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risk assessment. One background well, LM122A, will be sampled to determine whether hexavalent chromium is present in regional groundwater; this was the background well with the highest concentration of chromium in the October 1994 sampling round. The remaining five wells have had historically higher total chromium concentrations, or exhibited high total chromium concentrations in the October 1994 sampling round. Samples will be analyzed for total chromium by USEPA Method 6010 and hexavalent chromium by USEPA Method 7195 (the concentrations of trivalent chromium will then be obtained by arithemetic difference). 1.3 Soil Sampling - Chromium Speciation Four soil samples will be collected in the vicinity of each of the locations listed in Table 1-1 and analyzed for total and hexavalent chromium. These sampling locations are based upon the locations of the maximum detected chromium concentrations during the Phase I and II investigations, modified to provide spatial coverage of each site. Not all of these concentrations at each recommended sampling location are above the background threshold value for chromium in soil. The locations of the proposed soil borings are shown in Figures 1 through 5. Samples collected for chromium speciation will be taken from areas where the highest chromium concentrations were previously detected (i.e., concentrations above 50 mglkg). The purpose of the speciation sampling is to evaluate the percentage of chromium present as hexavalent chromium, and it is likely that the highest percentages will be associated with chromium that represents contamination. If the historical data indicate that chromium concentrations are present above 50 mg/kg at all depths at a SWMU, then the soil samples will be stratified among the following three depth intervals: 0 to 6 inches, 6 inches to two feet, and deeper than two feet. However, if all of the chromium concentrations in excess of 50 mg/kg at a SWMU are in one depth interval, then all of the speciation samples will be collected from that depth igterval. 1.4 Soil Sampling - SWMU 27 A total of five soil borings will be installed inside Building 206 to assess the horizontal and vertical extent of SVOC and pesticide contamination inside of the building. These borings will extend to the water table, and soil samples will be collected from each boring at depths of 3, 5, and 10 feet bgs. Samples will be analyzed for volatile organic compounds (VOCs), semivolatile organic compounds (SVOCs), OC pesticides/PCBs, OP pesticides, chlorinated herbicides, C/U pesticides, metals, total petroleum hydrocarbons as gas (TPH- G), TPH as diesel (TPH-D), and TPH as motor oil (TPH-MO). Two of the borings will be drilled inside of Building 206, in the vicinity of the two soil borings drilled between the northern set of railroad tracks during the Phase II investigation (SB451 and SB452). The proposed locations are immediately south of and adjacent to the tracks to avoid potential cross-contamination from creosote in the railroad ties. The remaining three borings will be drilled inside Building 206, oriented around the area of highest soil contamination as defined by the two Phase II borings located between the southern set of railroad tracks (SB470 and SB471). Because some lateral definition of site contamination is provided by the borings within the locomotive pit and the third Phase II soil boring (SB469), the Phase III soil borings are located around the south and southwest portions of the building. In addition, these borings will be located in an area in which the floor slab is D(Lt cracked or jointed, to determine whether contamination is spatially associated only with floor joints and cracks. The locations of the proposed soil borings are shown in Figure 6. 2