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Chevron U S A , Inc February 20, 1992 02320 2338 Page 2 fiberglass tanks and piping Since 1988, eleven additional monitoring wells were installed at the site to define the magnitude and extent of impacted soil and groundwater 2 2 Results of Sod Assessment A total of 14 groundwater monitoring wells have been installed at the site Soil beneath the site consists of clays, silty clays, and silty sands (Figure 3, Appendix A) No soil samples were submitted for laboratory analysis prior to 1988 Since 1988, a total of 23 soil samples, collected during the drilling of monitoring well borings have been submitted for laboratory analysis, none of the samples contained total petroleum hydrocarbon-as-gasoline (TPH-G) concentrations above the method detection limit (MDL) The only evidence of soil contamination at the site was based on strong gasoline odors noted by the field geologist during the drilling of the first four feet of MW-1 MW-2 and MW-3 had slight odors during drilling to a depth of 25 feet No quantitative analysis was conducted on samples retrieved during the drilling of MW's 1,2 and 3 drilled immediately adjacent to a fuel delivery line trench Soil from around and beneath the trench was removed in September/October 1988 All soil samples collected since the installation of MW's 1,2 and 3, and subsequent excavation, have been below the MDL for TPH-G Table 1 (Appendix B) summarizes the analytical data collected from soil samples collected since 1988 2 3 Results of Groundwater Assessment Groundwater at the site flows to the east-northeast at a hydraulic gradient of approximately 0 01 Potentiometnc surface maps constructed from data collected during high and low groundwater elevation periods reflect this trend (Figures 4 and 5, Appendix A) Historical analytical data for groundwater are summarized in Table 2 (Appendix B) Groundwater samples collected during the initial investigation in 1986 were above State maximum contaminant levels (MCL) for benzene Since each well contained detectable concentrations of TPH-G, additional monitoring wells were installed to determine the extent of gasoline impacted groundwater Due to the decline in groundwater levels, three additional monitoring wells (MW-2A, MW-4A and MW-5A) were installed and screened deeper than MW-2, MW-4 and MW-5 Concentrations of BTEX and TPH-G have fluctuated since additional wells were installed in 1988 oncentrations of TPH-G recorded in MW-2 during 1989 are thought to be reflective of unsatisfactory well construction which allowed the flow of surface water to enter the well through the seat Wellheads were replaced in June 1990 and are now sealed from surface runoff Generally,_ as groundwater has declined since 1986, concentrations have declined_I The historical analytical data indicate that the dissolved plumes of BTEX and TPH-G are laterally confined 2339CAP RPT vGROUNDWATER Ca ( _ TECHNOLOGY ` Wv~