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FIRST QUARTER,1998 <br /> GROUNDWATER MONITORING REPORT <br /> .. USA STATION#119-STOCKTON <br /> ' MARCH, 1998 <br /> Groundwater elevation contours constructed from the January 22, 1998 measurements indicate <br /> groundwater is flowing to the east-northeast at a hydraulic gradient of approximately 0 0013- <br /> feet/feet A review of the historical field data suggests that the groundwater generally flows to <br /> the northeast with minor fluctuations to the north and south, as shown on Figures 4 and 5 <br /> ' 3.2 Results of Groundwater Laboratory Analyses <br /> ' TPH G concentrations ranged from non-detect (ND) at the method detection limit of 50 <br /> micrograms per liter(µg/1) in MW-1-p, MW-4-P, and MW-6-P to 2,500 µg/1 in <br /> MW-2-P Benzene concentrations ranged from ND at the method detection limit of 0 5 µg/1 in all <br /> wells sampled except MW-1-P, MW-2-P and MW-1, where it was reported at 2 2, 310 and 2 3 <br /> µg/l, respectively Groundwater analytical results are summarized in'fable 2, and shown on <br /> Figure 6 Copies of laboratory reports and COC documentation are included m Appendix B <br /> 3.3 Conclusions <br /> Groundwater elevations have decreased approximately 1 89-feet since the October 31, 1997 <br /> sampling event The presence of higher levels of TPH and BTEX compounds in MW-1, when <br /> compared to nearby MW-3-P, may be explained by the well construction MW-1 ns perforated <br /> between 40 and 70-feet and MW-3-P between 55 and 90-feet With the groundwater at 65-feet <br /> bgs the analysis from MW-1 appears to reflect the concentration at the very top of the water table <br /> ' and MW-3-P is more representative of the water bearing zone <br /> It is our opinion that the impacted groundwater plume continues to be minimal in size and appears <br /> ' to be stable In November, 1997, the UST's and piping were replaced No obvious holes were <br /> observed during the retrofit Downgradient wells have been consistently ND at the method <br /> detection limit, which define a "zero-lune" closure With the UST and line replacement the <br /> ' potential risk at tins site has been significantly reduced- There does not appear to be any <br /> indication of plume migration after monitoring the groundwater for six years This site should be <br /> considered for no further action as a low nsk site <br /> 1 <br />