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r ' FOURTH QUARTER, 1997 . GROUNDWATER MONITORING REPORT USA STATION#1110- STOCKTON DECEMBER, 1997 Groundwater elevation contours constructed from the October 31, 1997 measurements indicate groundwater is flowing to the east-southeast at a hydraulic gradient of approximately 0 0016- 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 hter(µg/1) in MW-1-P, MW-4-P, MW-5-P, and MW-6-P to 1,800 µg/1 in MW-1 Benzene concentrations ranged from ND at the method detection limit of 0 5 µg/1 in all wells sampled except MW-1-P, MW-2-P and MW-1, where it was detected at 1 1, 150 and 0 6 µg/1, respectively 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 r3,3 Conclusions Groundwater elevations are slightly higher when compared to the October, 1996 levels The groundwater in the area is affected by groundwater pumpage in the area The presence of higher levels of TPH and BTEX compounds in MW-1, when compared to nearby MW-3-P, may be ' explained by the well construction MW-1 is perforated between 40 and 70-feet and MW-3-P between 55 and 90-feet With the groundwater at 67-feet bgs the analysis from MW-1 appears to reflect the concentration at the very top of the water table and MW-3-P is more representative of the water bearing zone It is our opinion that the impacted groundwater plume continues to be mrummal in size and appears to be stable During November, 1997 the UST's and piping were replaced No obvious holes were observed during the retrofit Downgradient wells have been consistently ND at the method detection hnut, which define a "zero-hne" closure These findings suggest that closure as a low risk site may be warranted a r r r r 3 r r