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® MONTGOMERY WATSON <br /> To: Steve Light, CEHND Date: May 12, 1995 <br /> From: Sue Tiffany Reference: 1587.1690 <br /> Subject: Summary of Phase III <br /> Investigation Activities, <br /> DDRW-Tracy <br /> As a result of the Phase II investigation, and the review of the Phase II results in the Draft <br /> Technical Memorandum (March 1995), several recommendations were made for the <br /> performance of additional sampling in support of the RDFS activities at DDRW-Tracy. In <br /> addition to specific recommendations included in the TM,the Phase III sampling effort will <br /> include sampling of background wells to provide more data for the calculation of <br /> background threshold calculations, and collection of samples for DI-WET analysis. The <br /> following discussion presents the planned sampling rationale and methods. <br /> 1.0 SAMPLING LOCATIONS <br /> 1.1 Groundwater Sampling - Background <br /> On May 19, 1994, the USEPA, DTSC, and Lahontan RWQCB agreed that four quarters of <br /> groundwater data would be needed to definitively establish background concentrations of <br /> inorganic parameters, metals, and pesticides in groundwater. With the proposed Phase III <br /> sampling of nine background wells, there will be a complete set of data for all but three <br /> wells, which are currently missing two quarters' worth of data for the following <br /> parameters: LM 078B (pesticides), LM 086B (pesticides), and LM 090C (pesticides, and <br /> dissolved metals). Each of these wells will then require one additional round of sampling <br /> for the listed parameters, at which time groundwater background threshold concentrations <br /> will be finalized; however, this final round of sampling will be conducted subsequent to the <br /> current Phase III effort, by another contractor. Background groundwater samples will be <br /> analyzed for miscellaneous organics, total and dissolved metals, organochlorine (OC) <br /> pesticides, organophosphorus (OP) pesticides/PCBs, chlorinated herbicides, and <br /> carbamate/urea(C/U) pesticides,as needed to-obtain the missing data <br /> 1.2 Groundwater Sampling - Chromium Speciation <br /> Six wells (LM16AA, LM21AA, LM37AA, LM097A, LM115A, and LM122A) will be <br /> sampled and analyzed to determine the relative presence of trivalent(Cr III) and hexavalent <br /> chromium (Cr VI), and the resulting data used to support the performance of the baseline <br /> 1 <br />