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' FOURTH QUARTER,1998 GROUNDWATER MONITORING REPORT USA STATION 0110-STOCKTON JANUARY,1999 ' Groundwater elevation contours constructed from the October 20, 1998 measurements indicate groundwater is flowing to the southeast at a hydraulic gradient of approximately 0 002-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 tTPH G concentrations ranged from non-detect (ND) at the method detection limit of 50 micrograms per liter(µg/1) in MW-2, MW-1-P, MW-3-P, MW-4-P, and MW-6-P to 3,700 Ag/l in ' MW-2-P Benzene concentrations ranged from ND at the method detection limit of 0 5 µg/1 in MW-2, MW-3-P, MW-4-P, and MW-6-P to 650 gg/I in MW-2-P Due to the possibility of a sampling error, monitoring wells MW-3-P and MW-6-P were re-sampled on December 18, 1998, ' results indicate that they were inadvertently switched 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 Groundwater elevations are decreasing to levels consistent with historic data TPH G concentrations have decreased in MW-2 and concentrations of TPH G and benzene increased 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 Ines Downgradient wells have been consistently ND ' at the method detection limit, which define a "zero-lme" 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 ' should be considered for no further action as a low risk site ' 3