ARCHIVED REPORTS_DRAFT - PHASE I REMEDIAL INVESTIGATION REPORT

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Kennedy/Jenks Consultants The distribution of TPHg in groundwater is presented on Figure 13, TPHd on Figure 14, and TPHmo on Figure 15. ' Figure 13 indicates that the lateral extent of TPHg in groundwater appears to be bounded on the north, east, and south by samples where TPHg was not detected at concentrations exceeding analytical reporting limits. The western extent of TPHg is not known and TPHg was detected in sample OU5-AO110-GW-14-20 at a concentration of 1,900 pg/I. The highest concentrations of TPHg were observed in the groundwater sample OU5-AO110-GW-4-20. Elevated TPHg concentrations and visible hydrocarbon product were observed in samples OU5-AO110-GW-14-20 and OU5-AO110-GW-17-20, suggesting that elevated TPHg concentrations may extend farther to the west or northwest. Figure 14 illustrates that the lateral extent of TPHd in groundwater to the north and east has not been characterized. The TPHd concentrations in the northernmost and easternmost samples are lower than TPHd concentrations in samples from the central portion of AOI 10 and, assuming a single TPH source area in AOI 10, it is probable that the TPHd does not extend much farther to the north and east in groundwater. Elevated TPHd concentrations and visible hydrocarbon product were observed in offsite samples OU5-AO110-GW-14-20 and OU5-AO110-GW-17-20, suggesting that elevated TPHd concentrations may extend farther to the west or northwest. Figure 15 illustrates that the lateral extent of TPHmo to the north and east has not been characterized. The TPHmo concentrations in the northernmost and easternmost samples are lower than TPHmo concentrations in samples from the central portion of AOI 10 and, assuming a single source area in AOI 10, it is probable that the TPHmo in groundwater does not extend much farther to the north and east. Elevated TPHmo concentrations and visible hydrocarbon product were found in offsite samples OU5-AOl10-GW-14-20 and OU5-AO110-GW-17-20, suggesting that elevated TPHmo concentrations may extend farther to the west or northwest. The distribution of TPH in groundwater illustrated on Figures 13 through 15 is consistent with a release in the OU5-AO110-GW-4 area (or near the pipe discovered by Geocon) and a northwest groundwater gradient. 5.8 Background Arsenic Concentrations The arsenic concentration in every soil sample exceeded the residential and industrial PRGs and CHHSLs, regardless of the sample location. This finding is typical of background arsenic concentrations in northern California soils. Background arsenic concentrations for California soils have been discussed in several publications. Using 50 benchmark soils from across the state, Bradford et al. (1996) estimated a mean arsenic concentration of 3.5 mg/kg and a median concentration of 2.7 mg/kg. The Lawrence Berkeley National Laboratory (LBNL 1995) determined a mean arsenic concentration of 5.39 mg/kg and a 95% Upper Confidence Limit (UCL) concentration of 14.0 mg/kg in colluvium and fill at the Lawrence Berkeley National Laboratory in Berkeley, California. Shacklette and Boerngen (1984) report a geometric mean concentration of 5.5 mg/kg for arsenic in western U.S. soils. DRAFT Phase I Remedial Investigation Report Page 37 Former Tracy Railyard, Tracy, California g:lis-groupladminl ob10510565777.18_uprr\09-feports\ri rptltexldoc