be adsorbed per cubic foot is equal to 60 g. If the concentration of heavy metals in the leachate was 100 mg/L, the heavy metals could be removed from about 600 L of leachate. If the permeability of the clay is equal to 1 x 10-7 cm/s, then 2.83 L would pass through 1 ft- each year. At this rate of percolation, it would take 2 12 years to saturate the original ft of clay. If the amount of leachate allowed to percolate through the liner were limited to one tenth of that value by designing the leachate collection system properly, then the time required lo saturate the ft of clay would be approximately 2000 years. Even with ail of the simplifying assumptions that went into the above analysis, it can be concluded that with a properly designed landfill cover and clay liner, heavy metals should not pose a problem Trace Organics. Adsorption is the most common way in which the organic constituents in leachate removed as it moves through a porous medium. If hydrodynamic dispersion is neglected, the materials balance for a contaminant subject to adsorption m a groundwater aquifer is given by the following modified Form of Eq (15-2) aS pb dC 15-2 where S= mass of solute sorbed per unit mass of dry soil, g/g C concentration of contaminant in the liquid phase g/m3 v,= average fluid velocity in z direction m/s The mass of material sorbed per unit mass of dry soil is related to the concentration of the contaminant in the liquid phase and the soil distribution coefficient, as described m the following equation S=KsD×C where Ksd=soil distribution coefficient, m:/g Retardation of the organic constituents found in leachate is important because the retained material can be subjected to biological and chemical conversion reactions, in some cases rendering the retained material hannless Control of leachate in landfills As leachate percolates through the underlying strata, many of the chemical and biological constituents originally contained in it will be removed by the filtering and adsorptive action of the material composing the strata. In general, the extent of this action depends on the characteristics of the soil especially the clay content. Because of the potential risk involved in allowing leachate to percolate to the groundwater, best practice calls for its elimination or containment. Landfill liners are now commonly used to limit or eliminate the movement of leachate and landfill gases from the landfill site. To date (1992), the use of clay as a liner material has been the favored method of reducing or eliminating the seepage(percolation) of leachate from landfills. Clay is favored for its ability to adsorb and retain many of the chemical constituents found in leachate and for its resistance to the flow of leachate. However the use of combination composite geomembrane and clay liners is gaining in popularity, especially because of the resistance afforded by geomembranes to the movement of both leachate and landfill gases. The characteristics, advantages, and disadvantages of the geomembrane liners(also known as flexible membrane liners, FMLs) that have been used for MSw landfills are summarized in Table 15-3. Typical pecifications for geomembrane liners are given in Table 15-411 be adsorbed per cubic foot is equal to 60 g. If the concentration of heavy metals in the leachate was 100 mg/L, the heavy metals could be removed from about 600 L of leachate. If the permeability of the clay is equal to 1 × 10~7 cm/s, then 2.83 L would pass through 1 ft2 each year. At this rate of percolation, it would take 212 years to saturate the original ft3 of clay. If the amount of leachate allowed to percolate through the liner were limited to one tenth of that value by designing the leachate collection system properly, then the time required lo saturate the ft3 of clay would be approximately 2000 years. Even with ail of the simplifying assumptions that went into the above analysis, it can be concluded that with a properly designed landfill cover and clay liner, heavy metals should not pose a problem. Trace Organics. Adsorption is the most common way in which the organic constituents in leachate are removed as it moves through a porous medium. If hydrodynamic dispersion is neglected, the materials balance for a contaminant subject to adsorption m a groundwater aquifer is given by the following modified Form of Eq. (15-2): The mass of material sorbed per unit mass of dry soil is related to the concentration of the contaminant in the liquid phase and the soil distribution coefficient, as described m the following equation: S = KSD×C (15-3) where Ksd = soil distribution coefficient, m3 /g Retardation of the organic constituents found in leachate is important because the retained material can be subjected to biological and chemical conversion reactions, in some cases rendering the retained material hannless. Control of Leachate in Landfills As leachate percolates through the underlying strata, many of the chemical and biological constituents originally contained in it will be removed by the filtering and adsorptive action of the material composing the strata. In general, the extent of this action depends on the characteristics of the soil, especially the clay content. Because of the potential risk involved in allowing leachate to percolate to the groundwater, best practice calls for its elimination or containment. Landfill liners are now commonly used to limit or eliminate the movement of leachate and landfill gases from the landfill site. To date (1992), the use of clay as a liner material has been the favored method of reducing or eliminating the seepage (percolation) of leachate from landfills. Clay is favored for its ability to adsorb and retain many of the chemical constituents found in leachate and for its resistance to the flow of leachate. However, the use of combination composite geomembrane and clay liners is gaining in popularity, especially because of the resistance afforded by geomembranes to the movement of both leachate and landfill gases. The characteristics, advantages, and disadvantages of the geomembrane liners (also known as flexible membrane liners, FMLs) that have been used for MSW landfills are summarized in Table 15-3. Typical specifications for geomembrane liners are given in Table 15-4. 15-2