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8 The total lead concentrations in groundwater samples collected from monitor wells <br /> MW2 through MW5 did not exceed the MCLS established by the RWQCB. <br /> 6.0 RECOMMENDATIONS <br /> The vertical extent of soil contamination and the downgradient extent of groundwater <br /> contamination have been defined Soil contamination is limited to the depth at which <br /> groundwater is first encountered (30 feet below ground surface), and the depth at which <br /> groundwater had historically been shallowest (25 feet below ground surface). Groundwater <br /> contamination is hr sited to the monitor wells installed mimediately downgradient of the <br /> northern end of the underground storage tank excavation (MW3 and MW4), and the <br /> f downgradient extent of groundwater contamination defined by non-detectable concentrations <br /> of petroleum hydrocarbons in samples collected from well MW5. <br /> Since TPH were not detected in any groundwater samples collected, but low concentrations <br /> of BTEX compounds were detected in three of the five groundwater monitor wells installed <br /> adjacent to the underground storage tank excavation, and groundwater elevations had <br /> flucted considerably in this region of Stockton, McLaren/Hart recommends that a quarterly <br /> groundwater sampling program be initiated and that three quarters of data collected. The <br /> review of three additional quarters of data would allow McLaren/Hart to deternnine 1) a <br /> more accurate groundwater flow direction, 2) whether groundwater elevation fluctuations <br /> affect groundwater contamination concentrations of 3) allow for the evaluation of data to <br /> better determine which remedial activity, if necessary, would be most appropriate <br /> 7.0 REFERENCES <br /> 1) State Water Resources Control Board "Leaking Underground Fuel Tank (LUFT) <br /> Field Manual", December 1987 <br /> 2) Regional Water Quality Control Board, 'Regional Board Staff Recommendations for <br /> Initial Evaluation and Investigation of Underground Tanks", June 1988 <br /> 3) California Code of Regulations, Title 22, 66699. <br /> 4) Lindsay, Willard L., "Chemical Equilibria in Soils", John Wiley & Sons, New York, <br />' 1979 <br /> 5) Staff Report of the California Regional Water Quality Control Board Central Valley <br /> Region, "A Compilation of Water Quality Goals", September 1991. <br /> riiisieai 18 <br /> 1 <br />