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additional landfill gas extraction wells were added to the system along the northeastern edge of the refuse prism. In accordance with the Water Code Section 13627 Order issued by the RWQCB on July 7, 2011, the County recently submitted a proposal to the RWQCB to extend the closure cover over approximately 1.3 acres of waste outside of the northeastern edge of the main landfill, including portions on adjacent Caltrans property. The proposed design includes installing two additional landfill gas extraction wells into refuse on the Caltrans property. 2.2 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 Tulare Formation in some literature) has been identified in exploratory boreholes and monitoring wells drilled around the northern third of the CHSL. Engineers visiting the site during landfill construction also reported observing the Corcoran Clay on the northeastern edge of the landfill. Logs of well borings indicate that the top of the Corcoran Clay dips to the north-northeast and the unit has a thickness ranging from about 20 to 40 feet. 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. 2.3 HYDROGEOLOGIC SETTING Two aquifers have been identified at the site. A shallow aquifer is present in the northwestern corner of landfill, wherein groundwater is found on or within clays of the Corcoran Formation at depths ranging from 12 to 80 feet below the ground surface. A deep aquifer 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 separated by the shallow aquifer by approximately 250 feet of unsaturated soils. Historical groundwater elevation data suggest that groundwater in the shallow aquifer flows to the northwest and northeast, while groundwater in the deep aquifer flows to the northeast. Engineering Feasibility Study Geo-Logic Page 1114 Corral Hollow Sanitary Landfill ASSOCIATES January 25,2013