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Sulfolane Remedial Investigation Worts Plan ARCADIS Former Occidental Chemical Corporation ' Lathrop, California • Collect additional soil data in the vicinity of well cluster PW26 where the highest ' concentrations of sulfolane were previously identified.The objective of the investigation will be to identify residual soil which exceeds the RWQCB approved site-specific soil cleanup level of 8.6 mg/kg. ' • Collect additional lithologic and shallow groundwater data in the vicinity of PW26 to assess the horizontal and vertical extent(to a depth of approximately 70 to ' 90 feet below ground surface[bgs])of the sulfolane affected source area.This data will be used to establish a basis for the design and expansion of the current Remedial Program in the shallow groundwater zone.This data will also ' be used to till data gaps created by the previous limited investigation and well installation activities above 70 feet bgs. 2.. Site Background 2.1 Geology Regional.The Site is located in the Great Valley Geomorphic Province(California . Geological Survey 2002). The Great Valley trends generally north-south and is ' bordered by the Coast Ranges to the west and by the Sierra Nevada to the east. The Great Valley is an asymmetrical synclinal trough about 50 miles wide and 400 miles long with its axis off center to the west. Sediments have been deposited in this trough almost continuously since the Jurassic Period(about 160 million years ago). Lithology of the Great Valley consists of alluvial sediments known as the Great Valley Sequence,which is comprised of interbedded sedimentary units.The thickness of the ' Great Valley Sequence exceeds 25,000 feet in some areas. These sediments rest on a basement complex of metamorphic and igneous rocks.The surficial deposits are primarily eroded rock from the Sierra Nevada to the east and the Coast Range to the west. Because the synclinal trough was previously under water,some of the sediments are of marine origin.A variety of unconsolidated Quaternary alluvial deposits have been mapped in the site area;these represent active and inactive river and stream ' channels deposits,flood plain deposits,and alluvial outwash deposits. Local.The vadose zone in the vicinity of well PW26 is composed of fine grained silty ' sands and sandy silts from the surface to 7 feet bgs, silty sand from approximately 7 to 11 feet bgs,and sand with silt,or fine to medium grained sand from 11 to 14 feet bgs. In 2008,groundwater was first encountered at approximately 14 feet bgs(CRA 2008). LFU ri, k* a=ICIM 3