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1 ranging from approximately 13 to 41 feet bgs Groundwater has risen approximately 25 feet over time at the site, from -30 55 feet (40 70 feet bgs) in October 1992 to -5 24 feet (12 72 feet ' bgs in June 2001 This general rise in groundwater over time is graphically presented on the hydrograph for MW 10 (Appendix C), which presents well construction details and groundwater elevations over time Data for the groundwater elevation during the period from 1970 to 1986 ' was inferred from several groundwater monitoring stations in the Stockton area (Figure 7) During this time period, groundwater elevations varied from 0 feet in the early 1970s to approximately-20 feet in 1978, and back to 0 feet in the mid-1980s 1 The historical onsite groundwater elevation high and low points are also presented on the ' geologic cross-sections (Figures 5a-5d), as are the analytical results for soil samples collected during past hydrogeologic investigations As presented on these cross-sections, petroleum hydrocarbons occur in soil at the site between approximately 20 and 46 feet bgs Currently the groundwater is at approximately 19 feet bgs, which means that these petroleum hydrocarbons entrapped in soil at the site are submerged This issue, coupled with soil types encountered at the site, will be discussed in greater detail in other sections of this report as it relates to past and ' future remedial efforts at the site 2.4 DISTRIBUTION OF HYDROCARBONS IN SOIL ' A majority of the lateral and vertical extent of hydrocarbons detected in soil samples are beneath and immediately downgradient (northeast) of the former USTs and former pump islands This lie area is graphically depicted in Figure 8 The location, depth, and analytical results of all soil samples collected during this investigation are also presented in Figure 8, and appended in Appendix B This area identified in Figure 8 approximates the area of soils containing ' hydrocarbons at residual saturation, or areas in which hydrocarbons are entrapped in pore space within the soil matrix The relationship between the historically highest and lowest groundwater elevations and the Iocation and concentration of hydrocarbons detected in soil samples are also ' presented on the hydrogeologic cross-sections (Figures 5a-5d) The highest concentrations of TPH-g were detected in soil samples collected from beneath the ' former tank field in well V4, at depths ranging from 20 to 40 feet bgs at concentrations ranging from 2,100 to 12,000 mg/kg Approximately 200 feet northeast of and downgradient of V4, TPH-g was detected at a concentration of 130 mg/kg in a soil sample collected at 40 feet bgs in ' RW4 Soil samples collected at 30 feet bgs from wells IW 1 and P2, installed upgradient of the former tank field and in the general vicinity of former dispenser islands, contained 530 and 210 mg/kg TPH-g, respectively ' It is assumed that the hydrocarbon mass remaining in the current vadose zone (0-20 feet bgs), is negligible relative to hydrocarbon mass below groundwater because of previous excavation ' efforts to 20 feet bgs during UST removal (Appendix B), soil vapor extraction (SVE) conducted at the site between 1994 and 1996, and natural processes ' Monitoring well MW8 was installed west of and upgradient of the ExxonMobil site across North • Pershing Avenue This location is downgradient of the Shell service station, approximately 25 ' G 1Projmts%739421MASTERIWPIWP03031WPtx dm 4