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February 14, 1997 <br /> Page 13 <br /> ti <br /> petroleum hydrocai bon concentration in the influent sti earn i cached an asymptotic <br /> condition <br /> The composition of the residual hydrocarbons in soils is characterized by the pi esence of <br /> the relatively high molecular weight compounds TPPH-g concentrations range in <br /> concentration from less than 10 to 10,000 ppm Benzene with few exceptions has not <br /> 1 ` been detected <br /> Residual hydrocarbons dissolved in groundwatei are priniiarily affected by <br /> (1) hydrocarbon-impacted vadose-zone soils typical of source areas, (2) residual hydro- <br /> carbons left in soil by groundwater fluctuation, (3) operation of the SVE system, and <br /> (4) intrinsic remediation (mechanisms including biodegradation, chemical oxidation and <br /> adsorption/absorption) Dissolved hydrocarbon concentrations in groundwater samples <br /> collected from Well MW-4, located in the source area, have significantly decreased with <br /> the overall increase in groundwater elevation This trend indicates that SVE was <br /> effective in remediating hydrocarbon-impacted soils in the source area <br /> Groundwater samples collected between July 1993 and January 1996 from Well MW-13 <br /> were representative of actualgroundwater conditions Throughout this period, <br /> contaminant concentrations varied between non-detect to 38 ppb for benzene and <br /> non-detect to 3,400 ppb for TPPH-g More recent measurements at this well (with <br /> . submerged screen conditions) demonstrate the same variability Throughout these <br /> concentration variations, there have been no discernible trends indicating any increase <br /> As such, the data demonstrates that the contaminant plume is stable (not migrating) and <br /> it can be inferred that the total extent is not much further beyond Well MW-13 <br /> No detectable concentrations TPPH-g or benzene have been reported from downgradi- <br /> ent Wells MW-11, MW-12, or MW-14 for at minimum five consecutive quarters, there- <br /> fore the wells define the extent of petroleum hydrocarbons in groundwater , <br /> MtBE has been detected in Well MW-9 and confirmed Groundwater monitoring <br /> indicates that MtBE is limited to one well and is defined to non-detectable concentra- <br /> tions Chevron gasolmes in northern California did not contain MtBE prior to 1992 <br /> The site operated as a retail gasoline station until 1988, therefore it is unlikely that a, <br /> residual source of MtBE exists on site and will not have an adverse impact to ground- <br /> water quality <br /> Spectfic observations regarding the secondary sources of hydrocarbons identified are <br /> listed below <br /> Despite an approximate 23-foot fluctuation in groundwater elevations <br /> at the site from February 1989 to October 1992, and a subsequent <br /> increase of approximately 10 feet over the past few years, hydrocar- <br /> 3201337B1CLQSl1RE3 <br /> I ; � <br />