water entering the landfill cell from above. the moisture in the solid waste. the moisture in the cover aterial, and the moisture in the sludge, if the disposal of sludge is allowed The principal sinks are the water leaving the landfill as part of the landfill gas (i.e, water used in the formation of the gas),as saturated water vapor in the landfill gas, and as leachate. Each of these components 'is considered below. Water entering from above. For the upper layer of the landfill, the water from above corresponds to the precipitation that has percolated through the cover material. For the layers below the upper layer, water from above corresponds to the water that has percolated through the solid waste above the layer in question. One of the most critical aspects in the preparation of a water balance for a landfill is to determine the amount of the rainfall that actually percolates through the landfill cover layer. Where a geomembrane is not used, the amount of rainfall that percolates through the landfill cover can be determined using the Hydrologic Evaluation of Landfill Performance(HELP)model Water entering in solid waste. Water entering the landfill with the waste materials is that moisture inherent in the waste material as well as moisture that has been absorbed from the atmosphere or from rainfall(where the storage containers are not sealed properly). In dry climates, some of the inherent moisture contained in the waste can be lost, depending on the conditions of the storage. The moisture content of residential and commercial MSW is about 20 percent, as reported in Table 15-1. However, conduct a series of tests during the wet and dry period (g the wet and dry seasons, it may be necessary to because of the variability of the moisture content durin Water entering in cover material. The amount of water entering with the cover material will depend on the type and source of the cover material and the season of the year. The maximum amount of moisture that can be contained in the cover material is defined by the field capacity(FC) of the material, that is, the liquid which remains in the pore space subject to the pull of gravity. Typical values for soils range from 6-12 percent for sand to 23-31 percent for clay loams Water leaving from below. Water leaving from the bottom of the first cell of the landfill is termed leachate. As noted previously, water leaving the bottom of the second and subsequent cells corresponds to the water entering from above for the cell below the cell in question. In landfills where intermediate leachate collection systems are used, water leaving from the bottom of the cell placed directly over the intermediate leachate collection system is also termed leachate In general, the quantity of leachate is a direct function of the amount of external water entering the landfill. In fact, if a landfill is constructed properly the production of measurable quantities of leachate can be eliminated. When wastewater treatment plant sludge is added to solid wastes to increase the amount of methane produced, leachate control facilities must be provided. In some cases leachate treatment facilities may also be required Fate of Constituents in Leachate in Subsurface Migration The major concern with the movement of leachate into the subsurface aquifer below unlined and lined landfills is the fate of the constituents found in leachate Mechanisms that are operative in the attenuation of the constituents found in leachate as the leachate migrates through the subsurface soil include mechanical filtration, precipitation and coprecipitation. sorption(including ion exchange).gaseous exchange, dilution and dispersion, and microbial activity. The fate of heavy metals and trace organics, the two constituents of greatest interest, is considered in the following diss eactions as leachate travels through the soil while trace organics are removed primarily by adsorption The ability of a soil to retain the heavy metals found in leachate is a function of the cation exchange capacity(CEC) of the soil. The uptake and release of positively charged ions by a soil is referred to as cation, or base, exchange. The total CEC of a soil is defined as the number of milliequivalents(meq) of cations that 100 grams of soil will adsorb. The CEC of a soil depends on the amount of mineral and organic colloidal matter present in the soil matrix. Typical CEC values, at a pH value of 7, are 100 to 200 meq/100 g for organic colloids, 40 to 80 meq/100 g for 2: 1 clays(montmorillonite minerals), and 5 to 20 meq/100 g for 1: I clays (kaolinite minerals). The reported CEC values are affected by the pH of the solution; they drop to about 10 percent of the given The capacity of a clay landfill liner to take up heavy metals can be estimated as follows. Assume the CEC of the liner material is 100 meq/100 g. If the density of the clay material used in the liner is 137 Ib/ft(specific gravity equals 2.2), then about 3000 meq of cations can be adsorbed per cubic foot of liner material. Using a typical value of 20 mg/meq for the heavy metals, the amount of metal that could10 water entering the landfill cell from above, the moisture in the solid waste, the moisture in the cover material, and the moisture in the sludge, if the disposal of sludge is allowed. The principal sinks are the water leaving the landfill as part of the landfill gas (i.e., water used in the formation of the gas), as saturated water vapor in the landfill gas, and as leachate. Each of these components 'is considered below. Water entering from above. For the upper layer of the landfill, the water from above corresponds to the precipitation that has percolated through the cover material. For the layers below the upper layer, water from above corresponds to the water that has percolated through the solid waste above the layer in question. One of the most critical aspects in the preparation of a water balance for a landfill is to determine the amount of the rainfall that actually percolates through the landfill cover layer. Where a geomembrane is not used, the amount of rainfall that percolates through the landfill cover can be determined using the Hydrologic Evaluation of Landfill Performance (HELP) model. Water entering in solid waste. Water entering the landfill with the waste materials is that moisture inherent in the waste material as well as moisture that has been absorbed from the atmosphere or from rainfall (where the storage containers are not sealed properly). In dry climates, some of the inherent moisture contained in the waste can be lost, depending on the conditions of the storage. The moisture content of residential and commercial MSW is about 20 percent, as reported in Table 15-1. However, because of the variability of the moisture content during the wet and dry seasons, it may be necessary to conduct a series of tests during the wet and dry periods. Water entering in cover material. The amount of water entering with the cover material will depend on the type and source of the cover material and the season of the year. The maximum amount of moisture that can be contained in the cover material is defined by the field capacity (FC) of the material, that is, the liquid which remains in the pore space subject to the pull of gravity. Typical values for soils range from 6-12 percent for sand to 23-31 percent for clay loams. Water leaving from below. Water leaving from the bottom of the first cell of the landfill is termed leachate. As noted previously, water leaving the bottom of the second and subsequent cells corresponds to the water entering from above for the cell below the cell in question. In landfills where intermediate leachate collection systems are used, water leaving from the bottom of the cell placed directly over the intermediate leachate collection system is also termed leachate. In general, the quantity of leachate is a direct function of the amount of external water entering the landfill. In fact, if a landfill is constructed properly the production of measurable quantities of leachate can be eliminated. When wastewater treatment plant sludge is added to solid wastes to increase the amount of methane produced, leachate control facilities must be provided. In some cases leachate treatment facilities may also be required. Fate of Constituents in Leachate in Subsurface Migration The major concern with the movement of leachate into the subsurface aquifer below unlined and lined landfills is the fate of the constituents found in leachate. Mechanisms that are operative in the attenuation of the constituents found in leachate as the leachate migrates through the subsurface soil include mechanical filtration, precipitation and coprecipitation, sorption (including ion exchange), gaseous exchange, dilution and dispersion, and microbial activity. The fate of heavy metals and trace organics, the two constituents of greatest interest, is considered in the following discussion. Heavy Metals. In general, heavy metals are removed by ion exchange reactions as leachate travels through the soil while trace organics are removed primarily by adsorption. The ability of a soil to retain the heavy metals found in leachate is a function of the cation exchange capacity (CEC) of the soil. The uptake and release of positively charged ions by a soil is referred to as cation, or base, exchange. The total CEC of a soil is defined as the number of milliequivalents (meq) of cations that 100 grams of soil will adsorb. The CEC of a soil depends on the amount of mineral and organic colloidal matter present in the soil matrix. Typical CEC values, at a pH value of 7, are 100 to 200 meq/100 g for organic colloids, 40 to 80 meq/100 g for 2:1 clays (montmorillonite minerals), and 5 to 20 meq/100 g for 1:1 clays (kaolinite minerals). The reported CEC values are affected by the pH of the solution; they drop to about 10 percent of the given values at a pH value of 4. As noted previously, the presence of carbon dioxide in the bottom of a landfill will tend to lower the pH of the leachate. The capacity of a clay landfill liner to take up heavy metals can be estimated as follows. Assume the CEC of the liner material is 100 meq/100 g. If the density of the clay material used in the liner is 137 1b/ft3 (specific gravity equals 2.2), then about 3000 meq of cations can be adsorbed per cubic foot of liner material. Using a typical value of 20 mg/meq for the heavy metals, the amount of metal that could