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FOURTH QUARTER,1997 <br /> GROUNDWATER MONITORING REPORT <br /> USA STATION#65-LODI <br /> 1 NOVEMBER, 1997 <br /> 5.1 Groundwater Flow and Gradient <br /> 1 <br /> Groundwater data collected at the site on October 6, 1997 indicate that the depths to <br /> groundwater ranged from 41 59 to 44 01-feet bgs The groundwater elevations ranged from <br /> 1 4 60 to 6 13-feet below MSL Groundwater elevations have lowered since the third quarter <br /> monitoring event <br /> 1 The direction of groundwater flow has been calculated to be to the south southeast with an <br /> average hydraulic gradient of approximately 0 0035-ft/ft across the site Historic field data <br /> 1 suggests that the groundwater generally flows southerly, with minor fluctuations to the east <br /> and/or west, as shown on Figures 3 and 4 <br /> ' 5.2 Results of Groundwater Laboratory Analyses <br /> The October 6, 1997 analytical results reported TPH G in four of the nine monitoring wells <br /> sampled (MW-3, MW-4, MW-8, and S-2) TPH G was non-detect (ND) at the method detection <br /> ' hmrt in wells MW-1,MW-2, MW-5, MW-6, and MW-7 Concentrations of TPH G for this <br /> sampling event ranged from 120 micrograms per liter(µg/1) in MW-3 to 6,000 gg/l in S-2 <br /> '. Benzene was detected in four monitoring wells(MW-3, MW-4, MW-8, and S-2)at <br /> concentrations ranging from 0 97 µg/1 in MW-4 to 200 µg/1 in S-2 Benzene was ND in wells <br /> MW-1, MW-2, MW-5, MW-6, and MW-7 MTBE was ND in all wells, except S-2, where it was <br /> 1 present at 32 µg/1 Analytical results are summarized in Table 2, and shown on Figure 5 Copies <br /> of laboratory reports and COC documentation are included in Appendix B <br /> 1 Monitoring wells MW-3 and MW-4 continue to have sporadic elevated concentrations of TPH G <br /> and benzene The chromatographic signature in Appendix B for samples from MW-3 and MW4 <br /> indicate an older weathered gasoline The high groundwater levels dunng the winter months <br /> appear to have resolubilized degraded petroleum hydrocarbon compounds, which have been in the <br /> capillary fringe or vadose zone The hmited lateral extent of impacted groundwater is <br /> ' demonstrated by the lack of TPH in the downgradient wells <br /> There does not appear to be any indication of plume migration At this time, GHH recommends <br /> 1 that monitoring be continued The tank retrofit program conducted and the removal of impacted <br /> soil may make this site a candidate for site closure <br /> 1 <br /> 1 <br /> 1 5 <br /> 1 <br />