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7 �• concentrations of 1,500 and 6 5 parts per billion (ppb), respectively The hydrocarbon concentration in groundwater samples collected from this well may be the result of hydrocar- bon residuals within capillary-fiznge soils or potential off-site sources located east and south- east of the site (Figure 2) The concentrations of dissolved hydrocarbons in Wells MW-12 and MW-14 are also inter- preted to occur as a result of hydrocarbon residuals within capillary-fringe soils Data suggest these wells are located near the plume edge During the second quarter 1995, TPPH-g and benzene were not detected in groundwater samples collected from Wells MW-12 and MW-14 Specific observations regarding the secondary sources of hydrocarbons identified are listed below • Despite an approximate 23-foot fluctuation in groundwater elevations at the site from February 1989 to October 1992, and a subsequent increase of approximately 10 feet over the past few years, hydrocarbons in groundwater samples collected in plume source areas have decreased This trend indi- cates that SVE was effective in remediating hydrocarbon-affected soils in the source area • As shown on the geologic cross-sections, soil analytical data collected from borings and wells indicate that soils beneath the site are defined to non- detectable levels for TPPH-g and benzene laterally and vertically • The dissolved petroleum hydrocarbons plume is essentially defined to non- detectable or very low levels by Wells MW-2R, MW-10, MW-12, and MW-14 Additional delineation is necessary to define the dissolved plume However, it should be noted that potential sources of hydrocarbons that may be contributing to hydrocarbons in groundwater exist to the south, southeast, and east (Figure 2) • Water quality data for on- and off-site wells infer that significant hydrocar- bon degradation occurs across the site from the former primary source area near Well MW-9 toward downgradient off-site Wells MW-12 and MW-14 The data suggest that the leading edge of the plume boundary is controlled by intrinsic remediation 2.3 Geology/Hydrogeology The site is located in the northern part of the San Joaquin Valley The valley fill consists of late Mesozoic through Eocene marine deposits overlain by late Tertiary and Quaternary non-marine deposits which usually contain fresh water In the Stockton area, late Cenozoic 3201337B/CAP—AMD DOC 5 November 30, 1995 1 /