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Ground water samples were analyzed for total petroleum hydrocarbons as gasoline <br /> (TPH-G), and benzene, toluene, ethylbenzene, and total xylenes (BTEX) at Sequoia <br /> Analytical Laboratory, Concord, California. Monitoring Wells MW-3, MW-4, and MW-6 <br /> were not sampled because of the presence of free product. Extraction Well MW-5 was not <br /> sampled Official laboratory reports and chain of custody records are included m <br /> Appendix C. <br /> DISCUSSION OF RESULTS <br /> Laboratory results for this and previous quarterly ground water monitoring and sampling <br /> events are summarized in Table 2. A ground water elevation contour map for June 1992 <br /> is shown in Figure 3 Results of this monitoring and sampling event are summarized as <br /> follows <br /> • During the June 1992 sampling event, free product was more prevalent than the <br /> previous event. Free product was observed in onsite Wells MW-4 (0 04 foot) and <br /> MW-3 (0 60 foot) The product thickness in MW-3 increased from 0 01 to 0 60 foot. <br /> • Free product was observed in offsite Well MW-6 (0 87 foot), located at the Exxon <br /> site across Tracy Boulevard. Thus free product appears to originate from a source <br /> other than the former Mobil station (as evidenced by the lack of free product in <br /> AMW-4, and its crossgradient position relative to the site). <br /> • Except for three wells observed to contain free product, concentrations of <br /> TPH-G and BTEX were not detected at or above reported detection limits <br /> in ground water samples from all other monitoring wells. <br /> • The average depth to water during this event is approximately 7.2 feet below grade <br /> (18.8 feet above mean sea level). This indicates a rise in ground water elevations <br /> relative to the March 1992 sampling event, and may be linked to the increase in free <br /> product observed at the site. The average ground water gradient direction is <br /> estimated to be toward the north/northwest, with a magnitude of 0.005 foot/foot <br /> across the site. Both gradient direction and magnitude are consistent with historical <br /> trends. <br /> • Through June 1992, approximately 6,900 gallons of ground water was treated and <br /> discharged under NPDES permit. This presents an average daily treatment and <br /> discharge of approximately 75 gallons. Treatment system and extraction well flow <br /> data are presented in Table 2. <br /> 2 <br />