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1 ' FIRST QUARTER, 1999 GROUNDWATER MONITORING REPORT USA STATION#110 - STOCKTON MARCH, 1999 ' Groundwater elevation contours constructed from the January 21, 1999 measurements indicate groundwater is flowing to the northeast at a hydraulic gradient of approximately 0 001-feet/feet A review of the historical field data suggests that the groundwater generally flows to the northeast ' with minor fluctuations to the north and south, as shown on Figures 4 and 5 3.2 Results of Groundwater Laboratory Analyses ' TPH G concentrations ranged from non-detect (ND) at the method detection hrmt of 50 micrograms per liter (ug/1) in MW-2, MW-1-P, MW-4-P, and MW-6-P to 390 ggll in MW-2-P ' Benzene concentrations ranged from ND at the method detection limit of 0 5 gg/1 in MW-1, MW_ 2, MW-3-P, MW-4-P, and MW-6-P to 18 pg/1 in MW-2-P Groundwater analytical results are summarized in Table 2, and shown on Figure 6 Copies of laboratory reports with COC documentation are included in Appendix B 3.3 Conclusions ' TPH G concentration have s a e not been detected in MW-2 for the last two quarters TPH G and benzene have decreased in MW-2-P The chromatographic signatures included in Appendix B for these samples indicate an older gasoline ' It is our opinion that the impacted groundwater plume continues to be minimal in size and is stable In November, 1997, the UST's and piping were replaced No obvious holes were observed during the retrofit in the tanks or lines Downgradrent wells have been consistently ND at the method detection limit, which define a "zero-hne" closure With the UST and line replacement the potential risk at this site has been significantly reduced There does not appear to be any indication of plume migration after monitoring the groundwater for six years This site tshould be considered for no further action as a low risk site ' 3