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F si-Annual Ground-Water Monitoring Report 13975 Highway 88-E Lockaford, caiifornis NGA Project No. 10-90-104 Page 3 arouneater„Fa1�ti�oal Results Ground water samples collected on April 16, 1991, were analyzed for. BTEX constituents and TPH by a California-certified laboratory. The laboratory results (attached) indicate that BTEX constituents and TPH were not detected at monitoring wells MW-1, MW-3, and MW- 4. Benzene and xylenes were detected at MW-2 at concentrations of 1..3 and 1.4 parts per billion (ppb) respectively. Results of historical ground-water sampling at the site indicate that TPH and B EX concentrations have been declining since the initial sampling event in June, 1987. Benzene has historically been the petroleum constituents detected at the highest concentration at the site. Figure 3 (attached) :;bows hi:atorical benzene concentrations at site monitoring wells plotted against aampling dates. This figure illustrates a dramatic decline in benzene concentrations in monitoring wells at the site. Petroleum constituents have not been detected at monitoring wells MW-1, NW- 3, and MW-4 for a periodr exceeding 19 months. During the past two sampling events benzene has been detecte at monitoring well MW-2 at a maximum concentration of 1.3 ppm, a concentration only marginally higher than the California action . level of 1.0` ppb for this constituent. Examination of Figure 3 indicates that benzene concentrations at . MW-2 have declined dramatically since the initial sampling event in June, 1987. Resultsof the past two sampling events suggest that benzene concentrations at this well will likely. remain at very low or non- detectible concentrations. at this well. CONCLUBIONB AM- RECOMMENDATIONS In our professionaw opinion, the results of seven ground-water sampling events (over a period of 48 months) indicates that site activities have not posed a meaningful threat to ground-water resources beneath the site. Petroleum constituents are not present inground water beneath the site at concentrations that would normally require California's regional water duality control boards to call for remediation activities. Based on 'these conclusions we recommend that ground-water *monitoring activities at the site cease and that the San Joaquin County Public Health Services close the site in regard to further ground-water monitoring activities.