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THIRD QUARTER, 1997 GROUNDWATER MONITORING REPORT USA STATION#110-STOCKTON SEPTEMBER, 1997 The groundwater elevations ranged from 39 47 to 39 94-feet below mean sea level Groundwater elevation contours constructed from the July 30, 1997 measurements indicate groundwater is flowing to the east southeast at a hydraulic gradient of approximately 0 003- 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-3-P, MW-4-P, MW-5-P, MW-6-P, and MW-2 to 15,000 gg/l in MW-2-P Benzene concentrations ranged from ND at the method detection limit of 0 5 µg/1 in all wells sampled excluding MW-2-P, where it was detected at 350 µg/l Analytical results are summarized in Table 2, and shown on Figure 6 Copies of laboratory reports and COC documentation are included in Appendix B 3.3 Conclusions The groundwater has risen since the October, 1996 measurements, but the site continues to be 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 65-feet bgs, the analysis from MW-1 appears to reflect the concentration at the top of the water table and MW-3-P is more representative of the water bearing zone Benzene was ND in both wells this quarter Levels of TPH G in MW-2-P have increased since the previous sampling event It is our opinion that the impacted groundwater plume continues to be minimal in size and appears to be stable Downgradient wells have been consistently ND at the method detection Imut, which define a "zero-line" closure These findings suggest that closure as a low risk site may be ' warranted 4 4 t ♦4: F 1 3 1