SITE INFORMATION AND CORRESPONDENCE

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FROM 1997. ty 17:48 #888 P.14/24 February 14. 1997 Paue 1 wl petroleum hydrocarbon concentration in the influent stream reached an asymptotic condition_ The composition of the residual hydrocarbons in soils is characterized by the presence of the relatively high molecular weight compounds. TPPH concentrations range in concentration from less than 10 to 10,000 ppm. Benzene with few exceptions has not been detected. Residual hydrocarbons dissolved in groundwater are primarily affected by: (1) hydrocarbon-impacted vadose-zone soils typical of source areas, (2) residual hvdro- carbons left in soil by groundwater fluctuation, (1) operation of the SVE system: and (4)intrinsic remediation (mechanisms including biodegradation, chemical oxidation, and adsorptioniabsorption). Dissolved hydrocarbon concentrations in groundwater samples collected from Well MW-4, located in the source area. have significantly decreased with the overall increase in groundwater elevation. This trend indicates that SVE was effective in remediating hydrocarbon-impacted soils in the source area. Groundwater samples collected between July 1993 and January 1996 from Well MW-13 were representative of actual groundwater conditions. Throughout this period, contaminant concentrations varied between non-detect to 38 ppb for benzene and non-detect to 3,400 ppb for TPPH-a. More recent measurements at this well (with submerged screen conditions) demonstrate the same variability. Throughout these concentration variations, there have been no discernible trends indicating any increase. As such, the data demonstrates that the contaminant plume is stable(not migrating) and it can be inferred that the total extent is not much further beyond Well MW-13. No detectable concentrations TPPH-g or benzene have been reported from downgradi- ent Wells MW-11, MW-12, or MW-14 for at minimum five consecutive quarters,there- fore the wells define the extent of petroleum hydrocarbons in groundwater. , MtBE has been detected in Well MW-9 and confirmed. Groundwater monitoring indicates that MtBE is lir sited to one well and is defined to non-detectable concentra- tions. Chevron gasotines 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. • 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- 3201337BC1AStWI3