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1 SECOND QUARTER,1997 GROUNDWATER MONITORING REPORT USA STATION#110- STOCKTON JULY,1997 ' The groundwater elevations ranged from 39 33 to 39 85-feet below mean sea level Groundwater elevations have increased an average of 1 65-feet since the last sampling event ' Groundwater elevation contours constructed from the Apnl 21, 1997 gauging activities indicate groundwater is flowing to the northeast at a hydraulic gradient of approximately 0 0014-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 at the reporting limit of 50 nucrograms per liter (jig/1) in MW-1-P, MW-4-P, MW-5-P, and MW-6-P to 3,300 pgll m MW-2-P Benzene concentrations ranged from non-detect (ND) at the method detection limit of 0 5 µg/l in MW-I-P, MW-4-P, MW-5-P, MW-6-P, and MW-2 to 460 gg/l in MW-2-P MTBE and total lead have been ND at the method detection limits in all the wells sampled since the March, 1995 sampling ' event Analyses for these constituents will not be continued, as described in the"First Quarter, 1997, Groundwater Monitoring" Analytical results are summarized in Table 2, and shown on Figure 6 Copies of laboratory reports and COC documentation are provided 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 66-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 Both wells contained low levels of benzene this quarter r-,, _ ' Levels of TPH G and BTEX compounds in M}�V. 2-P Have increased to,historically high levels This may be due to groundwater solubilizmg prQleum compounds in the vadose zone or capillary fringe ' It is our opinion that the impacted groundwater plume continues to be minimal m size and appears to be stable Downgradient wells have been co'iWtently ND at the method detection limit, which ' define a "zero-line" closure These findings suggest tlt closure as a low risk site may be warranted i 3