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t Mr. Marshall Cloud August 31, 2004 Page 2 of the six confirmation samples showed exceedences of the cleanup standards for lindane detected at 2 micrograms per kilogram (Ng/kg), 2,4,5-T was detected at 16 pg/kg, and dieldrin at 160 pg/kg. Based on these initial sampling results additional soil was removed from the northern and southern sidewalls of the excavation. Additional excavation was not completed on the western sidewall because a 4 ft diameter storm drain (SD) limited access. The final round of confirmation step-out sampling showed residual contamination remaining in both the eastern and western sidewalls due to residual contaminants underneath both the Building 28's foundation and immediately next to the 4 ft diameter SD. Specifically, the highest remaining concentrations detected at SWMU 6 were dieldrin at 160 pg/kg found at 10 ft below ground surface (bgs), lindane - 4 pg/kg at 10' bgs and 2,4,5-T - 12J pg/kg at 10' bgs. The residual concentrations of dieldrin, lindane, and 2,4,5-T detected at SWMU 6 above the ROD cleanup standards, were modeled with VLEACH and SESOIL to determine if any potential impacts from SWMU 6 exist for underlying ground water. Deionized Water (DI) tests and vadose zone transport modeling for 2,4,5-T, dieldrin and lindane indicated that residual concentrations left in place will not adversely impact the water quality goals. The modeling results indicated that the ROD cleanup goals were overly conservative supporting a revision of the cleanup goals for 2,4,5-T and lindane from 5 pg/kg to 13 pg/kg and 1.7 pg/kg to 5 pg/kg, respectively. For reference, the United States Environmental Protection Agency's Industrial Preliminary Remediation Goals (PRG) for Regional 9 soil for 2,4,5-T and lindane are 6.2E+06 pg/kg and 1.7E+03 pg/kg, respectively. Dieldrin was the only COC at SWMU 6 that exists above the Region 9 Industrial PRG of 110 pg/kg with a detection of 160 pg/kg. In May 2002, a supplemental investigation was conducted at SWMU 6 to determine the extent of residual contaminants in a discrete area west of Building 28 and immediately to the east of the 4 ft SD line. The close proximity of Building 28 and the SD prohibited the excavation of soil from these specific locations in the previous two rounds. The supplemental sampling collected samples from three depths at four locations, two east of the storm drain and two in a small area west of Building 28. The Direct Push technology also collected grab ground water samples from each boring along with one additional down gradient grab ground water sample to assess whether the CDCs had leached from the soil and were migrating with ground water. The analytical results from all the supplemental soil and ground water samples showed concentrations below ROD- specified soil cleanup standards and ROD-specified ground water beneficial use limits. The residual concentration of dieldrin Ieft.in place above the industrial PRG prompted the regulatory team to request Land Use Controls (LUCs) to protect construction workers at SWMU 6. The LUCs are documented in the DDJC-Tracy Final Explanation