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FIRST QUARTER, 1997 <br /> 1 GROUNDWATER MONITORING ONITO NG REPORT <br /> USA STATION#110 - STOCKTON <br /> MAY, 1997 <br /> Groundwater elevation contours constructed from the January 24, 1997 gauging activities <br /> indicate groundwater is flowing to the northeast at a hydraulic gradient of approximately 0 003- <br /> feet/feet A review of the historical field data suggests that the groundwater generally flows to <br /> the northeast with minor fluctuations to the north and south, as shown on Figures 4 and 5 <br /> 3.2 Results of Groundwater Laboratory Analyses <br /> TPH G concentrations ranged from non-detect at the reporting limit of 50 micrograms per liter <br /> (jLgll) in MW-1-P, MW-4-P, MW-5-P, and MW-6-P to 1,100 µg/1 in MW-1 Benzene <br /> concentrations ranged from non-detect (ND) at the method detection limit of 0 5 Ag/l in MW-1-P, <br /> MW-3-P, MW-4-P, MW-5-P, MW-6-P, and MW-2 to 100 µg/1 in MW-2-P MTBE and total <br /> lead have been ND at the method detection lirruts in all the wells sampled since the March, 1995 <br /> sampling event Analyses for these constituents were discontinued beginning with the January, <br /> 1997 sampling Analytical results are summarized in Table 2, and shown on Figure 6 Copies of <br /> laboratory reports and COC documentation are provided in Appendix B <br /> 3.3 Conclusions <br /> The groundwater has risen since the October, 1996 measurements, but the site continues to be <br /> affected by groundwater pumpage in the area The presence of higher levels of TPH and BTEX <br /> compounds in MW-1 when compared to nearby MW-3-P may be explained by the well <br /> construction MW-1 is perforated between 40 and 70-feet, and MW-3-P between 55 and 90- <br /> feet With the groundwater at 66-feet bgs, the analysis from MW-1 appears to reflect the <br /> concentration at the top of the water table and MW-3-P is more representative of the water <br /> bearing zone Both wells contained low to ND levels of benzene this quarter <br /> It is our opinion that the impacted groundwater plume continues to be minimal in size and appears <br /> to be stable Downgradient wells have been consistently non-detect at the method detection hmit, <br /> which define a "zero-line" closure These findings suggest that closure as a low risk site may be <br /> Lwarranted <br /> 3 <br />