Well

WELL HYDRAULICS AND CAPTURE ZONE MODULE

K. Thorbjarnarson

Dept. of Geological Sciences, SDSU

INTRODUCTION

Since ancient times, wells have been dug or drilled into the subsurface to access groundwater. Prior to the development of drilling technologies, buckets were used to collect water from shallow hand-dug wells (Figure 1). Modern groundwater wells can be thousands of feet deep and allow extraction of large quantities of water with electric pumps.

Drinking water is obtained in many communities from groundwater wells. As

water is extracted from a well, the water level within the well drops. Water in the surrounding aquifer flows towards the well causing a lowering of the water level extending outward from the well (Figure 2). The drop in water level is greatest immediately adjacent to the well and decreases radially outward creating a feature called the cone of depression. As pumping continues, the cone of depression extends out farther gathering water from a larger cylindrical volume surrounding the well. The expansion of the cone of depression will continue until the volume of water intercepted or drawn by the well equals the pumping rate. Besides aquifer water, the water drawn by a well can also be recharge from the ground surface, adjacent aquifers, streams, lakes or oceans. Impermeable boundaries formed by low hydraulic conductivity materials (bedrock, faults, etc.) will halt the progression of the cone of depression at their location.

Knowledge of the drop in water level and pattern of groundwater flow resulting

from well pumping is necessary for assessing environmental impacts in many situations. Excessive drops in groundwater levels over regional scales can result in adverse impacts to stream flows, vegetation and the use of shallow wells (Figure 3). At sites of groundwater contamination, the cone of depression can expand outward from the pumping well and capture the contaminated water (Figure 4). While the well capture of contaminated water is desired with a remediation well, the situation could be disastrous with a drinking water well. In a worst-case scenario, contamination of drinking water wells can go undetected for long time periods resulting in illnesses as in Woburn Massachusetts (Harr, 1995). Once detected, communities must deal with the loss of their drinking water source (Santa Monica, CA: www.epa.gov//region09/cross_pr/mtbe/charnock/drinking.html).

Figure 1. Hand-dug wells.

Figure 2. Cone of depression created by pumping a groundwater well (from Oregon State University Extension Service http://groundwater.orst.edu/under/wells.html)

Figure 3. Overpumping of groundwater depleting Streamflow and shallow well . (from Oregon State University Extension Service http://groundwater.orst.edu/under/wells.html)

Figure 4. Contaminants drawn into well (from Oregon State University Extension Service http://groundwater.orst.edu/under/wells.html) .

PROBLEM STATEMENT

Assessment of well capture is necessary for two scenarios:

Identification of potentially responsible parties (PRPs) for contamination of a drinking water well

Identification of regions for wellhead protection

In many contaminated sites, multiple possible sources or responsible parties could have caused the pollution. The need to identify the responsible party or parties results in litigation and assessment of groundwater flow patterns (see A Civil Action, the Woburn, Massachusetts case). Models are used to simulate the groundwater flow and to see whether groundwater underneath potential sources is eventually captured by the pumping well. To further assess the feasibility of a potential source, the travel time of contaminants from the source to the well is estimated to see if the timing fits with possible releases from the site. Well-capture models and numerical models (MODFLOW) can be used to evaluate the capture zone and travel times. Figure 5 shows the pattern of flow to a drinking water well with three possible contaminant sources. In this scenario, all three sites are within the capture zone and have a similar travel time.

Figure 5. Groundwater pumping well capture zone pathlines (brown lines).

Potential sources are purple symbols. In wellhead protection, the headwaters of the well or regions which supply water

to the well are identified. The community and regulators decide on appropriate land use for these regions which will have minimal impact on the subsurface groundwater.

The need for the ability to predict the drop in groundwater level, extent of the

cone of depression and the well capture zone has resulted in many models of well hydraulics and capture. The models vary in their level of complexity with the trade-off being the number of simplifying assumptions required for their solution. This module will start with a analytical solution for a single well (Theis solution) with many simplifying assumptions and progress to a more complex multiple well-capture zone model (Environmental Protection Agencys WhAEM model).

BACKGROUND Uniform Flow Field and Pore-Water Velocities

Under regional flow conditions, uniform flow can occur at local scales and is easily modeled by the one-dimensional forms of Darcys Law:

Q = K A I (1)

Where, Q = groundwater flow rate (L3/T) K = hydraulic conductivity (L/T)

A = cross-sectional area to flow (L2)

I = hydraulic gradient

V = Ki/ (2)

Where, v = average pore-water velocity K = hydraulic conductivity

I = hydraulic gradient = effective porosity

Assuming homogeneous conditions, the flow rate and average pore-water velocity are easily calculated for uniform flow. The regional hydraulic gradient, hydraulic conductivity, and effective porosity and resulting groundwater flow rate and velocity are all constant. Travel time of solutes can easily be found by dividing the travel distance by the velocity. Radial Flow Field and Pore-Water Velocities

Under radial flow conditions produced by pumping a well, the same equations govern flow conditions. Applying Darcys Law to radial flow, the cross-sectional area of flow is not a constant. Groundwater converges toward the pumping well producing decreasing cross-sectional flow areas until reaching the small cross-sectional area of the well screen. To maintain continuity, the flow across one cross-sectional areal face must be equal to the others. The same flow rate is maintained across a smaller cross-sectional area by an increase in the hydraulic gradient. The pattern of decreasing hydraulic gradient (potentiometric slope) with increasing radial distance from the well produces the characteristic cone of depression. With the hydraulic gradient decreasing away from the well, the pore-water velocity is nonuniform and decreases with increasing radial distance away from the well:

(2 )Q QvA rb

= =

Travel time of water from a given radial distance can be calculated by integration of this steady-state radial flow equation:

(2 )dr Qvdt rb

= =

Rearrange the equation and integrate:

2 2

1 1

2 22 1

2

( )2 2

r t

r t

Qr dr dtb

r r Q tb

=

=

where, t = travel time for water from r2 to r1 (towards the pumping well) Q = pumping rate b = thickness of aquifer = effective porosity Assumptions: Steady-state flow conditions have been reached

Q through saturated thickness, b, is realistic and will be dependent on the aquifer transmissivity, T

Within the radial extent of the cone of depression, the hydraulic gradients

direction is towards the pumping well. Groundwater within the areal extent of the cone of depression is flowing towards the well and ultimately can be captured (pumped) out of the aquifer. However, capture zones must be defined by the length of time of capture water at the farthest radial distance within the cone of depression will take longer to reach the well than water within close proximity of the well. Transient Drawdown of Groundwater Potentiometric Surface by a Pumping Well in a Confined Aquifer Theis Solution

Theis (1935) solved the radial flow equation with the following assumptions (Fig. 6):

Homogeneous confined aquifer of infinite areal extent Aquifer is compressible and water is released instantaneously from storage as the

head is lowered Constant pumping rate No source of recharge

Radial groundwater flow equation: 2

2

1h h S hr r r T t

+ =

Figure 6. Variables in radial flow toward a pumping well in a confined aquifer. Analytical solution for flow to a well: Groundwater pumped from a well in a homogeneous confined aquifer at a constant rate,Q. The horizontal groundwater flow is governed by the radial flow equation. The steps to obtaining the analytical solution (Theis equation) are:

1. Transform the terms in the radial flow equation using dimensionless variable, u

2

4r SuTt

= (1)

2. Rearranging the variables in Equation (1) gives us equations for r and t as a

function of u:

4TturS

= (2)

2

4r StTu

= (3)

3. Using the chain rule of differentiation,

2 24

h h u h rS u hr u r u Tt r u

= = = (4)

2

2 2 2 2h u h u h u hr r r u r r u r r u

= = + (5)

4. Using Equation (1) and the chain rule,

24 4u rS S u

r r r Tt Tt r = = =

(6)

2

2

2h h u h ur u u u r u r

= = (7)

5. Substituting Equations (6) and (7) into Equation (5),

2 2 2

2 2 2 22 4h u h u hr r u r u

= +

(8)

6. The right hand side of the radial flow equation can also be rewritten using the

definitions of r and t as a function of the dimensionless variable, u. Using equations (1), (2), (3) and the chain rule,

2 2

2 244h h u h r S h T ut u t u Tt u S r

= = =

(9)

7. Substituting Equations (4), (8) and (9) into the radial flow equation,

2 2 2

2 2 2 24 4 4u h u h u hr u r u r u

+ =

(10)

8. Simplify Equation (10),

2

2

1h h hu u u u

+ =

(11)

9. Simplify Equation (11) further,

2

2

1 1 0h hu u u

+ + = (12)

10. Define g(u) as

( ) hg uu

=

(13)

11. Substitute Equation (13) into Equation (12),

1 1 0g gu u

+ + = (14)

12. Equation (14) is a well-known first-order differential equation having the

solution,

( )ln( )( ) u u uh cg u ce eu u

= = =

(15)

where, c= constant

13. Integrating Equation (15),

( )

( )

( )h u

h u u

eh c u cW uu

= =

( ) ( ) ( )

( ) ( ) ( )

h h u cW u

h u h cW u

=

=

where, W(u) is the well function or Theis function which is equal to the exponential integral:

( )u

u

eW u uu

=

h(u) corresponds to h(t), the hydraulic head at time t h() corresponds to h(t=0), the initial hydraulic head

= Initial conditions must be known for this solution

14. Boundary conditions are used to determine the value of c. At the well face, the

rate of flow from the aquifer into the well must be equal to Q, the pumping rate. If the radius of the well is rw, Darcys Law will be:

(2 ) 2w wh h hQ KA K r B r Tr r r

= = =

where, A = cross-sectional area to flow direction

K = hydraulic conductivity of aquifer B = thickness of confined aquifer

15. Using Equations (4) and (15), the above equation becomes:

4 4 4

4

uu

u

h eQ Tu Tuc c Teu u

Qc eT

= = =

=

16. For most wells, rw is small and u(rw) is very small and eu = 1.

4QcT

=

17. Inserting the value of c for this boundary condition gives the Theis equation:

( ) ( ) ( ) ( )4Qs t h h u W uT

= =

where, s(t) = drawdown from initial potentiometric water level at time t since pumping

started

The exponential integral can be evaluated by polynomial and rational approximations given by Abramowitz and Stegun (1970, p.231, eq. 5.1.53 and 5.1.56):

2 3

4 5

0 1( ) 0.57721566 0.99999193 0.24991055 0.05519968

0.00976004 0.00107857

For uW u n u u u u

u u

= + +

+

4 3 21 2 3 4

4 3 21 2 3 4

1 1

2 2

3 3

4 4

1

( )

,8.5733287401 9.573322345418.0590169730 25.63295614868.6347608925 21.09965308270.2677737343 3.9584969228

u

For u

u a u a u a u au bu b u b u b

W uu e

wherea ba ba ba b

+ + + + + + + + =

= == == == =

where, 2

4r SuTt

=

S = aquifer storativity (dimensionless) T = time since pumping began

Steady-State Drawdown of Groundwater Potentiometric Surface by a Pumping Well in a Confined Aquifer Thiem Solution Assuming steady-state conditions, the hydraulic head will not change with time. The Thiem or equilibrium equation can be used to calculate hydraulic head distributions with distance from the pumping well. This equation is derived from the radial form of Darcys Law:

(2 ) 2h h hQ KA K rb rTr r r

= = =

Rearrange the equation,

2Q drdhT r

=

Integrating the above equation,

2 2

1 12

h r

h r

Q drdhT r

=

results in the Thiem equation for confined aquifer:

2

2 1 12 ( )rQT n

h h r

=

After pumping for a long period of time and drawdown has stabilized, the Thiem equation can be used with observations of hydraulic head in two observation wells to calculate the transmissivity of the confined aquifer. The Thiem equation for drawdown calculations is:

2Q Rs nT r

=

where, R = radial extent of the cone of depression

Multiple Wells In many cases, a field site may have more than one pumping well. Multiple cones of depression can intersect and result in greater drawdown than expected from a single well (Fig. 7). As the equation governing groundwater flow (Laplace equation) is linear, the drawdown at any point is found by summing the drawdowns produced by all the wells (Fig. 8).

Figure 7. Coalescing of multiple cones of depression (from Oregon State University Extension Service http://groundwater.orst.edu/under/wells.html)

Figure 8. Summing of individual calculated drawdowns for multiple well scenarios (from Bear, 1979) Hydrogeologic Boundaries and Image Wells All the solutions previously discussed assumed infinite horizontal extent aquifers. In reality, most aquifers will be bounded by recharge sources or boundaries (ocean, lakes, streams) or impermeable boundaries (alluvial sand-bedrock contact). Utilizing the Theis and Thiem equations will result in cones of depression which extend an unrealistic distance past these boundaries. Imaginary wells called image wells are used to simulate the boundary effects.

In the case of a recharge boundary, once the cone of depression reaches a recharge

source, the rate of drawdown decreases as the well now receives water from this source. The cone of depression will not extend past the recharge boundary and will have less drawdown than simulated by the Theis or Thiem equation. The effect of a recharge boundary is simulated by use of an image well which injects water into the aquifer:

1. The image well is placed across the boundary at the same distance the pumping well is from the boundary (mirror image).

2. At a given point of known radial distance from the pumping well, drawdown for the pumping well is calculated by the Theis or Thiem equation.

3. At that same point of known radial distance from the image well, the potentiometric buildup (or negative drawdown) from the image well injecting water is calculated by the Theis or Thiem equation (Q will be negative)

4. The drawdown and buildup are summed to give the drawdown expected in the presence of a recharge boundary

Note from Figure 9 that the drawdown from the pumping well (s) and the buildup from the image well (-s) at the recharge boundary are the same value with opposite signs resulting in zero drawdown.

Figure 9. Recharge boundary and image well (from Bear, 1979)

Impermeable boundaries will result in enhanced drawdown as the spread of the cone of depression is stopped and all water pumped is removed from storage in the aquifer. Using a pumping image well to simulate the impermeable boundary:

1. The image well is placed across the boundary at the same distance as the pumping well is from the boundary (mirror image)

2. At a given point of known radial distance from the pumping well, drawdown for the pumping well is calculated by the Theis or Thiem equation.

3. At that same point of known radial distance from the image well, the drawdown for the image pumping well is calculated by the Theis or Thiem equation (Q will be positive).

4. The two drawdown values are summed to give the drawdown expected in the presence of an impermeable boundary.

Note from Figure 10 that the drawdown from the pumping well and the image well at the impermeable boundary are the same value with the same sign resulting in twice the drawdown than anticipated in an aquifer of infinite horizontal extent.

Figure 10. Impermeable boundary and image well (from Bear, 1979)

Superposition on a Uniform Regional Gradient The solutions utilized so far have assumed an initial horizontal potentiometric surface. In reality, groundwater potentiometric surfaces will slope with the direction of the hydraulic gradient and in the direction of groundwater flow. A capture zone which shows the areas around the well which will drain into the pumping well will be asymmetric when a regional hydraulic gradient exists (Figure 11). The capture zone is bounded by the cone of depressions drawdown to the sides and downgradient of the pumping well. However, as the regional gradient is sloping toward the well in the upgradient well, groundwater within the sideways bounds of the cone of depression are captured for a large distance upgradient given enough time. Direction

of Uniform Regional Flow

Radial Flow R = radius of circle = extent of cone of depression and capture zone

Single Well in Uniform Regional Flow

ymax

xo

Figure 11. Flow lines or streamlines indicating direction of water flow to a pumping well superimposed on uniform regional flow conditions (from Bear, 1979) Under steady-state conditions, the capture zone envelope for radial flow is a circle with the radius of the cone of depression. Superimposing a single well on a uniform regional flow, the capture zone can be described by x0, the downgradient distance from the well to the stagnation point and 2*ymax, the maximum width of the capture zone envelope (Todd, 1980; Fetter, 2001):

0

max

/(2 )

/(2 )

x Q Kbi

y Q Kbi

=

=

where, ymax and x0 = defined by above figures K = hydraulic conductivity

b = aquifer thickness Q = well pumping rate i = regional hydraulic gradient Capture Zone Analysis and Well Head Protection With the passage of the Safe Drinking Water Act, the United States Environmental Protection Agency implemented the Wellhead Protection Program and the Source Water Assessment Program (USEPA, 1999). In this program, source waters to a well or well field must be identified (Fig. 12). Within the capture zone of wells, potential contaminant sources and transport must be evaluated. Regions within the capture zone can be identified as wellhead protection areas with designated land uses to reduce the potential for groundwater contamination. Guidelines for delineating protection areas by the USEPA Office of Ground Water and Drinking Water range from simplistic (fixed radius from the well) to complex (hydrogeologic modeling) (USEPA 1994). A public domain, ground-water flow model, WhAEM, has been designed by the EPA to facilitate capture zone delineation and protection are mapping. Modeling of steady-state pumping wells, including the influence of hydrogeologic boundaries such as rivers, recharge and no-flow contacts is accomplished using the analytic element method (USEPA, 2000). Analytical elements are singularity distributions used to model certain flow features in an aquifer, e.g. point sinks to model wells, linesinks to model streams and lake boundaries, areal sinks to model recharge due to precipitation, and line doublets to model no-flow boundaries. The software and manual is available by downloading from an EPA website.

Figure 12. Capture zone for a pumping well.

MODEL DETAILS

Drawdown of Groundwater Potentiometric Surface by a Pumping Well in a Confined Aquifer The Theis equation will be used to calculate drawdown and the radial extent of a cone of depression in a confined aquifer.

( , ) ( )4Qs r t W uT

=

( )x

x u

eW u dxx

=

=

2

4SruTt

=

2 3

4 5

0 1( ) 0.57721566 0.99999193 0.24991055 0.05519968

0.00976004 0.00107857

For uW u n u u u u

u u

= + +

+

4 3 21 2 3 4

4 3 21 2 3 4

1 1

2 2

3 3

4 4

1

( )

,8.5733287401 9.573322345418.0590169730 25.63295614868.6347608925 21.09965308270.2677737343 3.9584969228

u

For u

u a u a u a u au bu b u b u b

W uu e

wherea ba ba ba b

+ + + + + + + + =

= == == == =

USEPA Wellhead Analytical Element Model (WhAEM) The steady-state analytic-element model, WhAEM, uses mathematical functions that represent hydrogeologic features. The analytic elements and their terminology included in the model are uniform flow, wells (point sinks), recharge boundaries (line sinks), no-flow boundaries (line doublets) and areal recharge (sink disc). The model allows placement of these flow features on a basemap. We must input aquifer properties

and information on hydraulic head, h, at select locations (at one point in aquifer and along recharge boundaries or line sinks).

The model calculates the discharge potential contributed by all the analytic elements at each location and are added together (superimposed). The discharge potential, , is directly related to the hydraulic head in the aquifer, h. The derivative of the discharge potential with respect to the two (x and y) coordinate directions yields the discharge vector, which is the total groundwater flow rate per unit width of the aquifer.

x

y

hQ per unit width Kbx xhQ per unit width Kby y

= =

= =

cKbh C = +

where, Cc = constant

Substituting Equation into the steady-state confined aquifer flow equation,

2 2

2 2 0h hx y

+ =

2 2 2 2

2 2 2 2

1 0c cC Cx Kb y Kb Kb x y

+ = + =

2 2

2 2 0x y

+ =

The governing differential equation in terms of is linear and the same for both confined and unconfined flow. The principle of superposition can be used under both flow conditions. A more detailed discussion of these functions can be found in Javendel et al. (1984) and Strack (1989).

SOLUTION METHODOLOGY/IMPLEMENTATION

Radial Drawdown of Groundwater Potentiometric Surface by a Pumping Well in a Confined Aquifer

At time, t, a cross-sectional view of the cone of depression can be graphed by solving the Theis equation for drawdown at varying radial distances. EXCEL can easily be used for the calculations of drawdown and graphing and visualization of results. More advanced software can be used for programming and graphing of the cone of depression. A Visual Basic program, ConeExtent, is included for quick solution and graphing of drawdown versus radial distance in a confined aquifer. Students can easily program the Theis solution with any programming language or software of choice. WhAEM WhAEM 2000 for Windows WhAEM2000 is a public domain, ground-water flow model and can be downloaded from the Environmental Protection Agency (EPA) website. Currently the URL for access to the code, users manual and basemaps is http://www.epa.gov/ceampubl/gwater/whaem/index.htm. WhAEM2000 is a Windows 95/98/NT program with the following minimum system requirements:

Intel 80486 DX/66 or higher microprocessor system Windows 95 or 98, or Windows NT 4.0 operating system 25 MB free disk space 16 MB memory VGA display or better Mouse or other pointing device Windows compatible printer

WhAEM can be quickly learned and used for simple well flow scenarios as well as more complex situations. Example problems in this module range from the modeling of a single pumping well superimposed on a uniform regional gradient to more complex modeling using basemaps and hydrogeologic boundaries. WhAEM contours hydraulic head values and will plot travel times for visualization of all scenarios.

ASSESSMENT OF THE MODEL

The implementation of the Theis solution within EXCEL allows for assessment of drawdown caused by a single well in simple scenarios. These simple scenarios enable us to examine effects of varying aquifer parameters on the cone of depression. The ramifications of the infinite horizontal extent assumption can be clearly seen in a simple

problem set. Drawdown along an axis normal to a single boundary can be easily simulated along with the effects of single hydrogeologic boundaries. However, superposition of multiple wells and boundaries would result in an increased complexity not easily handled by EXCEL. WhAEM allows for simulation of multiple wells and hydrogeologic boundaries. Simplifying assumptions include uniform aquifer properties (thickness, hydraulic conductivity), wells and boundaries fully penetrating through the thickness of the aquifer and steady-state conditions. These assumptions allow for minimal required input data and quick solutions and visualization of groundwater flow patterns. However, in situations of heterogeneous aquifers, wells screened at different depths and multiple aquifer, more complex numerical modeling (MODFLOW with MODPATH) would be required.

PROBLEMS AND PROJECTS Sets of problems are presented as separate documents and include Problem Statement, Solution, Conceptual Questions and Empirical Data (if needed). The problem sets and projects included in this module are: Travel Time of a Contaminant Under Uniform Flow Conditions: See file Uniformtravel Travel Time of a Contaminant Under Steady-State Radial Flow Conditions : See file Radialtravel Radial Drawdown of Groundwater Potentiometric Surface by a Pumping Well in a Confined Aquifer: See file Drawdown Infinite Horizontal Extent Aquifer Assumption: See file Infinite Assessment of Boundary Effects: See file Boundaries

EPA WhAEM Computer Program for Simulation of Capture Zones Single Well in Uniform Regional Flow Scenario: See file WhAEMnomap EPA WhAEM Computer Program for Simulation of Capture Zones Temecula Case Study: See file WhAEMmap

SUGGESTIONS TO INSTRUCTORS

This module contains problem sets for a range of abilities and class levels. The suggested order for modules of increasing complexity and time commitment are: Understanding of the effect of differing aquifer parameters on water level drawdown and the extent of the cone of depression can be gained without computer programming by use of the Visual Basic program ConeExtent.. Understanding of the effect of differing aquifer parameters on water level drawdown and the extent of the cone of depression can be gained with computer programming by use of the EXCEL problem sets. Simple investigation of the ramifications of the infinite aquifer extent assumption Simulation of single hydrogeologic boundaries with EXCEL Simulation of single well capture zone in uniform regional flow with WhAEM End of class project with report: Simulation of the Temecula, CA case. Students can either build the simulation themselves with information provided OR students can be given the database files for manipulation of aquifer and well properties to evaluate the case study.

GLOSSARY OF TERMS

Aquifer rock or sediment that is saturated and sufficiently permeable to transmit economic quantities of water to wells and springs. Cone of depression area around a pumping well where the hydraulic head in the aquifer has been lowered. Confined aquifer an aquifer that is overlain by a confining bed. The confining bed has a significantly lower hydraulic conductivity than the aquifer. Drawdown a lowering of the water table of an unconfined aquifer or the potentiometric surface of a confined aquifer caused by pumping ground water from wells. Effective porosity the volume of void spaces through which water or other fluids can travel in a rock or sediment divided by the total volume of the rock or sediment Equipotential line a line in two-dimensional groundwater flow field such that the total hydraulic head is the same for all points along the line. Homogeneous uniform properties everywhere Hydraulic conductivity a coefficient of proportionality describing the rate at which water can move through a permeable medium. The density and kinematic viscosity of the water must be considered in determining hydraulic conductivity. Hydraulic gradient the change in total head with a change in distance in the direction of flow. Image well an imaginary well that can be used to simulate the effect of a hydrologic barrier, such as a recharge boundary or a barrier boundary, on the hydraulics of a pumping or recharge well. Isotropy the condition in which properties are equal in all directions Potentiometric surface a surface that represents the level to which water will rise in tightly cased wells. The water table is a potentiometric surface for an unconfined aquifer. Radial flow the flow of water in an aquifer toward a vertically oriented well Recharge boundary an aquifer system boundary that adds water to the aquifer. Streams and lakes are typically recharge boundaries. Recharge well a well used to pump water into an aquifer

Storativity the volume of water an aquifer releases from or takes into storage per unit surface area of the aquifer per unit change in head Transmissivity the rate at which water is transmitted through a unit width of an aquifer under a unit hydraulic gradient Unconfined aquifer an aquifer in which there are no confining beds between the water table and the ground surface. Water table the surface of an unconfined aquifer

REFERENCES

Cited Bear, J. 1979. Hydraulics of Groundwater. McGraw-Hill, New York, 569 pp. Fetter, C.W. 2001. Applied Hydrogeology (4th edition). Prentice Hall, 598 pp. Harr, J. 1996. A Civil Action. Vintage Books, New York, 502 pp. Javandel, I, C. Doughty and C.F. Tsang 1984. Groundwater Transport: Handbook of

Mathematical Models. Water Resources Monograph 10, American Geophysical Union, Washington, D.C. 228 pp.

Strack, O.D.L. 1989. Groundwater Mechanics. Prentice Hall, Englewood Cliffs, New

Jersey, 732 pp. Theis, C.V. 1935. The relation between the lowering of the piezometric surface and the

rate of duration of discharge of a well using groundwater storage. Trans. Amer. Geophys. Union, Vol. 16, 519-524.

Todd, D.K. 1980. Groundwater Hydrology (2nd edition). John Wiley and Sons, New

York, 535 pp. USEPA 1994. Handbook: ground water and wellhead protection. Technical Report

EPA/625/R-94-001, U.S. Environmental Protection Agency, Office of Research and Development, Cincinnati, OH.

USEPA 1999. Protecting sources of drinking water. Office of Ground Water and

Drinking Water webpage http://www.epa.gov/ogwdw/protect.html last update. USEPA 2000. Working with WhAEM2000: source water assessment for a glacial

outwash wellfield, Vincennes, Indiana. Technical Report EPA/600/R-00-022, U.S. Environmental Protection Agency, Office of Research and Development, Washington, D.C.