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November 12, 1996 Page I 1 Well MW�9 ddring i second quarter 1996, and were reported at low concentrations dunn the third quarter 1996 g q 1 Groundwater samples collected from Well MW-13,'during the second quarter 1995, contained TPPH-g and benzene at concentrations of 1,500 and 6 5 parts per billion (ppb), respectively The hydrocarbon concentration in groundwater samples collected r from this well may be the result of hydrocarbon residuals within capillary-fringe soils or potential off-site sources located east and southeast of the site (Figure 2) During the third quarter 1996, no detectable concentrations of TPPH-g or benzene were reported in Well MW-13 No detectable concentrations TPPH-g or benzene have been reported from downgrade- ; ent Wells MW-1 1, MW-12, or MW-14 for at minimum four consecutive quarters, there- fore the wells define the extent of petroleum hydrocarbons in groundwater MtBE was detected in Well MW-9 but not confirmed Initial groundwater morutonng indicates that MtBE is limited to one well and is defined to non-detectable concentra- tions Chevron gasohnes in northern California did not contain MtBE prior to 1992 The site operated as a retail gasoline station until 1988, therefore it is unlikely that a residual source of MtBE exists on site and will not have an adverse impact to ground- water quality Specific observations regarding the secondary sources of hydrocarbons )Identified are listed below I • 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, hydrocar- bons in groundwater samples collected in plume source areas have decreased This trend indicates 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 verti- cally + The dissolved petroleum hydrocarbons plume is essentially defined to ' non-detectable or very low levels, is stable, and decreasing in size i • Water quality data for on- and off-site wells infer that sigruficant hydrocarbon degradation occurs across the site from the former primary source area near Well MW-9 toward downgradient off-site 3201337B\CLOSI E2 I