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C: Natural Liner Systems, " Department of Civil Engineering, University of Texas, Austin, it states that "No detrimental effects were observed when a wide range of actual and simulated landfill leachates were passed through specimens of natural earthen liner materials." Attenuation Characteristics of Compacted Clay Liners - Natural attenuation refers to a reduction in concentration as contaminants migrate through an environmental media. 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). Review of professional papers by Shackelford (1993, 1998) reveals that transport of contaminants in coarse-grained, relatively high-flow situations is primarily controlled by advection, while diffusion is the dominant contaminant transport mechanism through low -permeability soils and barriers (Shackelford and Rowe, 1998). Diffusion of contaminants through a barrier (either natural or man made — such as a clay liner) is a function of the concentration gradient between the contaminant source (such as leachate) and unaffected media, the thickness of the barrier, and the availability of a consistent contaminant source. With regard to potential inorganic constituents, the compacted clay liner is ideally suited for degradation of leachate. Clay is favored in liner construction because of its resistance to flow (low permeability) but it also has an inherent ability to adsorb and retain many of the inorganic chemical constituents found in leachate. In general, heavy metals are removed by ion exchange reactions as leachate travels through soils. Tchobanoglous et al. (1993) demonstrate that the cation exchange capacity of typical clay liners are more than adequate to remove metals from vertically migrating leachate. As indicated in Appendix A, the time required for leachate under one foot of head to migrate through the clay (in an unsaturated condition) is approximately 9 years and at this time the concentration of inorganic constituents is anticipated to be less than 1% of their initial concentration. Trace organics can also be removed within the compacted clay liner, primarily by adsorption and biodegradation. Non-destructive processes that result only in the reduction of VOC contaminant concentrations, but not of the total contaminant mass, include dispersion, sorption, dilution, and volatilization. Destructive processes that result in the reduction of contaminant concentrations (and mass) include biodegradation and abiotic degradation mechanisms. Microbial degradation of VOCs is typically accomplished using organic carbon as the primary substrate nutrient source. Since landfill leachate typically contains high concentrations of organic carbon (reflecting the abundance of organic wastes that are commonly placed in landfills) and considering the high concentrations of organic constituents in wastes, it is unlikely that nutrient sources could become sufficiently depleted to affect microbial degradation rates. As described in Appendix A, a series of analyses were submitted earlier to the CVRWQCB to evaluate the fate of VOCs within a CCL at the Forward Landfill. While these analyses integrated conservative assumptions regarding VOC concentrations in leachate, leachate head on the liner, and potential biodegradation rates during leachate transport through the liner, in the absence of peer-reviewed field demonstrations, CAMG-04fforWerd Composite Uner V3.doe\09/19102 IM Geologic Associates