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' feet from the farmer tank field A soil sample collected from this well at 28 5 feet bgs contained 3,200 mg/kg TPH-g ' 2.5 DISTRIBUTION OF HYDROCARBONS IN GROUNDWATER ' Petroleum hydrocarbons in groundwater have been found in the dissolved phase in all wells at the site, and intermittently as LPH in wells MW3, MW6, RW1, RW3/UVB, UVB-MW1, V1, V2, V3, and V4, as documented in quarterly groundwater monitoring reports Historical ' groundwater analytical results as well as groundwater elevations and LPH thicknesses are presented in Table 1 The analytical results of the most recent groundwater sampling event (18 December 2002) are graphically presented in Figure 3 ' During quarterly groundwater monitoring events conducted during 2002, sheen has been identified once in wells V1, V3, V4,twice in UVB-MW1, and 0 01 feet LPH was identified once ' in V2 The highest concentration of hydrocarbons detected during sampling of these wells with sheen or LPH was 110,000 µg/L TPH-g and 9,460 µg/L benzene detected in well UVB-MW 1 on 10 September 2002 These recent occurrences of LPH represents a reduction in LPH at the site over time, as LPH was identified in wells RW3 and V4 at thicknesses of up to 1 10 feet in 1991 In order to assess the concentration of dissolved-phase TPH-g and benzene at the site, four time- synchronous data sets (May 1993, April 1996, August 1998, and March 2002) were used to assess the spatial distribution of dissolved-phase hydrocarbons over time To assess these data Ile sets, graphs were prepared which present the concentrations for TPH-g and benzene across the site This is accomplished by graphing groundwater concentrations against the lateral distance of each well from the most upgradient well (MW8) in the downgradient or general groundwater Iflow direction This graph represents a cross-sectional view of groundwater concentrations of hydrocarbons during each of the four selected time periods The line of cross-section onto which wells were projected for measuring the distance downgradient as well as the four time Isynchronous data sets are graphically presented on Figure 9, which allows the reader to compare all the data collected during each of the sampling events Two trends that can be inferred from Figure 9 are (1) the overall reduction in concentration of TPH-g and benzene in groundwater Iover time, and (2) the reduction in the length of the hydrocarbon-impacted groundwater in the downgradient direction These trends can also be inferred from the hydrographs prepared for wells MW8, MW10, RWI, and RW4 (Appendix C) Groundwater analytical results for wells ' MW11, MW12 and MW13 indicate the lateral extent of hydrocarbons in the downgradient direction is well defined 2.6 ENVIRONMENTAL ISSUES AT ADJACENT SITES ' Exxon installed offsite wells MW8 and MW9 on the southwest and northwest corners, respectively, of the intersection of North Pershing Avenue and Rosemarie Lane These wells ' were installed to assess the potential impact of the Shell service station and a 7-Eleven food and gasoline service store located on these corners, which are upgradient and crossgradient from the ExxonMobil site, respectively Historical groundwater analytical results as well as groundwater ' elevations for ExxonMobil wells MW8 and MW9 are presented in Table 1 A brief description of adjacent sites is presented below The locations of these sites are presented on Figure 2 1 ' G\Prgens573942MASnp,\WP\Wn303\WPtxdoc 5