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r <br /> Wendy L. Cohen -2- d 6 June 1994 <br /> wells installed by the Stockton Terminals Technical Committee (STTC) and quite distant from the <br /> former UST locations. <br /> Thus, it is highly likely that the chlorinated hydrocarbon contamination in UO/MW-5, if really <br /> present, was caused by other terminals and not by Unocal's UST operations. Based on <br /> nondetectable chlorinated hydrocarbons in PS/P-14 and PS/MW-15, which are adjacent to UO/MW- <br /> 5, chlorinated hvdrocarbons may no longer exist in UO/MW-5. The presence of high <br /> concentrations of petroleum hydrocarbons in UO/MW-5 were likely caused by, other terminals and <br /> not by Unocal's UST operations as evidenced by similarly high concentrations of petroleum <br /> hydrocarbons in PS/P-14, PS/MW-15, and other STTC wells. <br /> The detection of chlorinated hydrocarbons at different locations in the terminals suggests that some <br /> facilities may have occasionally used a large enough volume of chlorinated hydrocarbons in their- <br /> operations to affect the ground water but not enough to cause a concentrated plume. <br /> Summary <br /> 1. Based on current data, the soil and ground water contamination due to UST operations at Unocal <br /> have been remediated. <br /> 2. The residual petroleum contamination occasionally detected in UO/MW-3 appears to be the <br /> result of ST Services operations because this well is side gradient from the former UST locations <br /> and downgradient from ST Services. <br /> 3. The chlorinated and petroleum hydrocarbon contamination in UO/MW-5 are likely to have been <br /> caused by other terminals and not by Unocal's UST operations because UO/MW-5 is closer to <br /> PS/P-14, PS/MW-15, and other STTC wells and quite distant from the former UST locations. <br /> 4. The presence of 15 ppb cis-1,2-DCE in PS/MW-1M should be investigated further because the <br /> MCL for cis-1,2-DCE is 6 ppb and PS/MW-1M is a Zone B monitoring well (about 35 feet <br /> deep). <br /> 5. Unocal should be allowed to close the site but should be requested not to abandon the <br /> monitoring wells in case the STTC would like to use them. <br /> PSI:psi\ldj <br />