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February 14 1997 Page 8 �- 4000 lbs Figures 6 through 8 depict these conditions and show soil analytical data from soil borings and wells drilled after the operation of the SVE system The location of the residual hydrocarbons in soils al e characterized by soil borings located 1n two areas These include Soil Borings SB-2 and SB-12 located in the north- ern portion of the UST complex and Borings SB-3 and SB-13 located adjacent to the western product island Hydrocarbons in these two areas are well defined and appear to be isolated based on data collected from several borings as shown on the geologic cross- In sections (Figures 7 and 8) The composition of the residual hydrocarbons in soils is dominated by the presence of the relatively high molecular weight compounds TPPH-g concentrations range from less than 10 to 10,000 ppm Benzene with few exceptions has not been detected, and based on toxicity characteristic leaching procedure (TCLP) results is not leachable (Table 3) Benzene is the most water soluble natural hydrocarbon constituent of petroleum is a known carcinogen, and is the primary constituent of concern for cleanup of petroleum discharges in California The remaining elevated concentrations of TPPH-g tend to reside in relatively tight soils consisting of sandy to clayey silt, or silty sandy clay RESIDUAL PETROLEUM HYDROCARBONS IN GROUNDWATER 1 - Quarterly groundwater monitoring was initiated in November 1988 Dissolved hydro- carbon concentrations in groundwater samples collected from Well MW-4, located in the source area, have generaIIy decreased with the overall increase in groundwater elevation This trend indicates that SVE was effective in remediating hydrocarbon-affected soils in the source area Furthermore, during 1996, groundwater elevations in site monitoring wells have risen allowing collection of groundwater samples from wells which were previously dry (Wells MW-1; and MW-3 through MW-8) Fourth quarter groundwater monitoring (October 24, 1996, Attachment D) indicates the maximum residual concentration of TPPH-g and benzene in groundwater to be 560 ppb and 4 8 ppb respectively, (Well MW-13) Wells MW-4 MW-6 and MW-9 are the only other wells to report detectable concentrations of petroleum hydrocarbons The wells are located within and to the east of the former UST complex and product island areas Therefore, minor petroleum hydrocarbons exist in groundwater locally in the vicinity of the north- eastern corner of the former UST complex and east of the former product island The lateral extent of petroleum hydrocarbons in groundwater has been delineated to non- detectable concentrations with the exception of laterally to the east as evidenced by fluctuating concentrations of petroleum hydrocarbons in Well MW-13 "Stable or decreasing plumes often display short-term variability in ground-water concentrations This behavior should not be construed as evidence of an unstable plume, but may be the natural variations of a stable plume in the environment " (Supplemental Instructions to 3201337B1CLOSLTRE3