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0 overlying vegetative soil layer. In 2001, a landfill gas extraction system was installed to mitigate landfill gas migration. The original landfill gas extraction system included 39 gas extraction wells connected by header lines to a flare at the southern end of the site. In 2008, five additional landfill gas extraction wells were added to the system along the northeastern edge of the refuse prism. The County recently submitted a proposal to the RWQCB to extend the closure cover iover approximately 1.3 acres of waste outside of the northeastern edge of the main landfill, including portions of adjacent Caltrans property. This proposal includes installing two additional landfill gas extraction wells into refuse on adjacent Caltrans property. 3.2 GEOLOGIC SETTING Past drilling and geologic investigations at the site suggest that the CHSL is underlain by recent alluvium, Quaternary stream terrace deposits, and relatively unconsolidated sedimentary rock of the Plio-Pleistocene age Corcoran Formation. Lithologies within these three units are similar, and include assemblages of clays, silts, sands, and gravels. The Corcoran Clay member of the Corcoran Formation (also referred to as the Turlock Formation in some literature) has been identified in exploratory boreholes drilled around the northern third of the CHSL. The Corcoran Formation is characterized as moderately to highly plastic clays, and mixtures of clay with silts, sands, and gravels. Regionally, the presence of significant clays in the Corcoran Formation allows water to perch on it, though historical groundwater investigations at the site indicate that the shallow groundwater is locally perched on or entrained within these clay units at the ■ site. 3.3 HYDROGEOLOGIC SETTING Two aquifers have been identified at the site. An upper aquifer that has been identified in the northwestern corner of landfill, wherein groundwater is found on or within clays of the Corcoran Formation at depths ranging from 12 to 65 feet below the ground surface, and a deep aquifer that occurs at depths of 300 to 350 feet (corresponding to elevations ranging from -45 to -57 feet relative to mean sea level). The deep aquifer is approximately 250 feet deeper than the shallow aquifer. Groundwater flows in the deep aquifer to the northeast. Nine wells and one piezometer have been constructed at the site to monitor the two aquifers, though only six wells are presently used for groundwater monitoring as a result of decreasing water levels (Figure 2). Monitoring well construction information is summarized in the following table: r EMP Report — Phase IGeo-Loic Page ■ s Corral Hollow Sanitary Landfill A S s o c I AT E sg February 1, 2012