ARCHIVED REPORTS_XR0005194 - Laserfiche WebLink

Home Browse Search

I 1 THIRD QUARTER, 1998 GROUNDWATER MONITORING REPORT USA STATION 9110-STOCKTON AUGUST,1998 Groundwater elevation contours constructed from the July 17, 1998 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 limit of 50 micrograms per liter(µg11) in MW-1-P, MW-4-P and MW-6-P to 3,100 µg/l in MW-2-P Benzene concentrations ranged from ND at the method detection limit of 0 5 Vg/1 in MW-1, 1 MW-2, MW-4-P, and MW-6-P to 490 µg/1 in MW-2-P Groundwater analytical results are summanzed in Table 2, and shown on Figure 6 Copies of laboratory reports and COC documentation are included in Appendix B 3.3 Conclusions 1 Groundwater elevations have increased approximately 2 4-feet since the April 16, 1998 sampling event, and are the highest levels recorded TPH G and benzene concentrations have decreased in lie all wells except MW-1 and MW-3-P, where the concentrations have increased This minor increase is attributed to the high groundwater levels during the winter months and appear to have re-solubilized degraded petroleum hydrocarbon compounds 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 sue 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 Downgradient wells have been consistently ND at the method detection hmit, which define a "zero-line" 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 1 1 i 1 3