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ameO '-• TPHd at less than 1.0, and benzene at 0.78 mg/kg). With the exception of SB-1, where TPHg was detected at 120 mg/kg (46 feet bgs), only minor detections of petroleum hydrocarbons were found in the other borings. Although the results of these investigations have bounded v the limits of impacted soil in the source area, the exact location of the remaining petroleum hydrocarbon mass and possibly the highest petroleum hydrocarbon concentrations has not been defined. Between 1992 and 1996, a total of 12 groundwater monitoring wells (MW-1 through MW-12) and two vapor extraction wells (VW-1 and VW-2)were installed at the Site and have been used to monitor petroleum hydrocarbon concentrations in groundwater. Historic groundwater analytical results and elevation data are provided in Appendix B. The highest petroleum hydrocarbon concentrations in groundwater have historically been detected in MW-2 and VW-2. Between 2005 and 2007, the groundwater elevation rose approximately 10 feet, resulting in a condition where the entire screened portion of most of the wells (excluding VW-1 and VW-2) became submerged below the elevation of the water table surface. As is most evident in MW-2, this submerged well screen condition resulted in a measured large decrease in petroleum hydrocarbon concentrations in groundwater. Because petroleum hydrocarbons saveae &o are lighter than water, they will generally move upward to near the surface of the water table, w making samples collected from submerged wells screens unrepresentative of actual aquifer concentrations. Between 2008 and 2009, Caltrans installed 7 new groundwater monitoring wells (MW-13 through MW-19)to determine representative petroleum hydrocarbon concentrations in groundwater(Figure 2). Groundwater monitoring results from the new wells (Appendix B) indicate the petroleum hydrocarbons concentrations are higher than those from wells with submerged screens. The results also indicate that the lateral and vertical extent of the groundwater plume is defined within the property boundary of the Site. v.+ 2.3 SITE LITHOLOGY AND HYDROGEOLOGY The Site is located within the Tracy (Stockton) Fault Zone (Wagner et al, 1991). Surficial soils are generally comprised of Quaternary-aged unconsolidated sediments that were deposited during flood stages of major streams in areas between natural stream levees and alluvial fans (Weir, 1952). The surface of the Site is paved with 4 to 6 inches of asphalt concrete. Beneath the pavement, alluvial deposits consisting of interbedded silty clay, clayey silt, silty sand, sand and clayey sand were encountered to the total depth explored (approximately 80 feet) (Geocon, 2009a). The alluvium is generally a clayey silt and silty sand with lenses of silty clay and sandy clay encountered at depths between 3 and 25 feet below ground surface (bgs). Soils comprised of clayey sand and silty sand with lenses of sandy gravel, sand and clay were AMEC Geomatrix, Inc. P:\Project\140003\14443_Caltrans\TO 10\11000_PLAWAS WP_March 2010\AS WP.doc 3 V