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r November 12, 1996 Page 7 0calculate the volume and estimate average conceirtration of the remaining soil contaml- nation is also presented in Attachment A RESIDUAL PETROLEUM HYDROCARBONS IN SOIL Currently, residual petroleum hydrocarbons in soil occur below and northeast of the former UST cluster (installed in 1980 and adjacent product island The vertical and lateral extent of petroleum hydrocarbons in soil are shown on Figures 6 through 8 These hydrocarbon-affected soils occur in what can be described as two localized verti- cal soli columns Hydrocarbon-impacted soils in these two vertical soil columns occur from a depth of approximately 15 to 50 feet bgs The horizontal extent of hydrocarbon- affected soils within these soil columns occur primarily within a range of 5 to 10 feet, with occasional thin zones that extend horizontally an additional 10 feet from the soil column These soil columns occur within vadose zone soils, soils within the capillary fringe and zone of groundwater fluctuation, and currently saturated soils Figures 6 through 8 depict these conditions and show soilanalytical data from soil borings and wells drilled after the operation of the SVE system The location of the residual hydrocarbons in soils are characterized by soil borings located in 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- sections (Figures 7 and 8) The concentration of the residual hydrocarbons in soils are characterized by the presence of the relatively high molecular weight compounds TPPH-g concentrations range in concentration from less than 10 to 10,000 ppm Benzene with few exceptions has not been detected The remaining elevated concentrations of TPPH-g tend to reside in rela- tively tight soils consisting of sandy to clayey silt, or silty sandy clay RESIDUAL PETROLEUM HYDROCARBONS IN GROUNDWATER Quarterly groundwater monitoring was initiated in November 1988 Dissolved hydro- carbon concentrations in groundwater samples collected from Well MW-9, located in the TYewt f k7 source area, have generally decreased with the overall increase in groundwater elevation M V This trend indicates that SVE was effective in remediating hydrocarbon-affected soils in iidkLAA r the source area Furthermore, during 1996, groundwater elevations in site monitoring of b�T wells have risen allowing collection of groundwater samples from wells which were previously dry (Wells MW-1, and MW-3 through MW-8) Third quarter groundwater monitoring (July 30, 1996, Attachment D) indicates the maximum residual concentration of TPPH-g in groundwater to be'310 ppb (Well MW-4) The maximum residual 3201337B1CL0S1 RE2 t r