A Guide to the Revegetation and Environmental Restoration of Closed Landfills

Chapter 12: Some Problematic Conditions

Designing and installing a vegetative cover, whether it is a natural restoration or a landscaped terra-form using nursery plant stocks, requires detailed planning. Several issues as previously discussed must be addressed before construction begins. Some cases involving problems or successes with the more unusual issues will be covered in this section.

Irrigation Source Water Problems

For a landfill vegetation program to succeed, not only must the irrigation hardware design (sprinklers, piping, etc.) be considered, but the source water and its source(s) must be addressed. If the chemical makeup of the source water is not analyzed, there may be some complications that may require expensive remedial actions. Such a problem occurred with a landfill revegetation project at a BKK Class I landfill in West Covina, California. The landfill cover consisted of a 5-foot thick erosion (vegetative) layer that required irrigation to maintain proper moisture content. The landfill's surface features included several benches and sideslopes, totaling 118 acres with a top deck of 42 acres (160 acres total). Water for this moisture maintenance was procured by using effluent outflow from a nearby leachate treatment plant and a power plant cooling tower water system effluent.

Total Dissolved Solids (TDS). Sodium and boron are included in this water. Concentrations of these compounds leached into the soil during the irrigation program and attained dangerous levels that could adversely affect many less tolerant species of plants that had been planted on the site. Because of this contamination, a new revegetation (remedial) program was initiated. Plant selections for the new vegetation program were limited to those plants tolerant to the high boron and salt concentrations.

Existing grasses and small herbaceous plants would be removed from the landfill cover and replaced with ice plant initially, followed by larger shrubs and trees in test plots. All candidate plants are non-native. This choice was made as the salts left behind by the effluent water irrigation made the soil intolerable for other plant selections including native candidates.

A new potable water source was selected to replace the previous water source with hopes of avoiding continued salt contamination. Water would be distributed on the site with overhead sprinklers as well as drip systems. Water volumes for the new program would be controlled using a computation based on area, average evapotranspiration rate, plant water needs, gallons conversion, and irrigation efficiency.

This model would determine the volume of water required to irrigate the cover satisfactorily. Soil moisture sensors on site would provide feedback on the moisture content of the soil cover. This would enable the operator to monitor soil moisture content and to regulate the amount of irrigation needed to maintain the proper moisture.

This is intended to reduce the chance of excess moisture penetration into the waste, which could lead to unwanted gas production and leachate generation.

In tandem with this project is a series of proposed options to treat the effluent water from the leachate treatment plant and the power plant cooling water to irrigate the landfill. Five process options to treat the water for salts and boron are being addressed including reverse osmosis, electrodialysis, distillation, and ion exchange. The least costly process (reverse osmosis) is estimated at 1 million dollars to construct, while ion exchange would exceed 2 million dollars to build. (Evaluation of Treatment Options for Removal of TDS and Minerals from LTP and Power Plant Effluents, Invirotreat, Inc., June 17, 1996). The project was being reviewed in 1996.

This problem demonstrates the importance of source water quality testing to reduce the potential for salt contamination. Soil testing for salts would be recommended prior to planting. This will enable the operator to select vegetation according to salt tolerance capacity, to select a less salt-contaminated water source, or to apply remediation techniques to bring concentrations to more tolerable levels. As the project goes into the maintenance stage, continued soil sampling is advised. This will allow for potential salt build-up problems in the future, catching the problem before it causes damage to vegetation or the soil. This is most important in sites using recycled water or effluent water for irrigation.

Below are listed two methods for salt testing in soil.

1. Electrical Conductivity (EC).
2. Sodium Absorption Ratio (SAR).

Several grasses are good candidates for soils with high salinity. These are listed in the following tables.

Some of these grasses are not California natives.

Some tolerant legumes include:

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False Readings in Soil Water Samples

When monitoring soil water samples at a landfill as part of an irrigation maintenance program or other data gathering activities, attention must be paid to the characteristics of the landfill's cover soil makeup including any amendment materials added to the soil or plants and their debris. Note that some amendments like tree bark can generate chemical compounds that can mimic certain manmade hydrocarbons. Because of false results from natural compounds, they could lead to unnecessary testing and remedial activities to correct the "problem."

False sample results involving detected hydrocarbons in groundwater monitoring samples revealed the production of terpenoids by water passing through a natural tree bark fill material at Caspar Landfill, in Mendocino County, owned by Louisiana Pacific.¹¹ This condition produced chemicals that resembled diesel and hydraulic fluid that could be detected in groundwater samples. Thorough testing was conducted for certain components in the diesel test samples, the hydraulic fluid test samples, and the tree bark test samples. Test chromatograms for these samples revealed that the hydrocarbons generated at the Caspar site were of biological origin. Certain tree species with high terpene concentrations (eucalyptus) could create a similar circumstance if bark and plant debris is allowed to accumulate on the landfill.

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Drainage and Surface Settling

The efficient and complete removal of water on a conventional dry landfill is vital to its proper function and the integrity of the cap. The presence of excessive moisture in a landfill can lead to downward migration of water through the landfill cover into the underlying refuse. When the refuse is exposed to the invading moisture, accelerated degradation of the wastes can occur. Since this moisture would be uncontrolled, degradation of wastes will be uneven with varied settling rates across the landfill.

If a landfill experiences variable settling, the cap material may crack, compromising the function of the cap in containing landfill gas, moisture exclusion and surface runoff drainage functions. Low spots in a landfill cover can encourage ponding of water, causing added drainage problems and moisture invasion. Leachate can form as moisture collects. These leachates can seep out of the side slopes and contribute to soil weakness or erosion. Leachate with high concentrations of waste residuals can injure or kill vegetation if it penetrates the root zone. As with gas, leachates will damage plant roots, affecting viability. Leaves will brown and drop off. Growth will be impacted.

Sustained exposure to leachates causes defoliation and plant death. Remediation of the leachate problem would be an expensive project, requiring removal of the contaminated soil or possibly washing the soil and replacing it at the site.

Variable surface settling can create problems for surface facilities such as parks or golf courses located on the landfill. Gas collection and irrigation systems can be seriously affected by the differential settling of landfill covers. Piping for gas collection, surface water runoff and irrigation systems can be damaged if the top layers are severely distorted enough to break pipes or induce leakage. Surface drainages can be disturbed significantly, changing slopes and interfering with surface runoff. Extreme surface distortions resulting from settling can damage wires for lighting or electronic monitoring systems. Structures such as access vaults, maintenance sheds or other buildings that may be located on the landfill may suffer foundation damage or shifting, possibly encouraging gas entrapment in their enclosed spaces.

Golf courses can be significantly, adversely affected by improper irrigation and drainage control. If uncontrolled settling occurs on a golf course, the affected section can be rendered unplayable and require reconstruction. This can involve regrading and expensive reconstruction to correct.

Landfill cover integrity is highly dependent upon the moisture and drainage design elements in the final landfill plans. This moisture control should reduce the potential for uncontrolled settlement across a landfill.

A recreational facility at Industry Hills in San Bernardino County, California, is located on top of a closed landfill. The center includes a major hotel, swimming pool, tennis and gym facilities, two golf courses, horse and pedestrian trails. This extensive facility must interact with the postclosure landfill behaviors that accompany an aging landfill. Landfill gas is collected from wells throughout the site and is fed into their combustion facility for space heating and water use. All aspects of the facility are monitored and maintained through an aggressive program to maintain constant gas production, with consistent BTU (heat) production, irrigation and water control.

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Soil Methane Gas Concentrations

When landfills are completed, a clay moisture barrier layer or synthetic barrier and a final erosion or vegetative layer are placed on the structure to provide a means to keep excessive moisture from reaching the wastes contained within. In addition to keeping excess moisture out of the landfill, the clay and erosion layers are intended to provide a capture layer to prevent landfill gas from escaping uncontrollably into the atmosphere. If a gas recovery system is employed, the clay layer enhances the system’s efficiency.

Construction of the cap and incorporating a landfill gas collection system of wells and lateral piping systems to recover the gas for burning or flaring helps to reduce fugitive gas released to the atmosphere.

The erosion or vegetative layer is placed on top of the final landfill cap, serving both as part of the gas trapping function and as a substrate to support the planned vegetative cover. To comply with the minimum requirements of Title 27 and Subtitle D, in most instances, the vegetative cover may be a minimum of 12 inches thick. Due to leakages in the clay layer from non-uniform settling, side slope movement, cracking and breaches in the cover, volumes of landfill gas may escape through the clay layer and linger in the vegetative layer soil, as well as escape to the atmosphere. These gases can be produced in quantities to build up lethal residual concentrations that remain trapped in the vegetative layer. These soil gas concentrations can affect the root zone of the vegetation planted on the cover. If these gas concentrations exceed the tolerance levels of the overlying vegetative layer, the plants can be adversely affected to the point of impaired growth or death.

The concentrations of landfill gas can impact the natural soil gas concentrations that are essential to proper plant metabolism, displacing vital oxygen and other resident gases usually found in the soil and replacing it with methane and carbon dioxide.¹² This soil gas displacement can adversely affect the roots of plants and their symbiotic fungal associations that aid in nutrient and gas assimilation.

Initial signs of this impact on vegetation include stunted growth rates over time, brown patches in turf, partial loss of foliage, or total loss of foliage and plant death.

A golf course in Utah experienced a situation in which landfill gases were invading the vegetative layer and residing in the root zone of the cover. As the gas concentrations built up to critical levels impacting plant survival, there were apparent signs of damage to vegetation. Green turf areas became covered with brown and dead patches. Trees and shrubs also displayed signs of impaired growth and poor establishment, with partial loss of leaves. These signs revealed a potential gas leakage problem.

Gas concentration tests were performed at two locations in which extensive areas of brown patches were found in the turf. Studies were undertaken to observe methane flux emissions and methane oxidation rates in the soil. The greens were constructed with an 80 percent and 20 percent peat soil mixture, following United States Golf Association standards. To maintain gas exchange, greens management staff carried out remedial aeration procedures but this effort failed. After using a corer to prepare the holes, gas samplers were installed at 6 to14 inch depths. The soil plugs were reinstalled behind the gas probes to allow natural gas concentrations to reestablish.

Gas samples were taken from the plugs and they were "determined" with gas chromatograph tests. Laboratory testing on the samples revealed high methane concentrations where turf was injured. Diffusion rates at four sample sites reached 212 lb./ac/hr, 23 lb./ac/hr, 88 lb./ac/hr and 16.5 lb./ac/hr. Areas with no signs of turf damage revealed no methane concentrations in the soil. It was determined that methanotrophic bacteria were displacing oxygen in the soil. This eliminated oxygen from the soil environment, creating anaerobic conditions and resulting in damage to roots and turf. In situations where soil gas is found, usually methane is in the highest concentration with carbon dioxide, hydrogen, and nitrogen in lesser concentrations.

Correcting a soil gas problem is often an expensive and intensive operation for a golf course operator to undertake. Soil gas must be eliminated which requires installation of efficient gas collection systems. Retrofitting such a system will require trenching to install horizontal gas collection lines or drilling wells for vertical gas collection systems. These steps can be expensive and demanding of planners to work around existing surface features as are found in golf courses.

The best alternative to retrofitting is planning and installing the gas collection system while the final cover is being constructed or while the landfill is first being designed and constructed. Grading the waste to near final planned contours will reduce the need for using additional topsoil to build up to planned elevations. This will conserve soil resources for the project. Proper and complete compaction of wastes during filling may improve waste settling behavior, ideally promoting uniform settling of decomposing wastes and reduced breaches in the cap. Good design and installation of the final cover layer, and the erosion layer should provide the protection against escaping landfill gas and efficient recovery via the gas recovery system to prohibit soil gas problems.

Ample soil thickness can improve options for landscape design and selections for vegetation. Thicker soil can also promote better gas containment. A thin erosion layer will limit vegetation options to smaller grasses or herbaceous plants with shallow roots. This condition can also allow greater chances of damage to the clay layer as less differential shifting can more readily breach a thin soil layer. Potential for soil gas impacts on vegetation and the outside environment can be greater with a thinner soil layer by placing the plants in closer proximity to the underlying soil gas zones. Odor problems will be more possible, which is a concern when planning a golf course or other high use recreational facility.

Deeper soil overall or planned berms of deeper soil from 3 to 5 five feet thick, in addition to the 12-inch vegetation/erosion layer, will permit larger shrubs or trees to be used. This can also help buffer the root zone from the underlying barrier layer and the refuse or gas below. These soil enhancements should be large enough to accommodate the planned plants’ root systems.

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Additional Considerations

Cover minimizes the percolation of surface water into the waste layer. Since the applied demand for final cover is to prohibit excessive moisture entering the upper soil layer, some compaction is done on the erosion layer. The compaction will reduce porosity and downward water migration. This may interfere with irrigation of vegetation and promote surface water runoff.

Vegetation maximizes evapotranspiration with irrigation balancing evaporation. If soil moisture is critical to the integrity of underlying clay layers in the cover, vegetation can accelerate the evaporation of moisture back out of the cover and clay layer. Without proper monitoring of the soil/clay moisture, desiccation of the clay layer can lead to cracking and loss of the moisture barrier’s functional integrity. This can allow excess moisture into the landfill during wet weather seasons and landfill gas escape year round.

Vegetation selection should address fire safety issues. Certain vegetation can promote fire hazards, either due to growth cycles such as the dry period in grasslands in late summer or excessive plant debris accumulations such as bark, leaf and branch debris. Some deciduous species allow their leaves to dry and remain on their branches, prior to shedding. These materials combined with dry conditions can present a high fire hazard. An aggressive maintenance program of large sized debris removal can reduce this danger. In addition, the large debris (branches, broken-off treetops) can be ground up and recycled as mulch, a savings incentive for maintenance budgets.

The topsoil medium for plants to establish on is usually minimal in depth and of poor quality. Most closure projects are limited by the costs of the closure operation. In most instances, the cost of obtaining, transporting and installing quality topsoil for the vegetative layer can be high. Unless a free or low cost source of soil can be found close to the facility, the soil layer will be constructed to the minimum standard of 12 inches thickness in compliance with Subtitle D or Title 27 regulations. This thickness can support grasses and maybe smaller perennials such as vetch or lupine, but it would not be sufficient for larger shrubs or trees. Inferior soil quality will necessitate fertilizer and soil supplements to aid plant establishment and growth.

Compaction of the topsoil layer with a relatively smooth surface makes it difficult for roots to penetrate through the soil, and for plants to become established. Since the final erosion layer is placed on the final cover, the nature of its placement, grading, and sloping will increase the tendency towards compaction. The use of heavy earth moving equipment, inducing compaction, will reduce loft and the ability for the soil to absorb moisture and rapid root penetration, which is vital to plant establishment and growth. Texturing the soil with a sheep's foot or grid roller, or an imprinter in arid or desertified environments, can improve the soil conditions to some extent and will provide surface pockets to retain seed and fertilizer. Without texturing of the soil surface, water runoff and wind erosion can occur, causing soil loss, as well as loss of plant seed and accompanying nutrients.

Little organic material is available for plants in most landfill cover. In many cases, the "topsoil" used in the final erosion layer may actually come from subsoil layers moved in excavation. This material may not be of the same grade or quality of true topsoil, lacking the organic materials found in the original topsoil of the project area. With a lack of organic material or vital nutrients in the soil, the likelihood of a successful revegetation project is jeopardized. An option is to scrape the actual topsoil layer to the ‘A’ horizon (topmost layer) and to stockpile it, replacing it as a final layer when closure and final grading are completed.

Irrigation systems often can be poorly designed, providing uneven coverage. Germination and survival of the plants will closely reflect the irrigation pattern. Dry areas where water is not reaching will limit the possibility of new plantings to survive their initial establishment. Poor management of irrigation systems can create the same situation resulting in soil moisture loss, desiccation of vegetation and death. Excessive soil moisture can result in possible leachate and gas development or slope erosion or failure, as well as distressing young vegetation.

Invasion by weeds or other undesirable pest plants creates a competitive pressure, removing valuable water and nutrients from the cover vegetation. An active program of selective weed abatement can reduce the competitive pressures of weeds on the vegetation.

Side slopes at landfills can be steep, making irrigation and maintenance difficult. Steep grades, if not properly engineered or textured, can encourage excessive surface runoff. Steep slopes can make seeding and seed establishment difficult. Seeds can have a tendency to be washed off by accelerated water flows during storms and by strong winds.

Geotexturing can be used if site conditions require it or if the surrounding topography reflects an aggressive terrain. Geotexturing employs variable grade slope faces in its appearance; achieving 45 degree slopes. The variable grade creates a more natural face. These angles can be achieved through use of HDPE geogrids. Forming a subsurface foundation, the geogrids provide an internal subsoil structure to support the soil layers above, inhibiting downslope migration. Vegetation can be hand planted on these areas once they are prepared. A site at Spanish Hills Golf and Country Club, Ventura County employed this technique. Such slopes would require foundation soil analysis studies for slopes greater than 1.75 (H): 1 (V). Various soil stabilization products such as those produced by American Excelsior Company and others can assist in steep slope stabilization.

Soil temperature can vary throughout the soil cover and can be detrimental to healthy plant growth. In some areas where temperature extremes may occur, these extremes can affect seed germination, reducing seedling population. This variation in extremes can be affected by north-facing slopes (cooler, more moist) or south-facing slopes (hotter, drier).

External pressures from the local community can impact the aesthetics of the landfill site. A local community can elect to influence the vegetation choices a developer may put on a landfill. The choice, though functionally adequate, might not be aesthetically or, environmentally, the wisest choice. This must be emphasized when community members become involved in the landfill design process.

Cost Analysis: Determine the costs and availability of materials and resources. Conserving and stockpiling topsoil removed from the site will save future costs in procuring new soil and transporting it to the site. This may hold true for stockpiling vegetation stock. It may be less costly to transplant small trees and shrubs on the site to a holding nursery and then transplant them back at the closure phase. Different projects will be more costly than others will. Recreational parks and open or natural field projects may be cheaper than playing fields, or golf courses.

Two Golf Course Cost Analyses¹³

This scenario would involve a landfill in which most of the wastes are removed, "reclaimed" or "mined" for their recyclable value in metals, etc.

Costs of supplemental materials such as geotextiles and other slope re-enforcement/erosion control systems can vary for different systems that may accomplish the same things within the same performance standards. Below is a comparison of several erosion control systems from the least expensive and simplest up to the most complex and costly.

Long Term Erosion Control Systems ¹⁴⁽³⁾

(1) Some systems with greater mass and/or ground cover may exceed these upper limits.

(2) Depending on infill material.

(3) Shear stress ranges have been omitted because of comprehensive data for the range of materials is not readily available. Manufacturer may have specific performance and cost data.

PP, Polypro-= Polypropylene
PE=Polyethylene
UV=Ultraviolet

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Last updated: April 18, 2008