Guide to Revegetation and Environmental Restoration of ...

Guide to Revegetation and Environmental Restoration of Closed LandfillsA Guide to the Revegetation and Environmental Restoration of Closed Landfills
October 1999
S T A T E O F C A L I F O R N I A
Gray Davis Governor
•
Linda Moulton-Patterson Board Member
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Copyright 1999 by the Integrated Waste Management Board. All rights reserved. This publication, or parts thereof, may not be reproduced in any form without permission.
This report was prepared by staff of the Integrated Waste Management Board to provide information or technical assistance. The statements and conclusions of this report are those of the Board staff and not
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mention of commercial products or processes shall not be construed as an endorsement of such products or processes.
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Preface This guide provides landfill managers, owners, operators, and local enforcement agencies with information on revegetation and environmental restoration in the closure of landfills. These techniques also should prove useful to conservationists in restoration or other habitat reclamation.
The guide is intended to serve as a bridging document between two State publications. These publications are Guide to Vegetative Covers for California Landfills, published by the California Integrated Waste Management Board (IWMB); and WUCOLS, Water Use Classification of Landscape Species, prepared by the California Department of Water Resources. These three documents should provide the project coordinator with the essentials for revegetation or environmental restoration. This guide also provides listings of other references and restoration resources in California.
The guide distinguishes between revegetation and environmental restoration as follows:
Revegetation involves the placement of plants, horticultural or native, on a project site. Relatively few, if any, other environmental restoration techniques will be applied. The plants can be an arbitrary choice of the project coordinator, with no regard for native species, their distribution or plant community design. A landfill configured to engineering specifications and planted with non-native grasses in regulatory compliance illustrates simple revegetation. Consideration for county approval of species should be made.
Environmental restoration will invariably involve revegetation. But, it also involves the extensive design and naturalization of project site contours, soil content and vegetative communities. The intent of environmental restoration is to create a seamless “repair” by emulating and supporting the native floral and faunal communities adjacent to and on the project site. The ultimate aim is for the project to be “assimilated” back into the surrounding environment.
Environmental restoration is characterized by these elements:
• A detailed reconstruction of the project site topography (elevations). • Site geomorphology (surface features). • Soil types conducive to the native plants of the project area. • Surface hydrology (water features). • Native plant species, their diversity, and distribution.
Acknowledgement I thank my colleagues who volunteered their time and effort to review this guide. I especially give thanks to those who encouraged me to press on, to make this resource available to them.—Jacques Graber, Author, Associate Engineering Geologist, Permitting and Enforcement Division, California Integrated Waste Management Board.
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Acknowledgement i
Chapter 2. Regulatory Background 3 Regulatory Requirements for Vegetative Final Cover 4
Elements of Restoration 7Chapter 3. Definition of Vegetative Cover Layer 7 Role or Purpose of the Vegetative Cover 7 The Degrees of Vegetative Restoration 7 The Goals of Vegetative Cover Programs 9
Chapter 4. Types of Vegetative Communities 13 The Vegetative Zones 13
Chapter 5. Precipitation and Moisture 17
Chapter 6. Aspects of California’s Vegetation 21 Plant Assemblage Profiles 21 Plant Communities 22 Compatible Plant Associations 26
Chapter 7. Landfill Vegetative Design 31 Landfill Design Considerations 31 Additional Uses for Vegetative Cover 38
Chapter 8. Considerations in Vegetation Selection 41 Site-Specific Considerations 41 Planting Considerations 43 Vegetation Types and Considerations in Program Planning 45
Chapter 9. Planting of Vegetation 51 Seeding 51 Planting Small Seedlings, Cuttings or Saplings. 52 Transplantation 52 Sources for Vegetation 54
Chapter 10. Six Concepts for a Successful Restoration 55
Chapter 11. Maintenance of Vegetation 58 Irrigation 58 Water Supplies 59 Fertilizing and Plant Nutrition 62 Maintaining Plant Health 62
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Chapter 12. Some Problematic Conditions 69 Irrigation Source Water Problems 69 False Readings in Soil Water Samples 71 Drainage and Surface Settling 71 Soil Methane Gas Concentrations 72 Additional Considerations 74
Chapter 13. Conclusion 79
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Chapter 1: Introduction The management and final closure of solid waste landfills in American society is a relatively new applied science. In 1795, Georgetown, Virginia, enacted the first ordinance for waste management in the nation. The ordinance prohibited the extended storage of refuse on private property or the dumping of it on a public thoroughfare. In 1873, Los Angeles (population 6,000) established a garbage and dead animal plot with burial of these wastes to be three feet below ground level.1
In the 1800s, waste disposal sites were selected based on convenience, especially in the major metropolitan areas. Sites were not selected to avoid negative environmental impacts. In San Francisco (population 149,000), two good examples of unsound disposal practices could be found. One site was at a once existing bay at the foot of present day Market Street, and a second site was located at the north area Marina district. Ships were scuttled in place and wastes brought in to create the newly reclaimed waterfronts. This process continued until the desired fill area was constructed. Because no containment barriers for the waste products were installed, debris freely scattered into the bay. Planned compaction of the wastes was not practiced at these sites. This activity led to calamitous differential settling and damage to or destruction of streets and building foundations during the Great Earthquake and Fire of 1906. Evidence of this disposal activity is discovered with each new construction excavation that occurs in the Financial District of San Francisco.
Smaller rural communities inland generally located their waste disposal sites with an “out-of-sight, out-of-mind” philosophy. Often, these disposal sites would be located where wastes literally could be shoved over the edge of a canyon or ravine, lost from view and future concern. Eventually, this strategy would be outgrown as communities grew larger and the over-the-side technique to dispose of wastes became less manageable. Burning of wastes, especially at area fills, took on a more important role as a way to "reduce" the volume of waste remaining at a community disposal site.
As more municipalities applied this practice with its cumulative air impacts, and other generators of air pollutants became more prevalent, a new strategy in waste management had to be devised. Prompted by the development and implementation of the Clean Air Act of 1977, and the creation of local Air Pollution Control Districts, the practice of open burn dumps was brought to a close.
The Integrated Waste Management Act (1989) brought waste management in California to a higher technical level. The end of open burning, the closure of these sites, and the opening of new landfills created new demands on management policy. Managed and planned closure procedures had to be developed to assure consistent closure of landfills to protect the public health, safety, and the environment.
By the 1970s, the general public attained a heightened awareness of the environment, which led to a more critical perspective on the closure of disposal sites and their final appearance. The casual viewer sought a more harmonious result, visually and ecologically, from the closed landfill.
1 Dr. Aston, R. Lee, “A Brief History of the Disposal of Wastes by Earth Burial,” AEG News, Winter 1998.
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As a result, the concepts of revegetation and, finally, environmental restoration, including “bio-engineering,” are becoming an accepted part of final closure. Even the use of vegetation as a moisture-regulating mechanism for the final cover is gaining some serious consideration.
Today's landfills are found in a spectrum of sizes (1 to 700 acres) as the old ones close and newer, larger ones open. They are often located in more environmentally critical areas, either in sensitive habitat or near urban residential housing. Each facility, when it closes, results in a long-term visual and environmental impact on the neighboring community or region.
By current regulation, a newly closed landfill is monitored for various conditions (leachates, landfill gas, slope stability, etc.) for a period of 30 years, possibly longer, following its final date of closure per Title 27, California Code of Regulations (27, CCR), Section (§) 21180. These sites will remain as permanent monuments to our waste management practices unless restoration is achieved.
Environmental restoration is used in the mitigation and restoration of lands damaged by open pit or strip mining operations and other development projects involving sensitive lands. These techniques are coming into their own in landfill closure practices. Research into revegetation with native plants, and the concepts and practices of environmental restoration, as practiced in these other venues, are becoming important in the closure of landfills.
When a landfill closes, the primary intent of its design is to contain the waste and control the by-products resulting from its containment. These can include landfill gas, leachates, and the wastes themselves. The design insures the integrity of the external cover from settling, wind and water erosion, slope failure, and seismic damage. The design protects the public from exposure to the confined wastes.
If a planned postclosure land use is implemented, the landfill site can be designed to accept the appropriate land use. If no planned postclosure land use is intended (non- irrigated open space) or the postclosure project entails a parkland, preserve, or golf course, the final role of the cover layers is to provide a veneer to prevent erosion, support a viable plant community, and the chosen postclosure use facility.
Landfills located in arid and desert regions would impose different demands on the cover. These landfill covers are expected to support more sparse vegetative communities or, as an alternative, to be covered with rock cladding. Cladding helps protect the landfill from slope failures, or erosion, and in turn, can provide a limited aesthetic visual buffer. Even desert environmental restoration practices are being utilized with encouraging results.
With special soil surface treatment, using imprinters and appropriate plant types, a revegetation program in an arid or desert environment can provide a secondary use for the public in the surrounding region. Such a program could provide a desert wildlife area, and educational park for local schools and other visitors.
This guide is intended to provide practical information and methods in the concepts of revegetation and environmental restoration as applied to solid waste landfills.
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Chapter 2: Regulatory Background To assure some degree of consistency in the development of final vegetative cover in landfill closure design, regulatory standards were developed by both federal and State agencies. These standards primarily apply to the thickness of the vegetative cover soil layer and its performance, and the vegetation planting and maintenance protocols. These regulations are concerned with the use of the vegetative layer as a protective element in the long-term integrity of the landfill cover rather than as part of a holistic, integrated, visual, and environmental reconstruction.
The use of vegetation as a soil and slope-stabilizing component for final cover can be a reasonably economical and durable slope protection method. Current research reveals that vegetation can serve as an effective soil layer binder and moisture transpiration control system. Vegetation can extract excess moisture from the cover layer, reducing the potential for saturation and possible slope failure, especially at the soils interface between the moisture barrier and the erosion or vegetative layer. Employing a planned plant community and a successional plant population introduction technique may ensure successful establishment of the higher plant types, creating a naturalized vegetation community.
Developing a more complex landscaped, or ecosystem-based, plant community that is integrated into the surrounding natural vegetation ecosystem will require more advanced planning, research, and effort on the part of the operator. Ultimately, the result can be economically advantageous and aesthetically rewarding through reduced maintenance costs, improved plant survival, and possible wildlife habitat enhancement.
There is no current regulatory requirement that states that native plants must be used in final vegetative cover, or that the landfill slope profiling and vegetative cover must reflect the natural conditions in which the landfill is located. But, through practical application of more natural slope design and vegetative cover in mine reclamation projects, the natural and native configurations of plant communities can be more economical in the long run. Soil conditions and moisture may not support the non-native plants that are introduced. Natural pests attack and destroy non-native plants lacking natural defenses against these pests, or the costs and efforts of maintaining the non-native vegetation, through irrigation and pest control, are greater than they would be in using the native counterparts.
This practice should be applicable to landfills. In addition, the costs of configuring side- slopes and decks to more natural profiles should not introduce significant costs, if these details are designed in the early development of the landfill closure.
As the public's environmental awareness matures, and urbanization expands around existing closed landfills, or existing urban landfills close, placing greater demands on final closure appearances, the role of environmental restoration as an integrated part of final cover and vegetation design can assume greater significance. Additionally, new and larger landfills are being proposed in remote regions of greater environmental sensitivity. These restored areas could recover lost habitat or, increase available rare or endangered species habitat. This effort would not only improve the chances of survival of native or endangered species, but it could also enhance the public image of the agencies or operators that adopt this type of restoration program at these landfills. A
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project at Coyote Canyon, Orange County, is applying such a program for the California Gnatcatcher, (Polioptilia californica).
Regulatory Requirements for Vegetative Final Cover The primary regulatory sources for State and federal standards for closures are Title 27, California Code of Regulations (27 CCR), and 40 Code of Federal Regulations (40 CFR), Part 258 (Subtitle D).
State
Title 27, CCR Requirements (formerly 14, CCR and 23, CCR) Subchapter 5, Article 1, Section (§) 20950(e). For landfills and for waste piles and surface impoundments that are closed as landfills, all vegetation for the closed unit’s vegetative cover shall meet the requirements of Section 21090(a)(3)(A)1, in cases where the unit does not utilize the mechanically resistant erosion layer per § 21090(a)(3)(A)2.
Section 21090 (a)(3)(A)1. Closed landfills shall be provided with an uppermost cover layer consisting of either:
1. Erosion resistance via a vegetative layer. This layer consists of not less than one foot of soil which:
a. Contains no waste (including leachate). b. Is placed on top of all portions of the low hydraulic conductivity
layer described in § (a)(2). c. Is capable of sustaining native or other suitable plant growth. d. Is initially planted and is later replanted as needed to provide
effective erosion resistance with native or other suitable vegetation having a rooting depth not exceeding the depth to the top of the low hydraulic conductivity layer described in § (a)(2). For any proposed vegetative cover, the discharger shall propose a species mix which harmonizes with the proposed postclosure land use and which requires little long term maintenance as feasible by virtue of its tolerance to the vegetative layer’s soil conditions.
2. Mechanically erosion-resistant layer. An erosion and ultra violet light-resistant layer which, by virtue of its composition and finished-and-maintained grade, resists foreseeable erosion effects by wind-scour, raindrop impact, and runoff (e.g., a one-foot-thick layer of cobbles, the interstices of which are filled with gravel).
California Coastal Commission Should a closure project be located within the jurisdiction of the California Coastal Commission, the regulatory standards and requirements of that agency may have to be addressed (PRC 13053.5(a)).
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Department of Fish and Game Should a closure project be located within the jurisdiction of the California Department of Fish and Game, the regulatory standards and requirements of that agency may have to be addressed (PRC 13053.5(a)).
California Environmental Quality Act (CEQA) All projects in California must be reviewed in accordance with the California Environmental Quality Act (CEQA) to determine whether the project may have a significant impact on the environment. If the project might have a potential significant impact, mitigation measures may have to be incorporated into the project to avoid the impact.
Federal
Final Cover Design––40 CFR § 258.60, Subpart F 6.2.1 (a)(3). Minimize erosion of the final cover by the use of an erosion layer that contains a minimum 6 inches (60 cm) of earthen material capable of sustaining native plant growth.
6.2.3. Design criteria for a final cover system should be selected to ... improve aesthetics.
There are alternatives to the Subtitle D prescriptive standard cover designs which regulatory agencies can consider and approve (40 CFR §258.60(b)). The alternatives include:
1. An infiltration layer that achieves an equivalent reduction in the infiltration as specified in paragraph (a)(2) above.
2. An erosion layer that provides equivalent protection from the wind and water erosion as specified in paragraph (a)(3) above.
These alternatives provide an additional design choice that can broaden the vegetative design options available to an operator closing a landfill.
U.S. Army Corps of Engineers Because many existing landfills and sites for potential landfills are located near natural waterways or may be sites upon which are located sensitive wetlands or vernal pools, the Army Corps of Engineers may have jurisdictional involvement under section 404 of the Clean Water Act. This jurisdictional authority may require a closure project obtain a permit from the Corps, prior to initiating the project. Dispute over Corps permit jurisdiction is requiring more scrutiny of project content and project location.
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Chapter 3: Elements of Restoration Definition of Vegetative Cover Layer
Although there is no specific definition identified in the CCR, for the purposes of State regulation, the vegetative cover layer can be defined on the basis of compliance with all of the requirements of 27, CCR.
Role or Purpose of the Vegetative Cover When a landfill is closed, the final design of the structure must incorporate various elements to serve several functions. The cover layers form the containment and moisture barriers directly overlying the waste mass, providing the containment and barrier functions above the waste. This protects the contents from invasive moisture and protects the public from exposure. The final layer covering all these preceding elements is the erosion, or vegetative soil layer. This layer, with vegetation, helps to prevent erosion, supports the vegetation, and provides some additional moisture protection.
The operations and containment layers below the waste and the final cover foundation layer and moisture barrier layer above the waste are intended to serve as barriers to moisture and gas migration into or out of the landfill. The final vegetative layer’s intended purpose, in addition to preventing erosion and enhancing moisture protection, is to serve as a stable substrate for a surface-stabilizing plant community on the final cover. The minimum standard vegetative soil layer thickness in California’s Title 27 requirements is 12-inch minimum thickness. This layer can be thicker but it may not be any thinner than the minimum. This minimum standard supersedes the 6-inch federal standard for Subtitle D for landfills in California.
The vegetation that is planted on the final cover is intended to serve as a protective soil binding and stabilizing element. The vegetation can also serve as an attenuator; the canopy absorbing damaging rainfall velocity before it strikes the soil.
This function of the vegetation aids in reduced impact erosion on the soil layer and improved moisture capture. Vegetation also serves as a moisture control through evapotranspiration by removing excess moisture from the soil, an aesthetic mitigation and an ecological mitigation by providing a reconstructed vegetative habitat for local animal species as well as rare or endangered plant or animal species.
The Degrees of Vegetative Restoration When a landfill is finally closed and the operator is preparing the final vegetation layer and the vegetative cover, there are three options to consider: restoration, aesthetics, and function.
Environmental Restoration Environmental restoration is recreating, as completely as is practicable, that portion of the ecosystem that was displaced or disturbed by the project. Restoration takes into consideration the reconstruction or close approximation of the soil types and profile or topography of the area that was
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modified. The reconstructed slopes and terrain will mimic, as closely as possible, the natural features of the surrounding land. If the landfill were placed in a canyon, the slopes would be designed to mimic a shallower canyon or broad slope; or a ridge, if fill material overfilled the original canyon terrain. Surface landfills in flatter terrain would be profiled to emulate hill slopes, if hills are nearby, or to emulate a hill though none are in the area. Vegetation in such a restoration project would ideally reflect the proportions of plant species distribution reflected in the surrounding plant communities, utilizing the same species of native plants in the revegetation phase.
This type of restoration would serve three important functions. It would “repair” the ecosystem by replacing the project with the original environmental composition displaced while the project was operating. It would provide a new natural environment to enhance the local biotic community, improving species diversity and expanding available habitat. Restoration could also provide mitigative capacity in certain circumstances for mitigation of endangered species by allowing custom fitting of special localized habitats for endangered species in an area into the surrounding natural community. By using native plant species, the restoration project serves in contributing to the local species gene pool by providing more indigenous individuals to reproduce with the established local resident species.
Aesthetic Mitigation—Providing Compatible Postclosure Use Options If environmental restoration is not a viable option for the operator, or a proposed postclosure land use development is intended for the former project site, an aesthetically satisfying vegetation program can be implemented on the site that approximates the local vegetation community. Such a vegetation option would be available for recreational parks or golf courses, or business park campuses. In this application, horticultural or nursery plant types and aesthetically designed landscaping are planned, not necessarily to emulate the natural vegetation and local terrain. Native plants could also be utilized but with the landscaped accent required for the project plan. Generally, this type of project will serve two purposes:
• Space Use The postclosure project will provide a viable natural environment for the public’s enjoyment. It can provide an aesthetically pleasing landscape that will mediate visual impacts created by the closed landfill. The project can still satisfy native plant needs while exhibiting landscaped features.
• Mitigative Needs The project will provide an acceptable alternative that will satisfy the regulatory standards of 27, CCR and Subtitle D. It will also offset the past impacts of the previous landfill activities.
Regulatory Compliance—Satisfying Regulatory Standards A vegetative program that is designed to satisfy the requirements of Title 27, CCR and Subtitle D will employ the simplest and most basic elements of final cover design, landfill slope profiling, and vegetation types. Slope profiles will assume the most basic engineered forms in compliance with the closure requirements. Overall cover structure surfaces will generally be planar and
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obviously man-made. The primary functions of the vegetative cover will be to provide slope stability and soil binding, provide moisture control, control surface runoff flows, enhance evapotranspiration, reduce moisture intrusion and leachate production, and reduce landfill gas production. Vegetation will assume the more direct functional roles while providing the basic coverage to satisfy the requirements of Title 27.
In this application, landfill control systems will be least visually hidden. Gas control systems, vents, well heads, collection pipes, surface moisture control systems, and maintenance/access roads will be most visible. A general grass vegetative cover will be in place. Still, native grasses can be employed in this situation.
The Goals of Vegetative Cover Programs The goals of vegetative cover programs may be based upon or dictated by the financial resources and priorities established by each operator, while complying with the regulatory requirements of CCR Title 27 and 40 CFR, Subtitle D.
Restorative
The technically most complex project is the restorative vegetation plan. To properly implement restoration, the operator must construct (reconstruct) a final cover (erosion or vegetative layer) that provides soil conditions and topographic features closely duplicating the surrounding soil types and geography. These preparations are intended to increase the chances that the replacement native plant community that is reintroduced will survive. A restored vegetative plant assemblage must duplicate the native plant profile in terms of ratios of species occurrence (distribution), correct native species selected and distribution of these species across the project site to closely duplicate the plant distributions in the surrounding undamaged areas. Ideally, this restoration will create conditions that will provide a natural habitat to encourage re-population by native animal species. In theory when this project has matured, it should provide a seamless restoration with the surrounding land or create a natural native environment in mixed urban or suburban areas.
An alternative project may involve creation of special habitat for rare or endangered species that both mitigates the project and provides new habitat. Conditions may warrant preparation of the site with special vegetation types that are present in that area that are attractants of local rare or endangered species, especially insects, such as certain species of butterfly or beetle, small reptiles, or mammals.
Characteristics of a Restored Site Restored sites use native vegetation indigenous to the immediate area, or, for rare and endangered species mitigation, rare plant species that would be found within the ecological region. They provide vegetation or unique habitat that is depended upon by specific species of animals or insects for food or reproductive needs or as a mitigative effort to increase populations of a rare or endangered plant.
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• They provide for a natural plant community profile, with representative species distributions and correct profile of understory plants, intermediate shrubs and overstory trees.
• The reconstructed land surface closely mimics the surrounding natural land features. This is accomplished by using HDPE geogrid reinforcement, landform contour grading and importing large rocks or cobbles and placing them on-site, These practices can be effected if the surrounding terrain demonstrates these features and if they can be engineered into the final cover design without compromising final cover functions.
• Restored sites use bioengineering techniques for erosion repair and slope stabilization efforts including straw logs, wattles, revetments, and other surface stabilizing structures that can employ living plant materials in the structures, aiding in slope profiling and stabilizing.
• The primary projects employing habitat restoration or mitigation are wildlife preserves, natural parklands, wildlife management areas, rare or endangered species mitigation, or natural public or educational parklands.
• Restored sites do not have planned postclosure land uses beyond the role of parkland or preserve.
In terms of the restorative role of a site, a vegetation plan designed around a recreational use would rank as a close second for environmental value. A choice of either native plants or compatible nursery varieties would still provide a significant environment with both ecological as well as aesthetic merit.
A proposed postclosure land use following an initial vegetation phase would influence plant selections more toward a vegetation selection that would be less expensive to plant and remove. This cover type would be less environmentally mitigative than the first two options of natural parklands or native-or-non-native landscaped recreation area.
A site that is strictly designed to comply with the regulatory requirements of 27, CCR and Subtitle D regulations would employ the simplest vegetation plan and would be the least costly to install and maintain, while facilitating a visually pleasing cover. A grassland type cover could still provide a satisfactory mitigative result; if the site uses California native grasses and is located within grassland or mixed open lands and forests (glade) or savannah.
Vegetative restoration should be compatibly designed to fit in with the surroundings. No radical selection of plants should be made that will make the site stand out. It would not be prudent to place a grove of tall Eucalyptus (non-natives) on an above ground landfill in an open grassland environment. It is unnatural, a non-native, and the unprotected stand of trees could be rendered vulnerable to blow-down from strong winds over time, damaging the final cover and creating added repair and cleanup costs. Again,
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considerations must be exercised to fit the planting appearance and the plant selections in with the surrounding environment.
Aesthetic Restorations
These mitigative projects provide a vegetative cover that supports a plant community similar to surrounding native plant communities but which derives its plant makeup more from nursery plant species. The plant profile could employ trees, shrubs, and grasses assuming similar ecological roles as their native counterparts. The final result could range from natural appearing, to landscaped, both cases presenting a visually satisfying product. A compromise form would employ native plants, but with the landscaped appearance.
Characteristics of an Aesthetic Restoration
• The use of non-native plants compatible with the environmental conditions where they will be planted and/or use of native plants when desired. This cover could assume natural plant profiles (grasses, shrubs, and overstory trees) when appropriate.
• Application of landscape architectural techniques to create natural- looking or purposely designed landscapes.
• Aesthetically pleasing landscapes that serve man’s needs or requirements such as parks, golf courses, playing fields (baseball, soccer) or recreation areas, and/or minimized visual impacts to the surrounding community.
• Little potential for planned postclosure land use beyond the initial planned use (although a secondary or tertiary postclosure land use may not be ruled out).
Functional Sites
These landfill covers will have their primary function in ensuring their compliance with the regulatory requirements of 27, CCR and Subtitle D. This type of cover is the most commonly employed, using a standard hydroseed mix of annual and/or perennial grasses. Some smaller herbaceous plants such as legumes (vetch or lupine) may also be used. Natural invasion and succession by nearby plant species may play a role in the later years of postclosure maintenance. Aesthetic or environmental mitigations would be of a secondary importance in their design function.
Characteristics of a Functional Site
• The use of climatically compatible native or non-native plants in the vegetative cover. No significant effort is expected in plant community profiling. Possible use of grasses and planted or volunteer plants such as small shrubs and, eventually trees, if the cover can accommodate them.
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• Developing primarily engineered slopes and land features without attempts at duplicating or mimicking surrounding land profiles or aesthetic landscaping.
• Function takes precedence over form. The function of the final cover design is to be in regulatory compliance. There would be minimal land forming beyond required, engineered standards. Functional requirements would include:
1. Slope stabilization.
2. Moisture control. a. Water penetration into cover. b. Down-slope water flow control, drainage systems, etc. c. Leachate control.
3. Reduced maintenance demand. a. Low irrigation requirements. b. High reseeding characteristics or return seed replenishment. c. Minimal maintenance or cleanup requirements. d. High potential for postclosure use; the landscape materials are
“disposable” and can be easily removed should a future postclosure use such as office buildings or warehouses be placed on the site.
e. Minimum vegetation diversity. Grasses and possibly larger herbaceous plants such as legumes.
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Chapter 4: Types of Vegetative Communities A “community” is defined as “an aggregation of living organisms having mutual relationships among themselves and to their environment.”2 A plant community includes each element of the vegetation characterized by a dominant species. For restoration of a plant community to be successfully achieved for any project, an understanding of the basics of plant communities must be explored. For a project proponent to install a vegetation community that will have the highest chance of succeeding, the planner must be aware of the types of plant communities that exist throughout California and the one at his project site. The operator must consider climate conditions, soil types, and compositions in the project area and demonstrate an awareness of the surface topography of the area surrounding the project site where restoration will occur.
Even in using nursery stock instead of California native plant stocks in a revegetation project, soil types, climate, and equivalency in plant types are important to successful survival of the final planting.
Throughout the State of California, plant communities have developed and evolved into distinct assemblages of plants and distribution patterns. Coastal plant communities differ greatly from desert plant communities. Alpine conifer forests will differ from valley chaparral. Species of plants will differ from one northern oak woodland community in northern California versus a southern oak woodland community in southern California, although they may look superficially alike. Even the western coastal conifer makeup is different from the conifer forests in the western Sierras. An awareness of these subtle differences may help make the difference in a restoration or revegetation project being a success or a potential failure.
The Vegetative Zones California’s vegetative communities fall within four major vegetative zones.3 (Micro- environments are found within each major zone, containing their own distinctive plant communities.) Following are the four major zones.
Coastal Zone
This vegetation zone embraces the majority of northern California from Modoc County to the northeast, across the northern counties to include the mountainous areas of Siskiyou, Shasta and Trinity counties. This zone includes the coastal counties from Del Norte south to San Diego County, bounded by the coastal ranges on its eastern margin.
Plants in this zone are varieties that are highly moisture dependent, preferring a more temperate average climate ranging from the 50s (OF) to the 90s rarely. Rainfall is abundant to moderately available while frequent occurrences of fog from the marine air layer of the Pacific Ocean increase atmospheric moisture levels. The conifers dominate the northern forests, including the Redwoods
2 Munz, Phillip A. and David D. Keck, A California Flora and Supplement, University of California Press, 1959, p. 11. 3 Duncan, Craig Thomas, Guide to Vegetative Covers for California Landfills, Bryan A. Stirrat and Associates, 1994.
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(Sequoia sempervirons) and Douglas Fir (Pseudotsuga menziesii). These species dominate as the overstory species. Hardwoods (Western Hemlock, oak, and others) can be commingled in some areas, occupying the intermediate layer of vegetation.
Smaller shrubs fall in the understory at the closest to ground level. More southerly forests will be populated with coastal species of hardwood (deciduous) forests. A pocket of alpine desert plant communities can be found in northern and eastern Siskiyou County and Modoc County.
Interior Zone
The Interior Zone includes the entire Central Valley and a narrow band that follows along the eastern slopes of the Coast Ranges, including the west halves of Los Angeles, San Bernardino, Riverside and San Diego counties. Climatic conditions in this zone are drier than the Coastal Zone, being partially influenced by the initial rain shadow effect of the Coast Ranges. Temperatures can vary from the low 30s (OF) into the 100-plus degree range. Atmospheric moisture is generally dry during the summer and fall months. A short period of heavy rains occurs between September and March.
This region is dominated by grasslands and Oak Chaparral communities, often with higher concentrations of vegetation along river and creek channels (riparian environments). Many of these riparian environments are dominated by cottonwoods (Populus trichocarpa or tremuloides) as well as willow (Salix subspecies) in the overstory layer. Digger Pine (Pinus sabiniana) can be found in the drier hilly areas of this zone. Embracing the San Gabriel and San Bernardino Mountains, the Interior Zone holds distinctive mountain plant communities in these ranges.
Mountain Zone
This zone includes the region running along the western slopes of the Sierra Nevada, from southern Modoc County, across all western Sierra counties southerly to Tehama County and including north Kern County. Dry temperate to warm summers and cold, snowbound winters at the higher elevations generally dominate climatic conditions in the Mountain Zone.
The western Sierra receives high volumes of rain, and thunderstorms are frequent. Much of the eastward migration of storm moisture conveyed to this point is precipitated out before crossing to the east desert regions. Vegetation in this area is dominated again by conifers such as Ponderosa Pine (Pinus ponderosa) and Fir (Abies grandis or A. concolor) in the overstory. These species of conifers are more tolerant of dryer, hotter climates than the conifers of the coastal varieties. Oak savannah or chaparral may dominate the southern portion of this zone. Arid conditions may dominate in the extreme southern zone. Hardwoods such as Valley Oaks (Quercus lobata) are found more at the lower elevations and as intermediate species at middle montane elevations.
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Desert Zone
The Desert Zone takes in northern Modoc County, the areas north of Lake Tahoe, and all of the easternmost counties to the southern California border. This zone includes the area bordered by the east slopes of the Sierra Nevada. The climate in this zone ranges from cool winter days to extremely hot summer daytime temperatures. The climate is generally arid with relatively less snowfall than the Mountain Region. Soil conditions are dry, sandy or stony, often forming a “desert pavement,” creating harsh conditions for natural plant growth. Precipitation is limited because of the rain shadow of the Sierra. What rain there is may come primarily as cloudbursts creating brief flash flood events. Vegetation in this region consists of xerophytes—plants highly tolerant of harsh desert conditions. Junipers (Juniperis) including J. californica and J. communis in the north, and J. ostosperma in the Mojave region, succulents, creosotes and other shrubs, and assorted species of yuccas or other desert vegetation will dominate this plant community.
The desert environment is particularly sensitive to impacts from man. Desert regions, both low-altitude and alpine, possess very subtle features difficult to replicate. These regions take a long time to “heal“ after excavations have been performed, and the slow rates of growth and relative sparseness of native plant species in the desert region will reveal scarring longer than other impacted areas. Barren rocky regions may make restoration nearly impossible to achieve as desert pavement and the phenomenon of “desert varnish,” a dark glaze over the rocky surfaces, are difficult to reconstruct, requiring natural weathering to complete the process. Alpine desert areas as in central Siskiyou county display subtle signs of frost polygon-like forms in the soil, presenting cell-like arrangements of surface stones, surrounding low hummocks over large tracts of open grassland.
“Desert” refers to a natural environmental community created in evolutionary response to hot arid climates. Desertification is an environmental condition, usually resulting from the adverse activities of man. These activities, such as mismanagement of irrigation water, salt leaching, and concentration of other minerals in the soil and wind erosion of soils from tilling operations result from agricultural activities in fertile or marginally fertile lands of the desert regions. In the arid soils, chemicals accumulate and the soil surface forms a thin crust, relatively impermeable to the sparse available rains of those regions. The result is conditions that are hostile to plants and animals and loss of natural soil nutrients that inhibits revegetation efforts.
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Vegetative Zones
Page 16 Page 16
Chapter 5: Precipitation and Moisture The types of plants to be selected and the irrigation plan intended for a specific landfill site will depend upon the average natural precipitation in a particular area. The project planner must take this variation into consideration. The precipitation pattern of California is atypical of most precipitation and climate distributions worldwide.
Most climate patterns follow defined responses to geographic features, resulting in gradated changes across a climate regime. This usually results in wetter coastal regions gradating to drier or arid environments inland. In California, the association between weather and precipitation, the widely varied terrain and regional temperatures creates a far more intricate melange of environmental zones. Precipitation may vary widely in two different areas even though they may both be within the same vegetation zone and geographic regime.
California’s terrain is divided lengthwise by two major mountain chains running the length of the state. Two regional mountain systems are located in the north central state and a long, transverse range in southern California. The Central Valley occupies the mid-portion of the state, while low desert and high desert plateaus and mountain complexes occupy the northeast and easternmost margins of California. The state’s length results in a broad temperature range from the north latitudes to the south. These wide-ranging environmental influences result in wide variation in temperatures and precipitation, all within small distances.
As an example, in the coastal vegetation zone, precipitation varies from 10 inches average annually, southeast of Monterey, to 100 inches or more north of Eureka; a 90- inch difference. Temperatures can be very cold on the north coast, yet warm in Monterey. Precipitation in the Central Zone varies between 10 and 70 inches. The eastern flank of the Sierra and the south desert region (Desert Zone) range from 2 to 20 inches average annual precipitation.
A balance must be developed between the natural precipitation and average temperatures, and the planned irrigation volumes for landfill revegetation at a specific project site. (See Figure 2).
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\ I \~I ', __ /
of Active Landfills
Figure 3a
Distribution of California’s Landfills Californians live throughout the state, in the most remote areas of the mountains, to the farthest reaches of the desert. This distribution places these sources of waste within virtually every climatic, temperature and precipitation zone in the state. Figure 3a shows the locations of the State’s 186 active landfills. This wide dispersal of landfills demonstrates the diversity of waste management requirements and postclosure maintenance and land use demands placed upon operators and postclosure re-vegetation programs, both active and proposed.
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Figure 3b shows 262 landfills, active and inactive, within California. The majority of these landfills have not employed an environmental restoration program. Most employ programs compliant with regulations, employing the basic techniques of vegetative cover and standard engineering practice. Many employ aesthetic programs, incorporating golf courses or other recreational facilities in the postclosure use plan.
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Chapter 6: Aspects of California’s Vegetation Plant Assemblage Profiles
Just as plants develop associations based on climate, moisture, and soil type, the plant community can establish itself into a simple to complex interrelationship as a layered or stratified structure. As the natural succession of a plant community develops through time, the larger vegetation supersedes the previous pioneer plants. Pioneer weeds and grasses begin the succession, preparing the soil for the succeeding plants. The pioneer weeds and grasses are eventually shaded out by larger shrubs. These shrubs are displaced or dominated by the larger trees. This system of layering provides environmental levels for wildlife and plants alike (Figure 4).
The main plant layers include the understory, intermediate, and overstory layers.
Understory
This includes the smallest vegetation such as mosses, ferns, grasses, small wildflowers, and low ground covering varieties of herbaceous or woody plants.
Intermediate
This layer will include smaller and larger shrubs and smaller species of trees or young saplings of larger overstory tree species. These plants may be adapted to softer light and cooler temperatures created by the shading effect of the overstory canopy. Woody perennial plants dominate the intermediate story.
Overstory
This vegetative layer consists of the larger species of trees in the natural assemblage. This layer can create a canopy that influences the overall light availability and average temperatures at the lower levels. These trees can be sparsely distributed or closely growing together to create a tight canopy. Destruction of the canopy trees can adversely affect the understory environment, or provide a point of opportunity for saplings to fill in. Not providing these trees in a poorly planned restoration project may jeopardize the success of understory plant species growth and the project.
A landfill revegetation or restoration project would shorten some of this successional process, compressing the sequence into roughly one step, with grasses, shrubs, and trees planted at the same time. Invasive plants, including pest weeds, would impose on this plan if a maintenance program to remove these invaders were not exercised.
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Figure 4 Jacques Graber 1999
Plant Communities Within each of the four vegetation zones, plants have established themselves into assemblages or communities. Each community, when viewed as a whole, is an integrated system adapted to that particular environment. Similar plant communities may be found in the Coastal and the Mountain Zones, the Interior Zone as well as the other zones. Though superficially resembling each other in function, two similar looking plant communities will have entirely different species assuming similar ecological functions. Species aside, these vegetative communities follow several basic patterns, such as grasslands, wetlands, woodlands or forests, etc. Within these major patterns, though, is a whole spectrum of variation. Some of the major plant assemblages most found in California include4:
Valley Grassland and Savannah
This vegetation community consists primarily of annual or perennial grasses with, now, predominantly introduced annual grass species from Europe (Festuca and others), and annual or perennial wildflowers. Grassland or prairie generally lacks
4 Munz and Keck, p. 11-18.
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major trees and shrubs. Though species such as oaks may be dispersed throughout this community, they would generally not constitute a “forest.”
Major streams or river channels that traverse grasslands or prairies may support dense stands of hardwood trees and shrubs as Riparian lands, being restricted to available water from the stream. Grasslands and the remnants of a once expansive riparian environment that existed along the major rivers and streams dominate the Central Valley (Figure. 5a or b).
Coastal Prairie
Similar to prairie. Open temperate hill grasslands or glades, or bald hills on the west slopes of the outer and middle coast ranges in Mendocino and Trinity counties, north, and scattered to counties southward to San Francisco County. In the past, native bunch grasses and flowering herbs dominated coastal prairies. Because of overgrazing, these native bunch grasses have been displaced by annual grasses and by intrusion by non-native grasses.
Chaparral
This vegetation community consists frequently of an understory grass soil cover with wildflowers within which are distributed, in varying density, different species of shrubs and oaks or juniper as intermediate and overstory plants. Chaparral is predominantly a dry climate plant community. Many of the trees naturally found in this environment possess thick corky bark (cork cambium) and are adapted to the wildfires that raged through prior to human involvement. These trees may include Interior Live Oak, Blue and White Oak, Manzanita and chamise. This plant assemblage occupies areas along Central Valley and foothills regions of California from Redding, along the western Sierras. Chaparral can be found in the southern California counties and along the eastern flanks of the Coast Ranges from Redding to the San Bernardino Mountains, as well as distributions south to San Diego County (Figure 5b).
Forest
These vegetation communities consist of a complex understory and intermediate plant relationship, with high diversity in these layers in natural mature forests. Woodlands’ overstory trees are represented by two dominant tree types, conifers and deciduous, with a mixture of the two as environmental conditions may dictate. The dominant overstory trees will be made up of either species of conifers in the higher altitudes or deciduous trees at lower elevations. (Figure 5c).
Forest communities create the most impressive and oldest plant communities in their natural mature state as exemplified by the redwood and old growth forests of the northwest state, the Big Trees National Forest in the central Sierra Nevada and the Bristlecone Pines in the high Sierras. Conifers or deciduous (broadleaf) trees can wholly dominate the overstory canopy to the exclusion of the other, or they can share this niche in varying proportions depending upon climate, elevation and soil conditions. Forest or Woodlands command the Coastal and Mountain Zones as well as the riparian environments in the Central Valley.
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Several divisions of the forest community are listed here. Forest communities are characterized by the dominant conifer found in each of them:
Closed Cone Pine Forest. This community is found at intermittent locations along the California coast from Mendocino to Santa Barbara counties.
Redwood Forest. Located along the west slopes of the Coast Range from Del Norte to Santa Cruz counties. Some small areas are found in Monterey County.
Douglas Fir Forest. This community is found in the north Coast Ranges from Mendocino County southward, with scattered remnants to Sonoma and Marin Counties, easterly of the redwood forest regions.
Yellow Pine Forest. Found in the North Coast regions, to Southern California.
Red Fir Forest. Found in the North Coast ranges to Southern California.
Lodgepole Forest. Found in northernmost California to the central Sierra.
Northern Juniper Woodland. A variant of woodland community in which the dominant tree species is Juniper (Juniperus occidentalis). This community can be found in central Siskiyou County, easterly to Modoc County and south to Mono County.
Desert or Arid
This plant assemblage will be found in those areas of California where moisture and temperature are at their extremes. Soil conditions are harsh and difficult for plants to grow in. The dominant plants, known as Xerophytes are deep-rooted, and slow-growing. Many plants in the desert community are defensive; producing terpines that will keep invasive plants away from their roots; blocking competition for valuable water. Leaves are thick and physical defenses such as spines, thorns, and foul-tasting resins keep herbivorous animals from destroying their slow-growing foliage. Knowledge of these traits can help in planning desert restoration projects, so as not to put these highly competitive plant species too close to each other. Alkali sink vegetation is found in poorly drained alkali flats and playas on the floor of the Central Valley and arid regions on the east slopes of the Sierra Nevada. (Figure 6a).
Wetland or Estuarine, Riparian, and Vernal Pools
These three aquatic plant communities constitute the vegetation assemblages found along natural water bodies throughout California. Some are extremely small and fragile, such as vernal pools, their combined statewide total areas barely covering several acres.
Plants in these communities are highly moisture-dependent, yet they can be adapted to intermittent dry cycles, going into dormancy. Many of these species are microscopic and may be sensitive and vulnerable to minor changes in their conditions. They can also serve as effective bioremediation mechanisms, filtering out certain effluent components in sewage treatment projects (Figure 6b, c).
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Wetland or Estuarine. These plant communities include Coastal Salt Marsh and Freshwater Marsh. They can be found generally as bordering lands along bays (large or small) salt, and fresh water bodies, lakes and rivers. The largest single wetland environment in California can be found at the north shore of Suisun Bay northeast of San Francisco. This plant community can consist of vast expanses of rushes and grasses adapted to live in water- saturated soils and conditions of brackish to fresh water. Floating plants, including an introduced species of water hyacinth, can be found in areas of the Sacramento and San Joaquin River Delta. Sloughs, lagoons, or river channels are generally associated with the wetland or estuarine community. Many estuarine or wetland systems can be influenced by tidal conditions (Figure 6b, c).
Riparian. The riparian environment is an ecologic community including plants growing along an established stream or river channel and the floodplain associated with it. Riparian vegetation consists of rapidly growing, moisture dependent species such as poplars or willows, assorted thick-growing shrubs, vines and understory grasses or other smaller plants. The riparian environment can form a complex multi-layered vegetation community. Dense forests of deciduous trees, understory shrubs and grasses can occupy areas embracing the stream channel while small wetland environments may be interspersed along the river or stream.
Early in California’s history, the riparian environment dominated the Central Valley where flooding along the Sacramento River floodplain was uncontrolled and frequent. Since western man’s immigration to California began, approximately 98 percent of this vegetation community had been destroyed by flood control programs, and by agriculture and other development.
Vernal Pools. Endemic to California, the vernal pool is one of the smallest environments, and one of the most sensitive to damaging impacts. The vernal pool plays an elusive role in project development and restoration issues. The discussion of what constitutes a “vernal pool” has caused some debate and legal consternation among developers and environmentalists. The ultimate indicator of what constitutes a vernal pool is the presence of certain plants that are restricted locally or entirely by this type of habitat.5
The vernal pool vegetation community can be very small in scale. A vernal pool is a water body often only several dozens of square feet in area and only inches deep. Vernal pools are seasonally created when water collects in a natural surface depression that is rendered water retentive by hardpan, claypan, or other low porosity soils in the basin. The vernal pool is intermittent, evaporating several days or weeks after it reaches its fullest level. This characteristic makes defining it even more confusing. The vegetal assemblage can consist of very small species of grasses, mosses, and some associated trees and/or shrubs.
5 Pritchett, David A., ”Creation and Monitoring of Vernal Pools at Santa Barbara, California. ” See references in bibliography.”
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Some rare or endangered species of vernal pool life such as fairy shrimp are found on the macroscopic and the microscopic level. (Figure 6c). Many vernal pool inhabitants often hibernate during the dry seasonal cycles. Vernal pools create issue for their often being located on prime lands for development and for being temporary or “insignificant” causing debate over their importance both in their destruction as well as their mitigative significance. On the other hand, vernal pool “construction” can be a mitigative opportunity for a project proponent, as vernal pools are small yet often of environmental significance.
Compatible Plant Associations When a closure project features a golf course, business park, or recreational park, the operator may select nursery rather than native plants for the landscaping. Selection of plant communities for such projects would be based upon the dominant geographic, soils, and climatic characteristics in which the project is located. These conditions would then determine the dominant plant types used. The main difference is that the plants that are selected would not be California native species, but they would still be compatible with the environmental characteristics of the site to ensure survival.
This type of planting program may require a more intensive maintenance regimen to control invasive plant species, pests, irrigation, and nutrient provision. Using plant volunteerism by neighboring native species to stock the site would not be employed to avoid competition with the introduced plants. If an aggressive plant control program is not followed, invasive (volunteer) plants could still establish themselves. Using the natural overstory-understory distribution concept could be applied to cultivated non-native planting.
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- Grassland or Prairie - Figure Sa
~~,Cl.'\1flllM&AIJlt\'!Wll\ftl/W~'Mli~~~-JhJ~JIOi'AJIO'JOI\M/U~'L\118~!1D~Jla!Mll!M'Ol\ttlJ1~1\0Uall~l\tlllJla@!M'rtlill
- Chaparral - Grass with Valley Oaks or Juniper
or other dominant hardwood trees
Figure Sb
Mature Woodland With Ove rsto ry Trees and Understory shrubs
Hardwood. Conifer. or mixed populations
Figure Sc
Shrubs. Succulents. Grasses. Trees adapted for dry desert environment.
Figure 6a ~.~ -~
-=--"' ~~----....:::::!'?~~~~-, ,.-.:::·:b --........,.._ ~ --', .- - ···-· Wetlands or Estuarine
Water saturated soil. reeds. rushes. aquatic plants (hyacinth. etc.) some trees may be present
salt brackish or fresh water
Vernal Pool Commonly shallow result from
water collection on hard pan substrate.
Can be intermittant
Pioneer Grasses Small Shrubs Soft Hardwoods Weeds (Poplar, Willow)
Expanding Saplings
Figure 9
Mature Hardwoods
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Chapter 7: Landfill Vegetative Design As a landfill approaches its waning years of active use, the operator should consider the details of the final closure and postclosure land use planning for the facility. At this time, the operator must consider if the closure will include an integrated planned postclosure use such as creating undeveloped land, a park, a playing field or preserve, golf course, business park, or industrial park. These “uses” will make a difference in the planned final cover design and possible slope design or contouring design for the decks and side slopes, irrigation, and drainage control. The more complex or structured the final land use, the more complex will be the design requirements for the final cover and revegetation planning. An undeveloped, open grassland will demand less from the final cover design than a planned recreational park or a business park. The final cover planning will be dictated by other conditions such as the final cover and moisture layer components, physical aspects of the landfill site, and the surrounding environments.
Costs and availability of materials such as soil and plant stocks will dictate the proposed design characteristics. The elements to consider in the restoration or other final closure plan will include landfill design and additional uses for vegetative cover.
Landfill Design Considerations Proposed Postclosure Use
If there is an actual planned postclosure use or idea in mind for the site, this aspect will have to be analyzed prior to any actual design planning of the final cover. Different uses will impact the demands placed on the final cover, including its thickness, slope profiles, and, in considering revegetation or restoration projects, the quality and quantity of the vegetative layer soil that will be utilized.
Drainage and supplemental irrigation issues will have to be addressed. If the final plan is to install a simple grass cover on the landfill, a thin vegetative layer that is in compliance with the regulations (Title 27 or Subtitle D) may be all that is necessary. If more complex plantings are proposed, berms, supplemental soils on benches and other such enhancements to the vegetative layer may be required to support deeper-rooted plants and to protect the underlying moisture barrier from root penetration.
Some final closure plans are proposing the use of a monolithic cover instead of the current multi-layer design in use on most landfills. The monolithic cover proposals use a thicker, single soil layer that would provide sufficient depths to include deep-rooted vegetative plantings. One proposed plan incorporates moisture control elements into the monolithic cover by using poplars or other similar vegetation and a groundcover plant such as clover to wick off soil moisture by evapotranspiration.6
Whether planning for root depths to determine the vegetative cover thickness or planning root depths to best work with the planned soil thickness, either technique requires forethought in designing the cover and vegetative systems as a unit.
6 Lich, Lewis, Mark Madison and Alicia Lanier, Ecolotree and CH2M Hill, “Poplar Tree Technology Provides Cost and Management Advantage for Landfills,” October 1996.
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Development of contours of the final cover can be impacted by the final postclosure use. An operator who intends to create a restoration project may create more naturally compatible slopes and contours to the adjacent landscape. Or, the operator may require basic contours in compliance with current regulations and the engineering needs of the final project.
Natural Parks, Preserves, and Mitigation Sites. These uses require “permanent cover” of native plants that will not be displaced for a future land use such as a business park or other structures.
Recreational parks and golf courses. These uses will entail permanent or long-term post-closure use cover elements. These sites can be developed with “disposable” cover. The vegetation may usually be nursery plantings, though natural plant species are optional. A potential for their removal exists, allowing re-use of the site for a business park or other structured development at a later time.
Final Design of the Landfill
This will dictate what kind of planting and vegetative layer conditions will be placed on the landfill final cover. Steep slopes on final cover will require more aggressively rooted vegetative types than shallow slopes. Benches can provide planting areas for deep-rooted plants. These benches must be sufficiently wide to accommodate maintenance vehicles in addition to the proposed plantings such as trees or large shrubs. A thin vegetative cover will restrict plant options to shallow-rooted varieties that will not penetrate moisture barrier layers. Irrigation from natural rainfall will dictate the types of vegetation available for use in dry or moist conditions. This may require supplemental irrigation to support the desired vegetative cover, at least until the plants are strongly established. Slopes and naturalized contours can provide alternative planning options for revegetation projects with areas to enhance opportunities for vegetation planting.
Benches. Benches can provide deep soil zones for trees if they are wide enough to provide space for the vegetation and maintenance activities, usually with the vegetation at the outside edge of the bench, where soil is deepest. Grouping trees instead of lining them up creates more natural “groves.”
Decks. If top layers of vegetative soil are sufficiently supplied beyond the minimum 12-inch requirement (preferably 48 inches or more), larger plants with deeper roots can be supported.
Berms. Berms add small areas of thicker vegetative soils as hills or other raised land features (48 inches or more); the added soil can provide sufficient depths for trees or large shrubs to enhance the natural vegetative appearances of the final cover.
Advanced planning of the landfill, taking the postclosure land use into account at the very outset, can help the operator plan for adequate soil and topsoil supplies for the final cover. By planning the final elevations and contours of the cover layer in advance, the underlying waste volume can be graded to within more precise tolerances to conform to the projected design.
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By designing the waste contours to closely match the finished design specifications, less financial resource is expended on poor grade foundation soil built to grade and, instead, can be used more efficiently in providing greater volumes of adequate quality topsoil in the vegetative layer. This could result in additional capital for other closure project costs or savings to the operator overall. (Figure 7 a and b).
Jacques Graber 1999
Figure 7a. A landfill with wastes that are not closely configured to the planned final contours will require additional foundation soil to achieve design grades. Less funding will be available for the acquisition of useful fertile topsoil for the vegetation layer.
Figure 7b. A landfill with wastes more closely configured to planned final contours will require less foundation layer soil to achieve grade. This allows for more funding to be allocated toward better quality, thicker topsoil for the vegetative layer. A thicker topsoil layer will allow for more vegetation design options and improve chances for plant survival.
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Location of Landfill
Where the landfill is located will influence the type of vegetation that can be used on it. Landfills in hot, dry regions will support appropriately selected vegetation for those climates. Irrigation can provide added options for vegetation selection, but it is more expensive to include as a planned element in vegetation selection and closure maintenance plans. In addition, atmospheric conditions can affect plant choices. Revegetation in urban areas may pose a challenge because airborne pollutants can adversely affect some vegetation. Conifers in the Los Angeles basin and grape plants in the Central Valley have demonstrated this sensitivity by loss of foliage and higher mortality.
General wind conditions at site should be considered when designing a vegetation cover. Pollutant gases can collect in windward-facing valleys or pockets, creating adverse atmospheric conditions injurious to plants.
Excessively tall trees may prove vulnerable to blow-down (“wind-throw”) if left unprotected. This can cause damage to the final cover, should the roots peel the soil layer up with the toppled tree. Natural, established tree groves in areas with a prevailing wind tend to develop an airfoil-like profile, due to natural pruning. Smaller trees of the expanding grove tend to grow at the perimeter of the grove with larger, mature trees in the center area of the grove. This dome-like form encourages airflow gradually over and around the tree grove (Figure 8a). Single- line hedgerow-like plantings or isolated individuals, especially at the edges of top decks and maintenance roads or benches, place adult trees in a vulnerable position to strong winds, encouraging wind-throw (Figure 8b). Planting shorter trees at the perimeter of a grove around taller varieties or adult trees can provide a windbreak by slowing wind velocities and directing airflow over or around the taller canopy layer.
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Plant Community
When a vegetative cover is installed, extensive planning must be exercised in laying out the details of the vegetative layer and the final plantings. This can be most complex when all vegetation is planned at the initial planting. A strategy for interim maintenance must still be instituted when the natural population process of a vegetative cover is attempted. A planned vegetative community can be designed and installed in a variety of ways, with three suggestions as follows:
Developing and providing all of the major elements of the plant community, such as grasses, shrubs and trees, at the very outset of planting.
This procedure will require the most advanced planning but it should provide the greatest element of control in the overall plant community design and final outcome. The final plant community would be established and maturing early in the revegetation, and postclosure maintenance program. Some invasive volunteerism by outside plants could occur if the operator does not exercise aggressive control efforts by keeping them out. The initial hydroseeding of annual grasses, or hand planting of native perennial grasses can be introduced for slope stabilization with larger plants installed at later dates.
Providing the proper environment and soil conditions to encourage plant growth and allowing natural invasion (volunteering) by native plants adjacent to the site.
This procedure provides the lowest element of control on the types of plants that may be introduced to the site. This process is the most dependent upon the unpredictable phenomenon of natural plant establishment and succession that may take longer than the immediate planting procedure. Some sort of initial soil stabilization planting with a rapid growing annual and/or perennial grass or ground cover will still be required to prevent erosion of the soil cap. The plant succession process occurs as the selected area matures.
Naturally, the pioneer plants, most adapted to the harsh conditions of bare, usually poor quality soils, begin the process. This community usually consists of low growing or prostrate weeds and grasses with deep taproots. This initial plant association begins the soil nutrient construction and softening of the soil that provides the conditions more conducive to the later succeeding plants to establish themselves.
As the soil is broken up and softened, taller grasses gain a foothold and establish themselves. In time, legumes, herbaceous perennials and woody perennials can begin the larger plant occupation as soil quality and nutrient content improves. Eventually, shrubs and the larger trees assume the mature level on the location.
A landfill preferably will have an annual and perennial native grass planting in its earliest vegetation phase, which may skip the pioneer phase of the succession. Some stronger invasive weeds may still try to occupy the site.
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Shrubs may not be allowed on the site to avoid root penetration, but after the postclosure maintenance program is complete, shrubs and trees may complete the progression anyway (Figure 9).
Combining planned planting with volunteering by adjacent native species to create the final vegetation cover.
This technique can allow some control in the selection and establishment of the larger plants with other plant selections and distributions left to chance. Efforts may still be required to control undesired pest plant intrusion, especially plants with deep reaching taproots that could damage the moisture barrier.
Jacques Graber 1999
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Additional Uses for Vegetative Cover In addition to the obvious uses of vegetative cover that include regulatory compliance, soil stabilization and aesthetic contribution to the landfill site, vegetative cover has some specialized functions that can be used to advantage.
Bioremediation
Vegetative cover can be used in certain situations to attenuate concentrations of certain chemicals, salts, trace metals, and other toxic materials such as boron and selenium present in soils. Certain grasses have the capacity of surviving in higher concentrations of these compounds than other vegetation candidates. In addition these plants tend to store these compounds in their leaf and stem tissues while removing them from the soil. This process can be used to advantage to prepare contaminated soils at problem sites for future population with less tolerant plants. By planting with these salt-tolerant species and mowing them at maturity, the contaminants can be removed. The contaminated cuttings are disposed at a different, appropriate disposal site as fill. After repeated cycles, the salts are removed from the soil; the less tolerant vegetation selections can be planted.
A pilot project at Mountain View Sanitary district wastewater treatment facility employed conifer trees as a moisture exchanging evapotranspiration mechanism to process water. The trees transpire the water into the atmosphere, and create a harvestable revenue crop at maturity. This activity can provide a year-round operation as a serviceable alternative to conventional irrigation disposal systems that shut down during the winter months. This technique could be employed at landfill sludge or septage ponds or possibly leachate ponds, upon closure.
Cattails and similar estuarine plants can absorb certain materials in solution, utilizing them as nutrients. Where high concentrations of these substances in water from sludge or leachate could pose pollution problems such as in leachate and certain liquid waste ponds, cattails and related water plants can mitigate these situations. With naturalized leachate ponds and cattails or similar rushes, natural looking artificial “wetlands” could be created. The cleaned water from these ponds can be recycled as irrigation water after additional treatment. The City of Arcata, in Humboldt County, is employing this technique at the final stages of its wastewater treatment process.
Landfill Gas
Remediation of landfill gas and detection of landfill gas can be accomplished using surface vegetation. Some landfill gas in low concentrations in soil can be absorbed and attenuated by the nitrogen fixing properties of certain legumes and other plant species that possess such bacteria or fungi in their roots. If gas concentrations do become excessive, plants are sensitive to these gases and their abnormal appearance, loss of leaves, usually will alert the operator to a potential gas problem that will need attention. This condition must be responded to quickly before permanent damage is caused to the impacted vegetation, or the gas migrates and spreads further from the initial site.
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Extreme exposures of vegetation to high gas concentrations can lead to stunting of growth or defoliation in some instances, or plant death, requiring subsequent removal and replanting.
Leachates
Leachates that develop at a landfill and accumulate in areas shallow enough to impact plant roots can be detected by plants. Loss of leaves and die-off of vegetation on or in close proximity to the landfill site may indicate signs of a possible leachate problem. With the right conditions and leachate composition, bioremediation (see above) can be employed to reduce leachate impacts.
Remediation of Nitrogen Deficiency
Certain plant groups, the legumes particularly, have a natural ability, through nitrogen-fixing fungal symbiotes, to fix nitrogen in the soil. This nitrogen-fixing ability aids in improving the nutrient quality of the soil that will encourage other plants to grow.
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Chapter 8: Considerations in Vegetation Selection Prior to initiating a vegetation or restoration project, several criteria must be considered in selecting the plants that will be utilized in the final vegetation population.
Site-Specific Considerations Soil Characteristics
The plant assemblage to be selected must be compatible with the soils that will be placed as the vegetative layer. Most soils that are utilized for vegetative cover are generally not the original native topsoils and may not be quality topsoils, lacking the primary nutrients found in natural topsoils.
Borrow Soils
Borrow soils may require amendments. This can be accomplished by adding mulch or wood waste material that has been properly processed and sorted, to the soil. Soil chemistry may be considered to anticipate potential problems with trace chemicals or salts. Relatively sterile soils may require augmentation with fertilizers, and compost to satisfy plant growth needs. Stockpiling the original topsoil layer until the project is done is a sound plan; replacing it on the cap upon completion.
Soil compaction may be an additional consideration in providing the optimum soil conditions for vegetative cover. If soil is compacted excessively, this will inhibit root development of the vegetation. Some grasses can serve as soil softeners, which can allow succeeding vegetation to establish itself.
Moisture or Irrigation Requirements
When a landfill is closed, the final layers of cover material may include a clay or geomembrane layer in the moisture barrier. Often, when this clay layer is utilized, irrigation or soil moistening must be designed to maintain required moisture levels in the soil to safeguard against desiccation of the clay layer. This moistening activity will be conditioned upon the annual precipitation and climate of each individual site. The moistening function can be taken advantage of as a means to support vegetative cover that may be planted on the final cover. The moisture level must be designed to account for the climate of the site location including annual rainfall volumes and periods of occurrences, the type of clay material to be utilized in the barrier, and the planned plant community to be installed on the landfill site. Once these conditions are established, the volume of water required for proper moisture control can be determined. Water Use Classification of Landscape Species (WUCOLS) is a reference published by the University of California Cooperative Extension that discusses all of the parameters involved in designing a vegetation cover.
If artificial irrigation is planned for the site, the source water must be tested for contaminants and the chemical constituents in the planned water must be considered. The chemical constituents in some water may include salts and other
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trace materials that may accumulate in the irrigated soil which can cause salt or chemical buildup leading to plant damage or mortality. A project planned to employ irrigation water from a cooling plant was designed for a Southern California landfill.7 In time, the vegetation on the irrigated landfill site began dying. A study of the source water revealed high concentrations of boron and other dissolved substances. These salts were injuring the vegetation being irrigated on the landfill. To correct the problem, plans are being developed to process the effluent water from the power plant to remove these salts prior to irrigation.
In addition to moisture being introduced, control of drainage off site must also be properly designed. Some sites may require less artificial irrigation with more surface runoff facilities provided at the site than other locations.
Maintenance Requirements
Maintenance of the final cover and its accompanying vegetation can be influenced by the proper design steps taken in constructing the vegetative layer. Selection of slow-growing plants will reduce the numbers of times maintenance crews may be needed to thin or control growth of grasses or other foliage. Selecting clean plants will help in cutting cleanup costs.
Some trees such as eucalyptus are extreme generators of litter. They produce an abundance of debris from fallen branches, topping from high winds during storms, bark debris, a hard nut-like seed pod and the thick lignin rich leaves that do not decompose readily. Vegetation that produces such volumes of litter can require more frequent cleanup maintenance than most native plants require. In addition to the cleanup requirements, unmaintained litter buildup of this kind can pose a potential fire hazard that could present added emergency costs for fighting a fire. The high oil content in eucalyptus, in addition, creates hot fires.
Native plants especially adapted to the environmental conditions of the candidate project site can reduce irrigation requirements as well as maintenance and pest control. Native plants are adapted to defend themselves from indigenous pests.
These plants can possess defense mechanisms effective against native plant pests. Using native plants in final cover planting should only increase the chances of plant survival. This can help in reducing costs of cleanup and replacement of plants lost to pest infestations.
Plant Compatibility
Plants in their natural assemblages have formed compatible community associations. Some plants have special defenses, usually chemical, to promote their survival. Attempting to place competitive vegetation too close to such defensive varieties will reduce the chances of survival of the “invasive” plant. Usually the protective zone of a plant will extend to the edge of its drip zone; the area directly below the crown of the tree or bush. Eucalyptus (non-native), oaks, and California Bay (native) are some potent defenders of their root zones.
7 Class I Conceptual Landscape Plan, BKK Class I Landfill, West Covina, CA, 1996
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Climbing plants should be avoided because they can overpower the supporting tree they may establish in, by blocking the sunlight to the leaves. Certain California varieties of wild grapes can overrun a large tree in a matter of a few years.
Plant Chemical Products
An awareness of the chemical by-products plants produce (such as terpines) can help in determining the data that may be generated during water quality and soil quality analyses. Decomposing leaf detritus or bark debris can accumulate in the soil and percolate out as complex compounds that can mimic certain manmade hydrocarbons or leachate compounds. This can confuse data sampling and source determination. Eucalyptus, California Bay, and most conifers produce high concentrations of these chemicals. An understanding of this can reduce confusion when data sampling shows unusually high readings of hydrocarbons. The use of chips from these trees as wood wastes or soil amendments can create the same effects, leaving dissolved terpines in the soil.
Native versus introduced (non-native) plants. The role of the vegetative cover is to stabilize landfill slopes, mitigate or control moisture levels in the soil, control surface runoff and comply with regulatory requirements. The landfill designer must also realize that this cover will continue as a long-term impact on the environment (hopefully beneficial). The landfill cover also should ideally serve as a repair to the environment. When possible, the vegetation should provide a near reconstruction of the ecosystem that the landfill had displaced, providing the native plant makeup to maintain the plant and wildlife diversity remaining in California. A vegetative assemblage that most completely integrates itself with the surrounding environment under given circumstances is what an operator who is attempting restoration is trying to achieve. A natural restoration would be the ideal result for the vegetative cover project; providing renewed natural environments for future generations. A landscape planner must consider several things in the overall look of the plant cover.
Planting Considerations Plant Heights
When designing the final plant distribution, will the vegetation compliment or blend in with the surrounding vegetation and environment? Can it at least present an acceptable appearance? A landfill in open grassland with 40-foot tall trees would present an obvious visual impact inconsistent with the natural surroundings, focusing attention on the landfill r