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,._CV <br /> TC <br /> ow A S S O C I A T E S I N C <br /> 6- <br /> Utilizing the areal distribution patterns of TPHg and benzene as shown in Figures 6 and 7, the <br /> soluable mass of these constituents was calculated. The total soluable mass for TPHg and <br /> benzene was estimated to be 5,663 Kg and 13.63 Kg, respectively. Assumtions and calcualtions <br /> are included in Appendix D. <br /> �- 3.2 Groundwater <br /> Based on the analytical results from groundwater samples collected from the on-site monitoring <br /> boo wells in June 1999, and from grab groundwater samples collected from borings SB 1 through SB7 <br /> on September 24 and October 2, 1996, the lateral extent of groundwater contamination beneath <br /> the site appears to be essentially defined. The dissolved hydrocarbon plume appears to extend <br /> from the former UST locations north and northwest at a distance less than 160 feet. The <br /> groundwater sample collected from MW-4, located approximately 100 feet downgradient of the <br /> former USTs, contained only benzene at a concentration of 1.7 ug/L. The previous March 1999 <br /> water sample collected from MW-4 in did not detect the presence of benzene above the <br /> laboratory method detectection limit of 0.05 ug/L. <br /> bow <br /> The estimated lateral extent of TPHg and benzene impacted groundwater is shown on Figures 8 <br /> y and 9, respectively. As indicated on Figure 8, the highest concentrations of TPHg are present in <br /> the groundwater north to northwest of the former location of the gasoline USTs. Similarly, as <br /> indicated on Figure 9, the highest concentrations of benzene are present in groundwater north to <br /> northwest of the former location of the gasoline USTs. Well MW1 and borings SBI and SB7 <br /> define the extent of contamination to the south, although directly south of boring SB6 the extent <br /> is inferred. The extent of dissolved contamination to the north is defined by borings SB3 and <br /> MW-5. The extent of contamination to the west is defined by MW-6. The eastern boundary is <br /> y inferred based on the nature of the release and the groundwater flow direction. Dissolved <br /> petroleum hydrocarbons beneath the site appear to attenuate with distance from the central <br /> portion of the site as evidenced by analytical data summarized in Table 2. <br /> Historically, MTBE has been detected in groundwater samples collected from the site's <br /> borings/wells at concentrations up to 40 ug/L by laboratory EPA method 8020. However, the <br /> highest MTBE concentration detected by EPA method 8260, a more reliable method, did not <br /> exceed 7.6 ug/L which was observed in September 1996 in a sample collected from MW-2. The <br /> higher MTBE concentrations detected by EPA method 8020 appear to be the result of laboratory <br /> `r chromatogram interference which is often inherent when BTEX of often present in the sample <br /> undergoing analysis, resulting in measurements unrepresentative of the sample. A review of June <br /> 1999 groundwater analytical data indicated MTBE was not present in the dissolved hydrocarbon <br /> plume beneath the site(Table 2). <br /> In accordance with California Heath and Safety Code, Title 22, the on-site domestic well is <br /> sampled quarterly under a monitoring program administered by PHS/EHD. According to Dr. <br /> wA95122VcposU\Reque9t for NFAdoc 8 <br />