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C would be applied on the liner for a period of 200 years. The results of these analyses indicate that the vadose zone soils provide an efficient buffer to a leachate release. Based on the model simulations, leachate migration to groundwater at measurable concentrations would take more than 7000 years, and at its peak, would result in statistically insignificant changes to groundwater chemistry — even when the anticipated leakage rate is increased by a factor of ten. Considering these results and the assumptions integrated in the analyses (i.e., one -foot of leachate head for 200 years, no biodegradation or adsorption), any changes in groundwater chemistry releated to liner leakage at the Forward Landfill would be non- detectable. Groundwater below the Forward site occurs within coarser sections of the Victor Formation. The upper Victor formation is typically considered an unconfined aquifer, but locally exhibits a slightly confined condition associated with laterally discontinuous clay units. Analyses completed by Herst & Associates / Andrews Environmental Engineering (HA/AEE, 2001) indicate that the hydraulic conductivity of coarse-grained aquifers beneath the site are relatively high. Groundwater level fluctuate seasonally by approximately 5 to 10 feet due to a distinct rainy season and dry season and have declined over the last 20 to 30 years by approximately 10 feet due to agricultural pumping demand and drought (EMCON, 1972 and Kleinfelder, 1992). The unconfined aquifers are recharged by rainfall and local stream infiltration. The sedimentary formations underlying the Victor formation have many productive confined aquifers. Rivers, reservoirs and surface runoff along the slopes of the western Sierra Nevada Foothills recharge the deeper confined aquifers. In order to develop adequate flow, most of the agricultural irrigation wells in the area are at least 500 feet and screened in deeper confined aquifers that are separated from the first water system by a significant thickness of low permeability soil. In addition to providing a significant level of protection to the deeper regional aquifers, the natural characteristics of the generally fine grained Victor Formation soils are also expected to facilitate natural dilution and attenuation in the unlikely event that landfill impacts were to escape the single composite liner system and reach groundwater. In fact, even when leachate leakage is mixed directly to groundwater, the high permeability of water -flowing zones are likely to result in statistically insignificant (essentially non- detectable) impacts to downgradient groundwater quality (Appendix A). 5.0 EXISTING RELEASE CHARACTERISTICS Further substantiation of the inference that potential liquid phase release from a single composite liner constructed at the Forward Landfill will effectively attenuate before creating a risk to groundwater can be found in the nature of known groundwater impacts at the existing Austin Road Landfills. In September 2000, the RWQCB requested that Forward assess the source and potential impacts of low-level VOC concentrations in the groundwater adjacent to the landfill. The CO2000.048Torwsrd Composite Liner V3 .doc\09/19/02 52 Geologic Associates