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Sulfolane Remedial , <br /> Investigation Work Plan <br /> Ca ARCADIS Former Occidental Chemical Corporation <br /> Lathrop, California ' <br /> 2.2 Hydrogeology <br /> Regional.The Site is located within the Eastern San Joaquin Sub-basin of the San , <br /> Joaquin River Valley Groundwater Basin as recognized by the California Department <br /> of Water Resources(DWR 2006).Groundwater-bearing formations in the vicinity of the ' <br /> Site consist of the Alluvium and Modesto/Riverbank Formations, Flood Basin Deposits, <br /> Laguna Formation, and Mehrten Formation of Pleistocene to Late Miocene age <br /> alluvium. ' <br /> Local. Based on previous investigations,first encountered groundwater occurs <br /> between approximately 14 and 18 feet bgs.The unconfined aquifer system below the ' <br /> Site has previously been classified as a loosely coupled aquifer with three production <br /> zones. The three identified zones are referred to as the shallow(between 31 and 83 - <br /> feet bgs), intermediate(between 84 and 150 feet bgs),and deep(between 151 and , <br /> 218 feet bgs)zones(CRA 2008). <br /> Below the deep zone at depths greater than 300 feet lies the Corcoran Clay.The <br /> Corcoran Clay is a regionally extensive layer of impermeable clay that limits the vertical <br /> extent of the unconfined aquifer system at the Site. <br /> Groundwater elevations in the vicinity of PW26 during the December 14, 2010 <br /> sampling event were 0.49 feet below mean sea level(msl; PW26)in the shallow zone, <br /> 0.29 feet above msl(PW26)in the intermediate zone,0.97 feet below msl(PW 18)and ' <br /> 0.18 feet below msl(PW26)in the deep zone(CRA 2011). Shallow, intermediate,and <br /> deep groundwater flow directions in this area of the Site during the December 14,2010 <br /> sampling event were to the north-northwest towards extraction well EW-8a(shallow , <br /> and intermediate zone extraction)and EW-8b(deep zone extraction).Well EW-8a is <br /> located approximately 180 feet north-northwest of well cluster PW26. <br /> 3. Summary of Historic Investigations and Potential Data Gaps <br /> A summary of the PW-18 Vadose Zone Soil Investigation(CRA 2008)and sampling <br /> results in the vicinity of well PW26 along with an evaluation of potential data gaps are ' <br /> presented below. <br /> 3.1 2008 Soil Investigation ' <br /> During the 2008 investigation, 31 soii borings(SB-1 through SB-31)were advanced <br /> within the vadose zone to a depth slightly above the groundwater table for collection of ' <br /> 10 <br />