This is the fourth lesson of ten make up the
new correspondence course from the University of Wisconsin.

Last month's lesson provided an overview of gas movement and control. This lesson will focus on the second of the two most serious concerns at landfills: controlling leachate generation and movement.

One of the most important problems associated with the design, operation and long-term care of landfills is managing leachate that is formed when water passes through the deposited waste. The leachate generated from municipal solid waste is a mixture of organic and inorganic, and dissolved and colloidal solids. It contains products of decomposition of organic materials and soluble ions which present a potential pollution problem for surface and ground waters.

Leachate generation rates are primarily dependent the amount of liquid the waste originally contained, an quantity of precipitation that enters the landfill through the cover or falls directly on the waste. Chemical character will be affected by the biological decomposition of biodegradable organic materials, chemical oxidation processes, and dissolving of organic and inorganic materials in the waste leachate's chemical composition will change as the landfill goes through the various phases of decomposition similar the changes in methane production described in Lesson 3.

Objectives

1. In this lesson you will learn;

2. Factors influencing leachate generation rate and chemical characteristic;

3. Ways of controlling leachate

Leachate as a potential water contaminant

Water contamination is of concern when the solid is placed near surface or ground waters, and when unhindered flow occurs from solid waste to the surface or ground waters. Where leachate enters surface waters, visual observations and sample analysis can be used to verify its impact on water quality. Movement of leachate into subsurface formations is most of-ten detected with monitoring well or, if the situation is serious, when a drinking water supply well is contaminated. A typical subsurface migration pattern is shown in Figure 1. The appropriate leachate control method must be selected after the potential for leachate contamination of ground and surface waters has been studied.

The extent to which the addition of contaminants to ground or surface water is viewed as detrimental has been debated extensively. Many states have adopted ground water quality standards which specify maximum allowable concentrations of contaminants. The standards specify maximum ground water contaminant concentrations either under the disposal site or at the edge of a buffer zone around the site. The numerical values are often based on the National Drinking Water Standards. A portion of Wisconsin 's standards for preventive action and enforcement are shown in Table 1 (Wisconsin Department of Natural Resources, 1988). Monitoring wells are periodically tested to determine if compliance is being achieved.

The performance standard approach is in contrast to other regulations which specify minimum design, construction, and operating standards for landfills. These regulations man-date particular design and construction procedures, similar to a building code, for the leachate control system. These design standards are enforced by reviewing plans before the facility is built and through inspections during construction.

Factors affecting leachate generation

Factors influencing leachate generation and indirectly the potential for detrimental water resources impacts at a landfill are: 1. Climate; 2. Site topography; 3. Final landfill cover material; 4. Vegetative cover; 5. Site phasing and Operating procedures; 6. Type of waste materials in the fill.
The climate at the site significantly influences the rate of leachate generation All other factors being equal, a site located in an area of high precipitation can be expected to generate more leachate. Vegetation will, by evapotranspiration, re-direct a portion of the infiltrating precipitation back into the atmosphere.

Topography affects the site's runoff pattern and amount of water entering and leaving the site. Landfills should he designed to limit leachate generation from areas peripheral to the site by diverting runoff away from the site, and by constructing the landfill cover area in a way that promotes runoff and reduces infiltration. Table 2 shows the difference in runoff that will occur for different soils and slopes.

Soil types at the site also influence the amount of water percolating into the landfill and escaping through the bottom. As the permeability of the soils used to construct the final landfill cover increases, leachate production rates increase. Figure 2 shows the permeability of different cover soils. In areas where the naturally occurring soils are not suitable for preventing unacceptable percolation through the cover, other soils may need to he brought to the site or geosynthetic membrane covers installed in combination with me natural soils.

Table 1. Selected Wisconsin Ground Water Quality Standards

Table 2. Impact of Soil Surface on Waste Runoff

The final vegetative cover plays an integral part in leachate production control. Its basic functions are to limit infiltration by intercepting precipitation directly, thereby improving evaporation from the surface, and to reduce percolation through the cover material by taking up soil moisture and transpiring it back to the atmosphere. A site with a poor vegetative cover may also experience erosion which cuts gullies through the cover soil, allowing precipitation to flow directly into the landfilled waste.
Extensive research has been conducted on the use of layered covers. Two examples are shown in Figure 3. Combinations of clays, sands plus possibly other soils are layered in an effort to promote maximum evapotranspiration, surface drainage, and runoff so that percolation is minimized. Geosynthetic membranes, when used, are placed under a soil layer to protect it from the weather and to allow the establishment of a vegetative cover.

An intermediate or temporary cover is placed over the landfill working face prior to the application of final cover. Leachate generation may be increased by the infiltration of larger quantities of precipitation through the intermediate or temporary cover. Quantity of infiltration will depend upon the soil material, with sandy soils allowing the most infiltration and clay soils the least infiltration. The use of clay soils for intermediate cover will reduce infiltration and consequently leachate generation, but the presence of relatively impermeable layers of soil within the landfill can result in leachate internally pounding upon the intermediate cover as successive lifts are added to the landfill. This internal ponding has frequently resulted in leachate moving laterally through the landfill and discharging out the side of the landfill and becoming a source of surface water pollution The best approach when utilizing clay soil for daily cover is to remove a portion of the clay immediately before placing me next layer of waste. This allows downward drainage of leachate to the collection system. It also should be a design objective to configure the landfill so that final cover can he placed as soon as practical after waste is deposited in the landfill.

Although leachate is primarily generated by percolation of water through solid waste layers in the landfill, it can also he generated from water released from high moisture content wastes. The acceptance of high moisture wastes or liquids is discouraged or prohibited by state laws unless specific management plans are incorporated into the design and operation of the landfill. In recent years the general practice has been to discourage acceptance of wet wastes.

Predicting the amount of leachate is a critical design parameter when designing a landfill. The amount of leachate generated will impact operating costs for leachate collection and treatment. The treatment plant must he sized to handle the peak period of leachate flow. These calculations will also directly influence the amount of money that may have to he placed into an escrow account for long-term care after me landfill is closed. The quantity of leachate generated will also he a factor in determining the leachate system which is installed at the base of the landfill.

The water balance equation shown in Figure 4 estimates the amount of water which will percolate through the landfill cover. Over time, the volume of leachate produced is assumed to equal the volume of percolating water. A lag may occur between me time percolating water enters the fill material and the time leachate emanates continuously from the base of the fill. During this period the solid wastes are increasing in moisture content.

Initially, some leachate will be generated intermittently due to water channeling through the wastes. After a several year period, leachate production should be more consistent. Although the quantity of leachate may be predicted by the water balance equation, its time to reach the base of the landfill is less predictable and will lag behind precipitation events by a period dependent upon the moisture capacity of the waste. Also, the permeability of intermediate soil layers within the landfill will control the downward migration rate.

The actual detection of leachate reaching the landfill will depend upon the design of the site and the availability of monitoring equipment. At a site where leachate collection lines are installed, the onset of leachate flow can he detected by observing flow in the collection system. At other sites, observation wells have been installed inside the landfill to measure the depth of leachate buildup on the landfill base. Where no observation facilities are provided, the first signs of leachate accumulation may he a leak: out of the side of the landfill or the detection of contaminated groundwater in a monitoring or water supply well.

The dimensions of water balance equations are usually inches or millimeters with the calculations being done on a weekly basis. Inspection of the equation reveals that PERC (and eventually the quantity of leachate) will increase as P increases. PERC decreases when R/O, AET, and AST he-come larger. The term AST represents the change in water storage in the landfill soil cover from the beginning to the end of each week or month being studied. The amount of water stored cannot exceed the soil field capacity (soil moisture holding capacity). When field capacity is exceeded, percolation takes place.

The water balance for a landfill cover can he computed by hand for monthly time periods. The calculations are described in "Procedures Manual for Ground Water Monitoring at Solid Waste Disposal Facilities" (EPA/530/ SW-61 1, August 1977). The method requires, as input, monthly weather data arid evapotranspiration estimates. Weather data can he obtained from the National Oceanic and Atmospheric Administration.

The U.S. EPA, in cooperation with the Army Corps of Engineers Waterways Experiment Laboratory, has prepared a computer program which calculates the water balance arid simulates water movement through the landfill arid any underlying liner. The HELP Model (EPA/53O-SW-84-009, June 1984) has weather records in data files and offers options for predicting leachate generation under many combinations of cover conditions arid liner designs. A portion of the output from a typical computer simulation is shown in Table 3. The HELP Model is set up to handle layered cover systems, and multi-layer liners, so that me most effective combination can he quickly found. This program is available for execution on a personal computer.
The water balance method assumes:

• that the sole source of infiltration/percolation is precipitation falling directly on the surface of a landfill;
• all surface runoff from adjacent areas is diverted around the landfill surface;
• groundwater does not enter the, landfill through the sidewalls or bottom;
• all water movement through the landfill is vertically downward;
• the landfill is at field capacity;
• no recycling of leachate or co-disposal of liquid is occurring.

Table 3. Projected Average Monthly Totals in Inches Based on 20 Years of Weather Records

Use of the water balance method reveals that weather conditions have a primary influence over the amount of Potential percolation and possible leachate production. In humid regions where precipitation exceeds evapotranspiration, the amount of percolation will he much higher than in drier regions for similar landfill covers. However, percolation can still he expected in dry areas if the amount of infiltration into the cover exceeds the soil's moisture holding capacity and the local evapotranspiration rate. This condition may exist during a short intense wet weather period. Since precipitation cannon he controlled, minimizing the quantity of leachate generated must he accomplished by site design procedures that will increase runoff and evapotranspiration.

Landfilling techniques affect leachate chemistry

The actual leachate composition found at a landfill will depend upon waste composition and conditions within the landfill such as temperature, moisture content, moisture routing, depth of fill, stage of decomposition, ability of intermediate soil Iayers to remove contaminates, and quality of water entering the landfill. Ranges of leachate compositions tabulated by Ehrig (1989) are shown in Table 4. The leachate characteristics were separated in the acetic phase which occurs soon after waste placement and methanogenic phase which occurs a period of time after tile waste is landfilled.

Table 4. Leachate Analysis (Parameters with differences between acetic and methanogenic phase)

*adsorbable organic halogen
Source: Ehrig, 1989

Precipitation events and changes in seasonal temperatures will result in wide fluctuations in actual observations. For example, Figure 5 shows the COD data collected from a solid waste test lysimeter operated at the University of Wisconsin. The concentration fluctuated considerably for the first three years and then decreased to relatively low values. In contrast, modifying the physical characteristics of the waste by shredding and waiting six months to place an earth cover results in higher initial COD concentrations, but the decline in concentration occurs earlier. It is believed that this is the result, of increased particle surface area, better mixing of the waste, and more uniform movement of moisture.

The leachate composition will also be influenced by the types of commercial or non-hazardous industrial wastes that are placed in the landfill. A study of 12 landfills that received various quantities and types of wastes showed ranges of compositions graphically described by Figure 6 (Kmet and McGinley, 1982).

Accurately predicting the composition of leachate that will be discharged from landfilled solid wastes is a difficult task. Researchers use a variety of approaches to improve their ability to predict leachate composition. A prediction method prepared by Farquhar (1988) divides the landfill into cells within which he predicts the quality of leachate that will emanate from the base ,of each cell. Using an additive procedure this method can estimate leachate quality at the landfill base for a variety of site configurations and ages of waste within the landfill. A different predictive approach has been proposed by Ehrig (1989). Figure 7 shows a predicted concentration for leachate chemical oxygen demand (COD) for 100 years after a landfill closes.

Natural atteuation limited

The chemicals contained in leachate which escape from the landfill base may undergo a variety of conversion and destruction reactions as they pass through the soil and into the underlying formations. For example, as leachate moves through a clay soil, most of the heavy metals (such as lead, arsenic, zinc, cadmium, and mercury) will be retained by the soil. The effectiveness of each soil to attenuate leachate is different, and not all elements or compounds are equally removed or reduced in concentration.
A sand will he much less effective than a clay. Of particular concern are small but significant concentrations of complex organic chemicals. These chemicals such as the organic compounds shown in the lower half of Table 1 are consider a health hazard at very low concentrations. Also, tests show that the soil has only a limited capacity to remove certain chemicals from leachate. Once that capacity is exceeded, the chemicals are unaffected by the soil.

Factors which affect, the severity and extent of ground water contamination that leachate may cause include leachate quantity, component concentrations, patterns of ground water movement beneath and adjacent to the landfill, and intensity of attenuation processes which reduce contaminant concentrations during transport through the ground water system. Figure 1 shows how ground water flow patterns will determine the direction and rate of contaminant transport as well as the dilution rate.
Attenuation takes place through six major mechanisms (Diaz et al., 1982): 1. Mechanical filtration; 2. Precipitation and co-precipitation; 3. Adsorption; 4. Dilution and dispersion; 5. Microbial activity;, 6. Volatilization.

Mechanical filtration restricts the flow of suspended contaminants. The effectiveness of the soil layer in straining or removing suspended particles depends upon the type of soil and the thickness of the layer. Filtration is especially effective in limiting the travel of microorganisms. Removal of pathogens can be complete if the soil formation is sufficiently thick.

As the leachate moves away from the landfill, changes in the micro-environmental conditions such as temperature, pH, and solution composition can result in immobilization of some constituents of leachate which are then converted into insoluble compounds. The advantages of precipitation as a mechanism for contaminant removal are its high capacity and low reversibility. These mechanisms are especially important in the removal of heavy metals.

Adsorption processes result in the soluble contaminants being chemically bonded to, clay minerals, hydrous oxides, and soil organic matter. The degree of adsorption will depend on soil properties, such as cation exchange capacity. Soil column tests can be conducted with samples of leachate to predict the degree of attenuation, that might be accomplished by adsorption mechanisms.

Attenuation by dilution and dispersion reduces the concentration of contaminants by mixing the contaminants with greater volumes of ground water. The degree of attenuation accomplished this way can he significant but it should be noted that the total quantity or weight of contaminants moving in the ground water system, is not changed by this mechanism; only the concentration is reduced by mixing with mote water. If at some later time it is determined that a contaminant is detrimental to ground water quality the spreading out of the contaminant over a large area may result in it being impractical to recover the contaminant by remedial action measures.

Attenuation through microbial activity is accomplished by the uptake and utilization of organic and inorganic contaminants by the microorganisms in the soil. The microbial activity may affect the pH of the leachate and result in destruction of pathogenic microorganisms. The destruction of many compounds is accomplished by microbial activity.

Volatilization occurs when the migrating compound evaporates and moves into the soil atmosphere above the water table. Only compounds with a low vapor pressure will be volatilized. The compound may eventually migrate to the ground surface and move into the atmosphere. Otherwise the compound may become part of a plume of gases moving away from the landfill through subsurface formations above the water table.

It must be remembered that no one single attenuation mechanism will be effective in removing or reducing leachate chemical contamination. All the mechanisms will be operating to some degree at any landfill site from which leachate is moving trough the base. The mechanisms that are most active and the degree of attenuation accomplished will depend on the leachate soil's physical and chemical properties. Natural attenuation mechanisms may provide sufficient protection of ground water resources so that leachate containment at a landfill is not necessary. However, most regulatory agencies are mandating higher degrees of ground water quality protection, which may be difficult to comply with by natural attenuation. Also, the unpredictable concentrations of leachate chemical constituents, plus weather-related leachate generation surges and variations in subsurface conditions result in it being extremely difficult to predict the degree of protection that natural attenuation will accomplish. Many existing sites were built with the expectation that natural attenuation would prove sufficient to protect groundwater quality. At a significant number of sites, natural attenuation has not provided sufficient protection and groundwater pollution has occurred.
Computer models are available to predict contaminant migration patterns. However, these models still have only limited ability to simulate the chemical reactions. The net result is that almost all new landfills have incorporated into their design means for containing and controlling leachate within the site.

Plan for leachate control during site development

The movement of leachate from the landfilled solid wastes depends upon site circumstances and landfill design features. It is especially important to plan for leachate control during the development of the landfill rather than after the landfill is constructed, since the control techniques are usually employed beneath the waste. To select a site for maximum natural containment of leachate, interrelations between topographical, hydrological, and geological factors must be considered. These factors will be described in the next lesson on site selection.

When constructing a landfill on a site that does not have a suitable subsurface hydrogeology for natural attenuation, the bottom of the landfill can be lined with compacted clay or silt, bentonite, membrane liners, or other rather impermeable materials. This is done to prevent the movement of leachate into the soil beneath the landfill. A prime consideration when installing a liner is its effectiveness in preventing leachate movement. Even the most impermeable liner will not contain leachate if it develops holes, cracks, or breaks.

Extensive research has been done on liner technology. The principal concerns that will be considered in this lesson are hydraulic conductivity of soil liners, layered systems, liner efficiency, geosynthetic membranes, and chemical attack.

The first step in developing a soil liner is to conduct tests determining the physical properties of the soils in the area. Samples are collected with soil boring equipment and analyzed for grain size and other parameters which predict the ability of the soil to perform as a liner. The soils' permeability will control the rate that leachate migrates trough the liner.

Where a high degree of protection is needed, soils such as slow permeability clays are sought for use as a soil liner. Typical permeabilities for soils are shown in Figure 2. Regulatory agencies often require that the soil liner have a permeability of less than 10-7 centimeters per second. In order to achieve final liner permeabilities that are consistently this low, tests must be conducted to determine the optimum moisture content and degree of compaction effort needed during construction of the liner.
The best procedure is to prepare written specifications which describe to the construction contractor the procedure for placing and compacting the soil liner. Usually the liner is several feet thick with the soil being placed in a series of 12-inch-thick lifts. Quality assurance tests should be periodically done during the placement of the soil to ensure the engineering specifications are being' satisfied. More information about inspection and quality assurance during construction will be provided in a later lesson.
Leachate retained by the soil liner will accumulate above the liner unless it is removed. To accomplish removal leachate collection pipes are installed over the liner to remove the leachate. The collection system consists of perforated drainpipe which is covered with coarse gravel or stones. A typical layout is shown in Figure 8. The pipes carry die leachate by gravity to a storage tank from which the leachate is periodically removed for treatment.

Hydraulic calculations show that even a 10w permeability soil will experience some leakage. Liner efficiency can be calculated using soil data, the depth of leachate above the liner, and the hydraulic conductivity of refuse or soil layer such as gravel placed on top of die liner Typical liner efficiency estimations for Wisconsin installations have been 85 percent. Efficiency decreases when leachate collection liners are placed farther apart. The liner efficiency will increase when lower permeability soils are employed as liners and when the depth of accumulate leachate above die liner is minimized. Placing highly permeable soils immediately above die liner also will increase efficiency by providing a less restrictive pathway for leachate to flow toward collection lines.

When soil conditions are such that naturally occurring or imported soils such as bentonite are unavailable or uneconomical, geosynthetic liners can be employed to control leachate movement. A wide variety of geosynthetics is available for use in containing different types of liquid wastes. Considerations when using geosynthetics under landfills include providing a firm base under the liner, construction quality assurance, and protection of die liner after construction.

A concern when relying on liners is that chemical interactions may destroy the liner's integrity. Certain waste materials have been known to degrade a particular type of liner. There is concern that properties of certain clays may be changed if attacked by the chemicals present in some leachates. However research has shown that under normal conditions most clays will not undergo significant detrimental changes when subjected to municipal solid waste leachate exposure. In fact, reductions in hydraulic conductivity have been observed. Short duration testing of geosynthetic liners has shown that many materials can resist chemical attack under most conditions. The recommended design procedure for both soil and geosynthetic liners is to consult the results of compatibility tests if available, or to conduct short durations tests with liner samples and leachate generated by the waste. Since leachate production is a long-term concern, liners will need to be tested over a long period of time to de-termine their ultimate suitability.

Some landfills are constructed in fine textured soils which have shallow depths to the water table. The low permeability of these soils will often allow a landfill operator to excavate below the water table and deposit it the solid wastes before the ground water re-enters the area. State regulations may require the installation of a leachate collection system at the base of the zone of saturation landfill. A typical layout is shown in Figure 9. This type of collection system will recover leachate mixed with a quantity of ground water. This increases the volume of liquid requiring treatment and disposal, but soil conditions in some regions necessitate landfilling in the zone of saturation.

Leachate treatment processes

Leachate treatment options include recycling, on-site treatment, discharge to a municipal sewage treatment plant, or a combination of these approaches.

Leachate recycling is accomplished by collecting leachate at die base of the landfill and redistributing it over the top of the waste. Research both supporting and discouraging this approach can be cited. Recent experience, however, has shown that recycling can significantly reduce leachate chemical concentrations, even out the flow of leachate that must be removed from die landfill for further treatment, and possibly enhance die stabilization of die landfill. Several states have modified regulations to encourage research on leachate recycling. The principal objective of these efforts is to discover procedures which will result in the landfilled waste being decomposed more quickly, thereby releasing gas for energy recovery and shortening the duration of long-term monitoring and maintenance.
On-site treatment has been attempted with pond systems, conventional treatment plants, anaerobic treatment processes, and physical-chemical units. When compared to either municipal and most industrial wastewaters, leachate is a very high strength waste. Consequently die treatment process must he carefully developed to guarantee a successful system.

The most commonly used leachate treatment option is discharge to municipal sewage treatment plants. Since leachate strengths are significantly greater than normal municipal wastewaters, care must be taken to avoid 'overloading the plant. Studies have shown that a 5 percent loading of a sewage treatment plant with leachate will not disrupt its operation, but greater loadings may be detrimental. 1

Lesson Assigments

1. List at least four factors which influence die leachate generation rate.

2. Why is leachate control necessary?

3. List at least four conditions that will influence leachate chemical characteristics.

4. Propose at least two ways for controlling leachate produced in die landfills.

References

Diaz, LF., G.M. Savage and C.G.Golueke "Resource Recovery from Municipal Solid
Wastes", Volume II, Final Processing, CRC Press, lnc. 1982.

Ehrig. H.J., "Water and Element Balances of Landfills", in Lecture Notes in Earth
Sciences, The Landfill, P. Baccini, editor. p. 83-115, Springer-Verlag Press, 175 Fifth
Avenue, New York, NY 100910 (1989).

Farquhar, G.J., "Leachate Production and Characterization." in Can. J. Civ. Eng., 16, pp. 317-325, (1989).

Griffin, R.A. and N.F. Shimp. "Leachate Migration Through Selected Clays", in "Gas and Leachate from Landfills", Proc. Res. Symp., Rutgers University, New Brunswick. N.J., EPA-600/9-75-004, U.S. EPA, March 1975.

"Ground Water Quality Standards," Wisconsin Department of Natural Resources, 1988.

"The Hydrologic Evaluation of Landfill Performance (HELP) Model, Volume 1. User's
Guide for Version 1. "United States Environmental Protection Agency, EPA/530/SW-84-009, June 1984.

Kmet, Peter and Paul M. McGinley. "Chemical Characteristics of Leachate from Municipal
Solid Waste Landfills in Wisconsin." Presented at the Fifth Annual Madison Conference of
Apofied Research and Practice on Municipal & Industrial Waste, Madison, Wisconsin.
September 22-24, 1982.

"Procedures Manual for Ground Water Monitoring at 50111 Waste Disposal Facilities," "A Current Report on Solid Waste Management," "United States Environmental Protection Agency. EPA/530-SW-611, August 1977.

Date upgrated May/29/98
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