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ggg V'! . 3.3 Laboratory Analysis - Groundwater Laboratory analysis of �groundQiit'ej r7. recorded concentrations of tbonzene.�at aQ :�Ot billian '(Fpb) in well T-1. The three other wells did not record BTEX above the method detection limits as reported. The laboratory data sheets and chain of custody form are presented in Appendix IV. 3.4 Wel! Locations Research of Department of Water Resources records and _- discussions with the Tracy public works department indicates there a rip r P (Figure 5) . The two closest� .wells were used for municipal supply but were abandoned in November. 1987 due to poor yield and sand infiltration. It is reported that a replacement well is proposed in Lincoln Park, due west of the High School site. The five active wells , including one municipal suppler well, are-located in an up groundwater gradient direction from ;:he project site at distances ,-'-n excess of 1/4 mile. 4.0 CONCLUSIONS/RECOMMENDATIONS The hydrologic data for the site indicates a shallow groundwater aquifer is present with a northerly gradient of .20 to .35 - . percent. The dissolved concentration of benzene in the dowcigradient Well T-1 (0.907 ppb) is slightly above the 0.70 ppb action level for drinking water. Although present, the low level H„ of benzene within well T-1 and the absence of dissolved hydrocarbon constituents in wells T-2 and T-3 indicate that the 'Y contamination is relatively limited in, magnitude and .areal extent. Because of the fine grained nature .of the near surface sediments, natural groundwater velocities are relatively low; which acts to limit contaminant spreading. 4� � ' a't3 iiia°bf sQi l� from around and beneath the former tan,. y substantially raitigated the existing soil contamination as well as removing a major source of continuing groundwater contamination. Since no known active wells are present within a .1rte ' miYl �dgsy of the site no immediate threat to water supply exists. Moreover, these wells produce from below loo feet and are separated from the shallow groundwater zone by a thick section of low permeability material. .r Apparently contamination has not spread significantly away from the tank area, .which may be due to either the lack of enough time for contaminants to be detected 40-50 feet downgradient (unlikely) , or alternatively, natural biodegradation processes are mitigating the problem near-source. In order to confirm this process and insure that no contaminated water is moving beyond -� the site e r_gcommer d � .4.uarterly monitoring Rxog=am-ta pix c3=ode -- - werlaaf r FTX 'bysPA X02 ' As part of phis I�'94�tpt-�rel�.. levels��il.i�berrecordedwanc�r"ttie:�seasonal. 3�ehavior;�.af��s.:, 7