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0 t BACKGROUN t FORMATION2.1 LEACHATE • When water percolates through solid wastes that are undergoing decomposition, both biological materials and chemical constituents are leached into solution. While the chemical composition of leachate varies greatly depending on the nature of the wastes and the age of the landfill, leachates from "mature" landfills (e.g. > 10 years) typically have lower chemical concentrations than leachate derived from younger wastes (Tchobanoglous et al., 1993). Tables 1 and 2 list the median concentrations of select inorganic and organic constituents that have been identified in leachate by the U.S. EPA (1986) together with the average leachate chemistry for leachate samples collected at the Forward Landfill. Table 3 summarizes the average leachate chemistry at the Forward Landfill during the past 5 years. As more communities and operators of landfills institute programs to limit the disposal of hazardous wastes within municipal solid waste landfills, the quality of leachate within landfills has improved with respect to the presence of fewer VOCs. The Forward Landfill is a mature landfill and historically has had relatively benign leachate chemistry (Tables 1 -3). With the waste reduction and load check programs that are now in place at the facility, the potential for VOC production in leachate is expected to be even further reduced, and the quality of leachate within the planned landfill expansion area is expected to be even more benign. 2.3 LEACHATE ATTENUATION As leachate percolates through soils, many of the chemical and biological constituents that were originally contained in it are removed by filtering, adsorptive and biodegradation processes. Clay is favored in liner construction because of its resistance to flow (low permeability) and its ability to adsorb and retain many of the chemical constituents found in leachate. The clay source anticipated for construction of the Forward Landfill is a local one, associated with shallow -gradient fluvial processes that typically result in deposition of soils with a moderate to high organic carbon content. Since similarly clayey soils underlie the proposed expansion areas to a depth of at least 17 feet, it is expected that both the clay liner and the vadose zone soils would reduce leachate concentrations by filtering, adsorptive and biodegradation processes. Natural attenuation refers to the reduction in contaminant concentrations as contaminants migrate through an environmental media. In groundwater, this reduction is due to a number of fate and transport processes including: simple dilution, dispersion, sorption, volatilization, and biotic and abiotic transformations Wiediemeier et al. (1999). In recent years, natural attenuation processes have become recognized as important processes for reduction of VOC concentrations in soils and groundwater. Studies C:\2002-002\Revised Appx A - Leakageldec\09/19102 Geologic Associates