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I <br /> Mr. Marshall Cloud <br /> April 17, 2007 <br /> Page 2 <br /> 2. The source of dieldrin in the northwest corner (NWC) of DDJC-Tracy was shown <br /> to be a result of contaminated ground water re-injected from the Interim <br /> Remedial Measure occurring from 1992 through 1995. The dieldrin <br /> contamination in the NWC is well defined both laterally and vertically and is <br /> shown to be sorbed to fine suspended solids in ground water. Field filtered <br /> ground water samples versus non-filtered ground water samples showed most of <br /> the dieldrin is sorbed to fine-grained soils/colloids greater than the filter size. The <br /> Dieldrin Natural Attenuation Investigation Results Report data showed dieldrin in <br /> the NWC is undergoing non-destructive natural attenuation mechanisms, <br /> dispersion and adsorption. Dieldrin concentrations in the NWC and Sanitary <br /> Sewage Lagoons are attenuating as demonstrated by the contracting plume <br /> geometry and decreasing concentrations in monitoring wells. It was agreed to by <br /> all stakeholders that a Human Health Risk Assessment will determine whether <br /> Institutional Controls for the NWC are necessary for the long-term protectiveness <br /> of human health. <br /> 3. In 4Q05 monitoring results for Trichloroethene (TCE) and Tetrachloroethene <br /> (PCE) in soil vapor showed concentrations below ROD cleanup levels for Areas <br /> 1, 2 & 3. TCE was detected at inlet concentrations of 0.34 parts per million <br /> volume (ppmv) with PCE at 0.31 ppmv. Further SVE optimization efforts maybe <br /> recommended including air inlet wells or focused extraction. A work plan <br /> proposing the next course of SVE actions will be submitted to the Agencies prior <br /> to implementation. <br /> Recommendations <br /> 1. DTSC recommends DDJC-Tracy continue with the remedial process optimization <br /> efforts in order to maximize remaining TCE mass removal from both OU-1 and <br /> OU-2. <br /> 2. DTSC recommends that DDJC-Tracy provide an agenda item at the next <br /> Remedial Project Managers meeting to discuss the results from the recently <br /> submitted Dieldrin NWC Human Health Risk Assessment with the regulatory <br /> Agencies. <br /> 3. DTSC recommends that optimization efforts for the SVE system allow adequate <br /> rebound time to fully assess the remaining volatile organic mass and locations <br /> before proposing the next course of action. <br />