excavation and placement of
the waste material are the principal items of work in building an off-channel pond. The kind of
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excavating equipment used depends on the climatic and physical conditions at the site and on what
equipment is available.
In low-rainfall areas where water is unlikely to accumulate in the excavation, you can use almost any kind
of available equipment. Tractor-pulled wheeled scrapers, dragline excavators, and track-type tractors
equipped with a bulldozer blade are generally used. Bulldozers can only push the excavated material, not
carry it; if the length of push is long, using these machines is time consuming and expensive. The
contractor can excavate and place the waste material as close as possible to the lines and grades staked
on the site to minimize double and triple handling of the material and prevent the construction costs from
escalating. If a dragline excavator is used, other equipment will generally be required to stack or spread
the waste material and shape the edge to an irregular configuration. Bulldozers are most commonly
used. Graders, either tractor-pulled or self-propelled, can be used to good advantage, particularly if the
waste material is to be shaped (USDA NRCS, 1982).
5.8. Reclamation
Once CBNG operations and productions cease, the operator may be required to reclaim the impoundment
site to pre-existing conditions if the landowner does not wish to take responsibility for the impoundment.
The operator can minimize the exposure to risk at the end of the project by planning for the reclamation
of the site from the onset by stockpiling the topsoil for use in the reclamation phase. Reclamation of the
impoundment site to pre-existing conditions consists of (1) providing appropriate erosion controls (see
Section 5.1.2) during the reclamation process; (2) draining the impoundment; (3) breaching the dam; (4)
sampling and analyzing the soil on the floor of the reservoir (if required by state and/or federal
regulations)[ (5) excavating layers of salts and mineral residue that may exceed acceptable agronomic
concentrations and isolating this material so it will not leach into the surface or groundwater in the future
(if required by state and/or federal regulations); and (6) re-countouring the surface to existing conditions
so that the channel is able to pass water during rain events at a depth and velocity equivalent to the
existing conditions (on-channel impoundments), or to prevent ponding by filling in low spots (off-channel
impoundments). In order to achieve this, the side slopes of the breach can be excavated to a slope that
is stable and consistent with the natural angle of repose of adjacent material abutting the dam (MDNRC,
1989). This may require the entire width of the dam be removed and re-contoured. Once the dam has
been removed and re-contoured to match existing conditions, all areas can be re-vegetated as described
in Section 5.5.4 to prevent future erosion and aid in the stabilization of the banks and channel.
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6. PREPARING AND MAINTAINING A WATER BALANCE A CBNG field will produce water in variable quantities over its life depending upon drilling schedule and
the performance of the CBNG reservoirs. The accounting of the water production and management as it
changes over time is known as a water balance. A Plan of Development (POD) for a CBNG field may
include a water balance giving a forecast of the number of CBNG wells to be placed online, their average
water production rates over time, and the operator’s water management plans. Across the PRB,
impoundments are important factors in water management, and by using and understanding the water
balance for a given CBNG field, the operator can plan for the number of impoundments and total storage
capacity required to be installed to allow operations to continue without having to shut in wells due to
lack of planning.
A water balance can be prepared during the planning and design phase of the impoundment and
maintained or adjusted as required during operations to optimize the potential discharges into the
impoundment such that the system will continue to accept produced water. Furthermore, by forecasting
the water balance for each impoundment, the operator may be able to minimize the number of
impoundments required on a larger scale through flow-line planning and maintenance, thus saving time,
money, and potential impacts to the surface.
This section will discuss management areas that need to be addressed when preparing a water balance:
irrigation, livestock consumption, other beneficial uses, infiltration, evaporation, and discharge/storage.
The water balance can be set up on a month to month basis prior to operations, and once operations
begin, the month to month monitored water use data can be incorporated (by adding flow volume
meters, or by simple calculations based on actual site conditions) to aid in the future planning of
discharges to the impoundment.
6.1. Irrigation
Irrigation is a common practice in industry, as the water and soil conditions allow (Harvey and Brown,
2005). Some operators manage up to 30% of their produced water through this beneficial use by
applying an average of 12-24 inches of water on irrigated land over the span of a year. If an operator is
irrigating 1,000 acres, he can plan to manage roughly 1,000 – 2,000 acre-feet of produced water (about
7.5 – 15 million barrels) in the course of a year. This section will discuss how this annual amount can be
broken down to a monthly basis, thus enabling the operator with a more effective planning tool for water
management.
Section 2 discussed the beneficial use of produced water to irrigate pastures and crops when the quality
of the CBNG water is high enough that it will not negatively impact the vegetation or the soils where it is
being irrigated. The feasibility of irrigating with produced water can be studied in regard to the potential
impact to vegetation and soil prior to irrigating with produced water so that the impacts can either be
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Table 6.1 Kc Values for the Different Growth Stages of Selected Crops
Crop
Initial Planting –
Rapid Growth (Kc1)
Rapid Growth – Midseason
(Kc2)
Mid/Late Season – Harvest
(Kc3) Alfalfa Hay 0.30 1.25 1.10
Barley, Wheat, Oats 0.30 1.15 0.20 Beans, green 0.40 1.00 0.90
Beans, dry and Pulses 0.40 1.15 0.35 Beets, table 0.40 1.05 0.95 Corn, Field 0.40 1.15 0.60, 0.35
Cotton 0.40 1.20 0.65 Grass Pasture 0.80 0.80 0.80
Millet 0.30 1.10 0.25 Onion, dry 0.50 1.05 0.80
Potato 0.40 1.10 0.75 Soybeans 0.35 1.10 0.45 Spinach 0.30 1.00 0.95
Sugar Beet 0.30 1.15 1.00 Tomato 0.40 1.20 0.65
Winter Wheat 0.30 1.15 0.20 Source: Selected from http://www.wateright.org/rmm/faokc.html
mitigated through the use of amendments (such as gypsum), or they can be avoided by not irrigating the
water at all. If the feasibility study for using the CBNG produced water for irrigation is favorable, then
the design and planning of the impoundment can include irrigation volumes to the water balance to
account for the withdrawal of water for use in irrigation, which can provide a higher amount of produced
water managed for CBNG operations.
Once the produced water has been deemed fit for irrigation, the amount of water that can be irrigated is
dependant on the evapotranspiration of the crop (ETc) that will be irrigated each month. ETc is calculated
by first determining the evapotranspiration, ETo, of the area from a local weather station, and then
multiplying the ETo by the appropriate crop factor, Kc, given in Table 6.1. The following equation can be
used:
ETc = Kc * ETo
Once ETc has been calculated (in inches) the irrigation requirement can be determined (in inches) by
subtracting the rainfall as shown in the following equation:
Irrigation Requirements = ETc – Effective Rainfall
Once the irrigation requirement for one
irrigation cycle is determined, it can be
converted to gallons by multiplying the
irrigation requirement by the area of the field
(in square feet) to be irrigated, and then
converting cubic feet to gallons. The operator
can communicate with the landowner to
determine how many irrigation cycles to
perform each month, and then a water
volume can be calculated for each month that
the produced water will be used for irrigation
and incorporated into the water balance for
planning and design purposes. Table 6.2
provides an example of an irrigation
calculation requirement for a 1,000 acre grass
pasture.
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Table 6.3 Daily Water Consumption by Adult Animals1
Livestock Average Maintenance
Hot Weather
Beef cattle 8-12 20-25 Milking cow 20-25 30-40
Sheep 2-3 3-4 Swine 6-8 8-12
Beef calf 4-5 9-10 Horse 8-12 20-25
1Approximate amounts in gallons per day (gpd) Source: Iowa State University, 1995.
Table 6.2 Irrigation requirement Example Monthly Calculation
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year 1Mean ETo 0.7 0.9 1.8 3.3 4.7 5.6 7.2 6.3 4.0 2.6 1.2 0.8 39.1 2Mean ETc 0.00 0.00 0.00 2.64 3.76 4.48 5.76 5.04 3.20 0.00 0.00 0.00 24.88 3Mean Rainfall 0.2 0.2 0.7 0.5 1.9 1.9 1.9 1.1 0.7 0.8 0.4 0.7 11 4Irrigation Requirement (inches)
0.00 0.00 0.00 2.14 1.86 2.58 3.86 3.94 2.50 0.00 0.00 0.00 16.88
5Irrigation Requirement (million gallons)
0.0 0.0 0.0 63.9 55.6 77.1 115.3 117.7 74.7 0.0 0.0 0.0 504.2
Sources: 1 Pochop, et al., 1985.
2 During growing season, 0.8* ETo, 3 http://www.worldclimate.com 4 Irrigation Requirement (inches) = (ETc, -Rainfall)*0.9 5 Convert inches of water to million gallons irrigated over 1000 acres
6.2. Livestock Consumption
As discussed in Section 2, consumption of produced water by livestock is a beneficial use that can be
realized if the water quality is high enough to meet livestock watering standards (less than 5000 TDS is
generally consider suitable for cattle (Olson and Fox, 2002 GPE-1401)). Typically, produced water in the
PRB is suitable for livestock so long as it is not concentrated in the impoundments due to evaporation.
The amount of water consumed by livestock is in direct proportion to the headcount and number of
days/year the herd will use the impoundment as a watering hole. Seasonal considerations that impact
water consumption can also be included in the water budget on a month to month basis. Table 6.3
depicts daily water consumption rates for various types of livestock for average conditions and hot
weather conditions. This table, along with
the time of year, and headcount per days
per month that the impoundment will be
an active watering hole (this can be
estimated by coordinating with the
landowner), can be used to prepare the
anticipated livestock consumption
component of the water balance.
6.3. Other Beneficial Uses The operator may be able to arrange for other beneficial uses for his produced CBNG water; this could
include oil and gas operations, coal mining uses, road application to mitigate dust, and vehicle washing.
Like irrigation, these beneficial uses may be seasonal in their ability to take CBNG water and the
withdrawals from the irrigation system can be accounted for in the water balance.
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Figure 6.1 Examples of Soil StructuresGRANULAR BLOCKY
PRISMATIC
MASSIVE
Source: FAO, 1985.
6.4. Infiltration
Local infiltration rates will be dependent on the initial soil conditions (both at and below the bottom of the
impoundment) and how the soil conditions at the bottom of the impoundment change as a result of
infiltration; both of these conditions are determined by the hydraulic conductivity of the soil. The
hydraulic gradient created between the surface of the water in the impoundment and the groundwater
elevation in the shallow aquifer also impacts the rate of infiltration.
6.4.1. Initial Soil Conditions
“Soil conditions” refer to the soil composition, profile, texture, and structure (FAO, 1985). Soil
composition simply refers to the various particle
sizes and pore spaces in the soil. The soil
profile is defined as the various horizons of soil
types that are layered beneath the ground
surface (FAO, 1985). Soil texture is determined
by the amount of sand, silt, and clay in the soil
(FAO, 1985). Soils with high clay content will
typically have a lower infiltration rate than soil
with a high sand content. Although the soil
texture is permanent (i.e. cannot be changed
unless the soil is removed and replaced), the soil
structure can be altered to provide a higher
initial infiltration rate (FAO, 1985). Soil structure
refers to the in-situ grouping of soil particles into
aggregates, and the pores and cracks that are
formed by the arrangement of the aggregates
(FAO, 1985). Soil that is considered massive or
compacted will yield lower infiltration rates than
soil that is prismatic, granular, or blocky (see
Figure 6.1). The operator can treat the bed of
the impoundment by disking it and breaking up
the top layer of soil to improve initial infiltration.
6.4.2. Potential Changes to Soil Structure During Operations
Operators and ranchers have noted that the soils in the PRB have a tendency to “seal” themselves off
over time, based on observations from various existing impoundments and stock ponds. This self-sealing
is caused by the dispersion of the clay soil particles or de-flocculation, which results in the re-
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Table 6.4 Hydraulic Conductivity for Different Soil Types
Soil type Initial Hydraulic Conductivity, K,
(gal/day-ft2) Gravel 104 – 106
Gravelly Sand 103 Clean Sand 102
Dense Shale or Limestone 1 Granite or Quartzite 10-2
Clay 10-2 Source: Lindeburg, 2003
arrangement of the clay particles into nearly flat sheets, as depicted by the massive/compacted soil
structure in Figure 6.1. In a dispersed soil, the clay particles plug soil pores, reducing permeability. The
nature of the flocculated, undispersed soil structure is for the clay particles to lie at angles and in various
directions, such as the blocky soil structure in Figure 6.1. Random stacking of the clay particles
maintains soil permeability. Dispersion can be caused by application of low salinity, high sodium waters.
In the PRB, CBNG waters can have relatively low salinity but high sodium content and can cause
dispersion in soils.
By treating the bed of the impoundment prior to discharge of CBNG produced water the operator may be
able to extend the usable life of the impoundment. Additionally, analysis of the infiltration water quality
may identify chemical qualities in the water that are conducive to clay particle de-flocculation which may
require additional steps to prevent the bottom of the impoundment from self sealing. The chemical
changes which the soil and water undergo which result in the de-flocculation of the clays is a result of
cation exchange between the clay surfaces and the infiltrating water that result in sodium ions replacing
calcium ions on the clay surfaces.
6.4.3. Hydraulic Conductivity
As discussed above, the hydraulic conductivity of a soil can change over time as a result of infiltration
activities that result from pit operations. These changes to hydraulic conductivity can be difficult to
accurately predict; however, changes can be accounted for in the water balance by making some
assumptions based on the observations from
existing impoundments constructed over similar soil
materials. Table 6.4 depicts the ranges that can be
used for hydraulic conductivity for initial conditions.
Field observations have noted that the infiltration
rate gradually decreases over the period of two
years to approximately 20 to 40 percent of the initial
infiltration rate for clay bottom impoundments
(DeLapp, 2005)
6.4.4. Calculating Infiltration Rates
Infiltration volumes can be accounted for in the water balance by preparing infiltration rate calculations
using Darcy’s Law; however, first the assumption must be made that the soils will quickly become
saturated in order for Darcy’s Law to apply. As mentioned in Section 3.7, Darcy’s Law is defined as:
Q = K*i*A Where:
Q = Infiltration flow rate (gal/day)
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Table 6.5 Evaporation Data for Sheridan, Wyoming, in inches/month and inches/year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year
Mean 0.7 0.9 1.8 3.3 4.7 5.6 7.2 6.3 4.0 2.6 1.2 0.8 39.1 Std Deviation 0.2 0.3 0.4 0.7 0.7 0.9 0.7 0.7 0.6 0.6 0.4 0.3 2.6
High 1.5 1.9 2.5 4.6 6.7 7.7 8.5 7.9 5.0 3.6 2.2 2.0 44.2 Low 0.3 0.4 1.3 2.0 3.6 3.6 5.7 4.9 2.4 1.7 0.5 0.4 36.5 Source: Pochop, et al., 1985.
K = Hydraulic Conductivity (gal/day-ft2, see Table 6.4) i = Hydraulic Gradient (ft/ft) A = Area of the impoundment (ft2)
The hydraulic gradient is simply the difference between the water surface elevation in the impoundment
and groundwater surface elevation of shallow aquifer observed during the baseline study divided by the
distance from a measuring point within the impoundment to the groundwater surface as observed, for
instance, in a monitoring well.
Once the infiltration flow rate has been calculated in gallons/day, it can be multiplied by the days in each
month to calculate the gallons/month for the water balance.
6.5. Evaporation
The amount of water lost due to evaporation can be estimated in different ways. Pochop, et al.
(1985),evaluated eight different evaporation calculation models as they apply in Wyoming and compared
them to known evaporation pan results. Pochop’s study concluded that the resulting corrected "lake
evaporation" values are considered applicable to nearby small water bodies with negligible heat storage,
such as a CBNG impoundment (Pochop, et. al, 1985). Table 6.5 provides an example of monthly
evaporation data for Sheridan, Wyoming, in inches/month and inches/year that can be used in a water
balance to account for evaporation losses each month. The inches lost to evaporation can be multiplied
by the area of the impoundment to get volume.
6.6. Discharge and Storage
Impoundments can be designed for the ability to discharge to the surface in the event the design storage
is exceeded during an emergency, such as a major storm event (typically a 50 yr or 100 yr storm event),
to prevent the overall system’s failure (i.e. breach of the dam due to overtopping and excessive forces on
the dam). Furthermore, if the produced water is of sufficient quality as compared to the surface water
quality, discharge over the spillway of the impoundment may be an additional water management
process that can be employed during months when other seasonal water uses are not as high (such as
irrigation and livestock consumption during the winter months). The spillway can be designed in such a
way as to lower outlet velocities and minimize erosion impacts downstream of the impoundment.
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83
6.7. Other Water Management Schemes
The operator may choose other temporary or long-term water management options such as deep or
shallow injection, or water treatment and beneficial use. These options can be input into the water
balance to keep an accurate account of the water management within a CBNG field.
6.8. Example Water Balance
Based on the information provided in this section, an operator can prepare a water balance for an
impoundment for use as a planning and design tool. Figure 6.2 is a hypothetical example of a water
balance that was estimated for the first two years of operations.
The following site-specific assumptions were made for this hypothetical example:
• The volume of the pond is 65.4 million gallons (roughly 20 acres in area with an average depth of 10 feet)
• 100 acres of grass pasture will be irrigated, as necessary, year round • 1,000 head of cattle will be grazing the pasture around the pond an average of 15 days a month.
o December, January, and February the cattle will consume 8 gallons/head/day o March, April, October and November the cattle will consume an average of 12
gallons/head/day o May, June, and September the cattle will consume an average of 20 gallons/head/day o July and August the cattle will consume an average of 25 gallons/head/day
• The soil at the bottom of the pit is clayey with an initial blocky soil structure and infiltrates an average of 300,000 gallons/day initially
o The infiltration rate gradually drops 1.5% each month until at the end of year two the infiltration rate is approximately 220,000 gallons per day
• Average evaporation data for Sheridan, Wyoming is typical of the site • No water is discharged from the pond during operations and all storm water is diverted around
the pond • At the beginning of year one, there are no existing wells producing water to the pond. Wells are
put into operation on the following schedule: o 5 wells in March of Year 1, o 15 wells in April of Year 1, o 10 wells in May of Year 1, and o 5 wells in April of Year 2. o All wells pump initially at 14.661 gpm each, and their production rate declines according
to the following empirical equation (ALL, 2001):
Q(t) = 14.661 e-0.0242t
Where:
Q(t) = produced water flow rate at time t, in gpm
t = time, in months
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As can be noted in Figure 6.2, the pond becomes full in December of the second year as the infiltration
rate continues to decrease, and so the amount of water the pond can manage during subsequent years
will be lower. This information can benefit the operator by allowing the operator to understand the
limitations of the pond, given the site specific baseline study performed, and informs the operator that
additional storage or other beneficial use options will need to be evaluated. By having a thorough
understand of the pond’s limitations, the operator can plan more effectively on a larger scale and stage
construction in such a way that minimizes the occurrence of shutting in a well, or reducing field
production due to limitations from the water management planning. This hypothetical water balance is
for one impoundment managing 35 CBNG wells over a two year span of CBNG production. By staging
construction, the operator can install a new impoundment every year until the de-pressurization phase of
the project is over and produced water volumes decrease. Once the produced water volumes decrease,
the network of installed impoundments may be able to collectively handle the produced water for the life
of the CBNG project, even with declined infiltration rates.
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Figu
re 6
.2 E
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Mon
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Wat
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Year
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ear 2
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Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Mon
th
Water Production and Use(Million Gallon per Month)
010203040506070
Net Storage (Million Gallons)
Dis
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geIn
filtra
tion
Con
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ptio
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igat
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ater
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7. REGULATORY REQUIREMENTS The use of impoundments and impoundments associated with CBNG development is regulated by a
variety of agencies at the state and federal level. This section will focus on the regulatory permitting
requirements of the various agencies that have jurisdiction specific to impoundments in the Powder River
Basin. A more detailed regulatory discussion can be found in the Construction, Operation, and Modeling
of Impoundments for Managing CBM Water in the Powder River Basin report (ALL and MBOGC, In Press).
7.1. Permitting in Wyoming
The WSEO, the WDEQ, the Wyoming Oil and Gas Conservation Commission (WOGCC), and the Bureau of
Land Management Buffalo Field Office (BLM-BFO) all have varying levels of jurisdiction related to the
construction and operation of CBNG impoundments. Figure 7.1 is a decision flow diagram that can be
used to better understand each agency’s role in the permitting process of an impoundment.
Sections 7.1.1 through 7.1.7 provide details about each step in the permitting process and each permit
that may be required for an impoundment, as outlined in Figure 7.1. Monitoring and reporting
requirements for each permit are also discussed, where applicable.
7.1.1. Impoundment Determinations
The operator can benefit during the permitting process by having a good understanding of how
impoundments are defined by the regulatory authorities. Typically, four determinations are made by the
regulatory authorities in defining an impoundment: 1) whether or not the CBNG produced water
managed in the impoundment includes a beneficial use, 2) whether the impoundment is on- or off-
channel, 3) whether the impoundment is managing produced CBNG water from federal minerals or is
located on federal land, and 4) whether the impoundment volume is greater than 50 acre-ft, or the
embankment is greater than 20 ft. If the operator understands each of the concepts behind how the
agencies make these determinations, it can aid in acquiring the appropriate permits and speed up the
permitting process.
Beneficial Use Determination
If the water stored in the impoundment will be used for any inactive use, such as stock, wildlife, and/or
wetlands and/or active uses such as land application, leach fields, irrigation, and/or dust abatement, then
there is a beneficial use associated with the CBNG produced water (WDEQ, WOGCC, BLM, WSEO, 2002).
If a determination is made that there will be no beneficial use of the CBNG produced water, then a permit
will either need to be filed with the WOGCC or the BLM, dependant on the determination of the lease
type (see Lease/Land Determination discussion below).
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On- or Off-Channel Determination
An on-channel impoundment refers to an impoundment that receives CBNG produced water that is sited
within a designated water feature or within the floodplain or alluvium of a water feature including
perennial or intermittent streams, dry washes, bank stream channel, pond, reservoir, wetland, or lake as
defined on a USGS 1:24,000 scale topographic map (WDEQ, 2005). An off-channel system refers to any
impoundment that is not sited within a designated water feature as defined on a USGS 1:24,000 scale
topographic map (WDEQ, 2005).
No
Yes
State/Fee
Federal
No
Yes, On-Channel
Yes, Off-Channel
Figure 7.1 Decision Flow Chart for Permitting in Wyoming
Beneficial Use?
Permits Required: 1) Individual WYPDES Permit 2) WSEO Reservoir Permit Process
Mineral lease/ land type?
Permits Required: 1) General or Individual WYPDES Permit 2) WSEO Reservoir Permit Process 3) WDEQ Compliance Monitoring for Unlined Impoundments Permit Required:
1) BLM Permit, Impoundments
Permit Required: 1) WOGCC Permit for Produced Water Storage
Volume over 50 acre-ft, or embankment over 20 ft?
Construction and use can begin
WSEO Safety of Dam Law requires a PE to certify design
Start
Ground-water less than 150 ft
deep?
Ground-water less than 200 ft
deep?
Yes
No No
WDEQ Compliance Monitoring Plan Required for GW
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Prior to proceeding with the permitting process for an on-channel system, the WDEQ requires that
evidence (such as a subsurface investigation, modeling that utilizes site specific parameters, or other
evidence, such as determination of the groundwater gradient, that protects the surface waters of the
state) be presented that demonstrates that there will be no direct hydrologic connection from the
produced water in the impoundment to surface water of the state, or to areas outside of the
impoundment (WDEQ, et al, 2002).
If a beneficial use determination is made, off-channel impoundments require either a general or
individual WYPDES permit and on-channel impoundments require an individual WYPDES permit (see
Section 7.1.1) to be filed with the WDEQ. In this instance, both on- and off-channel impoundments
require a reservoir permit (see Section 7.1.2) to be filed with the WSEO. Furthermore, off-channel
impoundments may require an impoundment construction permit (see Section 7.1.3) be filed with the
WDEQ.
Lease/Land Determination
Regardless of whether the impoundment is on- or off-channel, permitting is different depending on the
owner of the mineral estate (Federal, State, or fee) and the owner of the land surface where the
impoundment is to be constructed (Federal, State, or fee). State and fee mineral leases and/or surface
construction are permitted through the WOGCC. See Section 7.1.5 for a description of what is entailed
with filing a permit with the WOGCC. If the produced water originates on a federal mineral lease, or if
federal land is disturbed as a result of the construction of an impoundment, then a permit must be filed
with the BLM. See Section 7.1.6 for a description of what is entailed with filing a permit with the BLM.
Volume and Embankment Height Determination
If the reservoir is constructed with an embankment (or dam) height greater than 20 feet (measured from
the crest of the dam to the low-flow channel downstream of the dam), or the volume of the reservoir is
greater than 50 acre-feet against a man-made embankment, then the safety-of-dams requirements may
apply as established by the WSEO. See Section 7.1.7 for a more detailed discussion of the safety-of-
dams requirements (WDEQ, et al, 2002).
7.1.2. General or Individual WYPDES Permit
If a WYPDES (formerly known as NPDES) permit is determined necessary based on the decision process
presented in Figure 7.1 and the above discussion, and the impoundment is off-channel, then the operator
has two options to be in compliance with the WYPDES program. The operator can either request
authorization to be covered under the WYPDES General Permit by filing a Notice of Intent with the
WDEQ/Water Quality Division (WQD), or the operator can file a WYPDES individual permit. There are
four major classes of surface water in Wyoming with various subcategories within each class. Class 1
waters are those waters that have been specifically designated by the Environmental Quality Council.
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Class 2 designations are based upon the fisheries information contained in the Wyoming Game and Fish
Department’s “Stream and Lakes” inventory database as submitted to the Department of Environmental
Quality in June, 2000. Class 4 designations are based upon knowledge that a water body is an artificial,
man made conveyance, or has been determined not to support aquatic life uses through an approved
Use Attainability Analysis. All other waters are designated as Class 3. The general permit was created in
2002 specifically for discharges to class 4 “off-channel containment units”, and in some cases, for indirect
discharges (infiltration) to class 2 and class 3 waters provided there will not be an impact to the water
body.
If the impoundment is on-channel, or if the effluent limits contained in the general permit are not
considered to be adequate for protecting water quality and designated uses for an off-channel
impoundment, then an WYPDES individual permit is required to be in compliance with WDEQ/WQD. A
WYPDES individual permit is filed by completing the Application Form for WYPDES Discharge of Produced
Water from Coal Bed Methane Production. The Construction, Operation, and Modeling of Impoundments
for Managing CBM Water in the Powder River Basin report (ALL and MBOGC, In Press) can be referenced
for a more in-depth discussion on WYPDES permitting and the different options available in the individual
permit.
Status: Watershed Based Permitting Approach
On June 14th, 2004, the WDEQ WQD announced that a watershed based permitting approach would
apply to future CBM WYPDES permit applications. The announcement stated that “new CBM produced
water WYPDES permits will not be issued in any Hydrologic Unit Code (HUC) 10 level drainages that
currently do not have CBM WYPDES permits until a watershed management plan for the drainage has
been written and accepted by the WQD. In addition, the WQD will set common expiration dates for
existing CBM permits within each HUC 10 level drainage, and will require approved watershed
management plans before re-issuing those permits” (WDEQ/WQD, 2004).
A follow up announcement from the WDEQ/WQD on January 4th, 2005, stated that due to the fact that
the permitting process development for the watershed approach is moving at a pace slower than
anticipated, and that the current permitting method has been deemed protective of state’s surface water
quality standards, the existing WYPDES permitting process will stay in place until the watershed based
permitting process is finalized, and the WDEQ/WQD reserves the right to require a watershed
management plan for any CBNG WYPDES permits that are applied for in a HUC 10 watershed where no
current WYPDES permits exist.
7.1.3. WSEO Reservoir Permit Process
Once the operator determines that the proposed impoundment will require a WSEO Reservoir permit, the
Category and Method (see Figure 7.2) that the impoundment falls under will determine what WSEO forms
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need to be filed to remain in compliance with the WSEO. Figure 7.2 depicts the decision flow diagram for
the reservoir permitting process of the WSEO. This figure does not represent the entire WSEO permitting
process, as a Form U.W.5 is required for each proposed CBNG well, and further requirements exist for
groundwater injection/aquifer storage. Contact the WSEO for more information about additional
permitting requirements outside of the reservoir permitting process.
7.1.4. WOGCC Permit for Produced Water Storage
WOGCC requires, at a minimum, that Form 14A be submitted and approved prior to construction of
impoundments on fee or state leases. The Supervisor may request information in addition to what is
Figure 7.2 Decision Flow Diagram for WSEO Reservoir Permitting Process
No. Method A.
Yes. Method B.
Yes. Method B.
Yes. Category 1. No. Category 2.
No. Method A.
It is determined that an WSEO reservoir Permit is required prior to commencement of construction of the impoundment.
Will the impoundment capacity be 20 AF or less AND will the dam height be 20 ft or less?
Will the reservoir remain after CBNG operation cease?
Will the reservoir remain after CBNG operation cease?
Fill out SEO Form SW-CBNG. No map is required. Applicant is operator producer, use is CBNG storage only.
Fill out SEO Form SW-3, with a qualifying map (see instructions for SW-3). Applicant is operator/producer, use is CBNG water storage only.
Follow the Special Application procedures with USGS quad map and form SW-3 and SW-3A. If water will be used for stock, the SW-4 application form should be used, map not required. Other beneficial uses may require the filing of form SW-3.
Fill out SEO Form SW-3, with a qualifying map (see instructions for SW-3). Landowner is the applicant or co-applicant.
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required on Form 14A. The operator must demonstrate that the operation of the impoundment will
comply with the water quality standards of the WDEQ in order for the Form 14A to be considered in
compliance. Approval for impoundment construction is handled administratively and WOGCC staff
routinely pre-site pit locations prior to taking action on applications (WDEQ, et al, 2002).
In addition to Form 14A, the WOGCC may require the operator to provide a bond for closure of the
impoundment once operations have ceased. Separate bonding amounts for the impoundments are set
by the Supervisor following evaluation of site-specific conditions and circumstances. The operator should
provide a written cost estimate, prepared by a Wyoming registered PE experienced in pit remediation, for
closure of the pit and reclamation of the surface and access areas closely adjacent to the pit. A copy of
the cost estimate must also be provided to the surface landowner. Bonding may be waived, however, if
the impoundment is to remain in place after operations have ceased. In this instance, a notarized
statement of acceptance signed by the surface landowner must accompany the Form 14A when it is
submitted to WOGCC. Specifics of the requirements of the letter of acceptance can be found in Chapter
3, Section 4 (WDEQ, et al, 2002).
7.1.5. BLM Permit for Impoundments
An Application for a Permit to Drill (APD) and a Plan of Development (POD) are required prior to
producing CBNG or CBNG produced water. A Water Management Plan (WMP) is a required attachment to
the APD/POD submittal. Information required in the WMP for approval of an impoundment includes, but
is not limited to:
• a representative onsite water analysis (WDEQ’s WYPDES analytical suite) for CBNG produced water;
• size of impoundment; • freeboard capacity; • method of disposal of produced water; • maximum fluid level above native soil on down gradient side; • soil characteristics; and • depth to shallow groundwater (with analytical results from subsurface investigation).
In order to remain in compliance with the BLM, the subsurface investigation should provide adequate
information to insure that the shallow aquifer will not be degraded below its existing class of use and that
infiltration will be primarily downward and not migrate laterally entering “surface waters of the state”.
7.1.6. WSEO Safety of Dam Law
If a dam or embankment is proposed to be constructed for the impoundment then the Wyoming Safety
of Dam Law is applicable. The Wyoming Safety of Dam Law (W.S. 41-3-307 through 41-3-318) was
enacted in 1977 and was amended in 1992. The permitting requirements are summarized in the
following way:
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"... duplicate plans and specifications showing the proposed work on any facility, meeting
the criteria of the Safety of Dams Law, shall be prepared by or under the direction of a
registered professional engineer licensed in Wyoming and experienced in dam design and
construction. These plans and specifications must be submitted to the State Engineer for
review and approval. No work shall begin until the submitted plans and specifications are
approved by the State Engineer. The term "work" includes the following activities, among
others: construction of a new facility and any repair, alteration, or rehabilitation which
may affect the safety or size of an existing facility. Once the plans and specifications
have been approved by the State Engineer, a qualified engineer shall be in charge of and
responsible for the proposed work. The engineer in charge shall inspect the work and
submit reports to (the state engineer's) office detailing the information obtained during
the inspection and on the progress of the work." (W.S. 41-3-308).
7.1.7. WDEQ Compliance Monitoring Plan
If groundwater is detected at a depth of 150 feet or less (200 feet or less for an impoundment that has a
capacity of 50 acre feet or more), then a Compliance Monitoring Plan (CMP) must be provided to remain
in compliance with WDEQ groundwater requirements. The CMP must be submitted to the WDEQ/WQD
and approved prior to any discharge into the impoundment in order to remain in compliance. For a
complete discussion of the CMP requirements, see the Compliance Monitoring for Groundwater Protection
Beneath Unlined Coal Bed Methane Produced Water Impoundments (WDEQ, 2004). Quarterly monitoring
samples and reporting is required as a part of the Compliance Monitoring Program. Further discussion of
the monitoring requirements can be found in the above referenced WDEQ/WQD document.
7.2. Permitting in Montana
Jurisdictions over impoundments that contain CBNG water are currently a matter of dispute within the
state of Montana. The MDEQ maintains that they have jurisdiction since these impoundments are
considered “waters of the state” because they are filled with water having beneficial uses (for example,
stock watering and irrigation). The Montana Department of Natural Resources and Conservation
(MDNRC) maintains that these impoundments are filled with “waste” as determined legislatively,
therefore, the impoundments are not “waters of the state” and no MDEQ permit is required. In this
instance, the Montana Board of Oil and Gas Conservation (MBOGC), the part of the MDNRC that governs
oil and gas operations, maintains that they have jurisdiction over the permitting process. Figure 7.3
illustrates the steps and decision-points in the permitting process for Montana CBNG impoundments.
7.2.1. Impoundment Determinations
The same impoundment determination criteria that applied in Wyoming can be used for Montana in terms
of the beneficial use determination and the on- or off-channel determination. In absence of detailed
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94
design calculations, the capacity of the impoundment can be calculated in acre-feet, by multiplying the
proposed surface area (acres) by 0.4 times the maximum height (feet) of the dam, as measured from the
crest to the downstream toe of the dam (Montana Code 36.14.102.3).
7.2.2. MBOGC Permit for Storage Impoundments
If a no beneficial use determination is made for the produced water to be stored in the impoundment,
then Form 23 (Application For Permit to Construct or Operate an Earthen Pit or Pond) must be filed with
the MBOGC. For a new impoundment, diagrams depicting a top view and two side views of the system
are to be attached. The diagrams must follow the requirements listed on the permit application. For an
existing pond that is to be converted, pictures of the existing pond need to be submitted along with the
application, as discussed on Form 23.
Yes
No
Yes
Yes
No
Off
No Beneficial Use of Water?
Start
MDEQ Individual MPDES Permit
(General MPDES Permit in preparation)
MBOGC Permit for Storage
Impoundments
On- or off-
channel?
Site-specific CBNG water quality and
watershed conditions
Site-specific CBNG water quality and receiving water
assumed to be G-1
Construction and Use can Begin
On
Capacity more than
50 AF?
MDNRC WRD Application for
Hazard Classification
Deemed high-hazard
classification?
MDNRC WRD Construction and Operation Permits
Figure 7.3 Decision Flow Diagram for the Montana Permitting Process
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7.2.3. MDEQ Individual MPDES Permit
If a beneficial use determination is made for the water in the impoundment then the impoundment is
considered “state waters”. Montana’s MPDES program (ARM: Title 17, Chapter 30, subpart 12-14) is
applicable to owners and operators who plan to discharge certain point source discharges into “state
waters”. Operators or owners applying for a MPDES permit are required to submit an application to
MDEQ at least 180 days before any discharge to a state water is planned to commence. Upon receipt,
MDEQ has 60 days to review new permit applications for completeness and 30 days for completeness
review of deficiency responses. During the application process the department will determine discharge
limitations and the length of mixing zones to ensure water quality standards are met. Information
required to complete an application for an individual MPDES permit, and a more detailed discussion on
the MPDES program can be found in the Construction, Operation, and Modeling of Impoundments for
Managing CBM Water in the Powder River Basin report (ALL and MBOGC, In Press).
7.2.4. Status of MDEQ General MPDES Permit
Pursuant to public approval for incorporation into by Montana’s MPDES program, operators or landowners
will be able to discharge CBNG produced water resulting from natural gas production to holding ponds for
the purpose of a pre-determined beneficial use, e.g. stock pond or wildlife watering, under a general
MPDES permit. Under this permit produced water discharges to surface “state waters” is not authorized
unless defined and approved during the permit process. In addition, discharges to holding ponds for
beneficial use is subject to the effluent limitations and monitoring requirements as described in the
Administrative Rules of Montana (ARM) Title 17, Chapter 30. The discharge permit (and accompanying
MDEQ authorization letter) is valid for five years from the date of issuance.
A discussion of the effluent limitations, special considerations, and monitoring requirements of the
general MPDES permit can be found in the Construction, Operation, and Modeling of Impoundments for
Managing CBM Water in the Powder River Basin report (ALL and MBOGC, In Press).
7.2.5. MDNRC WRD Permitting Requirements
If the operator estimates the proposed impoundment capacity to be greater than 50 acre-feet, then an
application to the MDNRC for hazard classification must be filed.
Hazard Classification
The determination of hazard involves an evaluation of the area downstream from the dam that would be
flooded if the dam fails. If the flood would be likely to cause a loss of life, the dam or reservoir is
classified as a high-hazard structure. Note: This hazard classification is not an assessment of the safety
of the structure instead is based on the potential loss of life at a point that may be flooded downstream.
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To apply for a dam hazard classification, the owner should write or call any MDNRC office and request an
application form. A hazard classification will be made within 60 days after the department receives a
complete application (Montana Dam Safety Act 85-15-209).
Construction Permit and Inspections
In the unlikely event that the dam is determined to be a high-hazard dam, a construction permit is
necessary before construction begins. "Construction" includes construction of a new high-hazard dam or
a major repair or alteration, enlargement, or removal of an existing high-hazard dam.
An application for a construction permit includes an application form, construction plans and
specifications, and an engineering design report. The plans and specifications must be prepared by an
engineer experienced in dam design and construction. Within 60 days of receiving a completed
application, the department will issue or deny a construction permit (Montana Dam Safety Act 85-15-
210).
The following inspection requirements exist for high-hazard dams during construction (Montana Dam
Safety Act 85-15-211):
• An engineer must be in charge of and responsible for inspections during construction of any high-hazard dam.
• Inspections during construction must be performed at intervals necessary to ensure conformity with the permit. The engineer in charge or a qualified designee shall perform the inspections.
• The department shall set procedures and requirements for reporting information obtained from, during, or as the result of an inspection. The engineer in charge shall certify all reports to the department.
• The department may also inspect the high-hazard dam during construction to ensure conformity with the construction permit.
• If the department finds that construction of the high-hazard dam does not conform with the construction permit, it may order that construction be stopped until changes are made in conformity with the permit
Operations Permit
An application for an operation permit for a high-hazard dam should include an operation plan and a
licensed professional engineer's inspection report of the high-hazard dam. The operation plan should
include reservoir operation procedures, maintenance procedures for the dam and appurtenant works, and
an emergency procedures and warning plan. Preparation of and adherence to the plan will aid in the
safe operation and maintenance of the high-hazard dam. A more detailed description of an operation
plan can be found in the administrative rules for dam safety. The inspection, which is the responsibility
of the dam owner, must be conducted by a licensed professional engineer. The minimum frequency for
inspection is set in the operation permit and is not less than once in five years. If the existing high-
hazard dam is found safe and can be operated and maintained safely as well, an operation permit is
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issued by the department. Renewal of the operation permit is granted by the department upon
satisfactory periodic inspections (Montana Dam Safety Act 85-15-212).
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8. REFERENCES
ALL Consulting and MBOGC. In Press. Construction, Operation, and Modeling of Impoundments for
Managing CBM Water in the Powder River Basin. ALL Consulting. October 2004. Coal Bed Natural Gas Handbook, prepared for the US DOE NETL, 2004. ALL Consulting. 2003. Handbook on Coal Bed Methane Produced Water: Management and Beneficial
Use Alternatives, prepared for the US DOE, July 2003. ALL Consulting. 2001. Water Resources Technical Report. Bureau of Land Management, Miles City
Field Office. Bartos, T.T. and K.M. Ogle. 2002. Water Quality and Environmental Isotopic Analyses of Ground-Water
Samples Collected from the Wasatch and Fort Union Formations in Areas of Coal bed Methane Development – Implications to Recharge and Ground-Water Flow, Eastern Powder River Basin, Wyoming. U.S. Geological Survey Water Resources Investigation Report 02-4045. 96pgs.
Bureau of Land Managment. 1999. Wyodak Coal bed Methane Project Final EIS, Buffalo Field Office.
October 1999. Blue Ridge Energy. 2005. http://www.br-energy.com/completion.html#liftingwell Centers for Disease Control and Prevention (CDCP), United States Department of Health and
Human Services. 2003. Epidemic/Epizootic West Nile Virus in the United Sates: Guidelines for Surveillance, Prevention, and Control. 3rd Revision. http://www.cdc.gov/ncidod/dvbid/westnile/resources/wnv-guidelines-apr-2001.pdf
Collins, A.G., and Carroll, H.B., Jr. 1987. Subsurface injection of fluids for the recovery of petroleum:
International Symposium on Subsurface Injection of Oilfield Brines Proceedings, Underground Injection Practices Council.
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