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• •hue 11 Environmental Site Assessment Heritage Square Page 9 typical soils in the vicinity of the site impede contaminant mobility and can often result in a highly localized zone of impacted soil, such as appears to be present in association with TKI. The contaminant plume in the vicinity of TK2 appears to be more mobile than that near TKI as the bottom of TK2 is at approximately four feet bgs and the laboratory analytical results indicate detectable concentrations of hydrocarbon constituents at depths of 1 1 feet bgs and 15.5 feet bgs. No hydrocarbon constituents were detected in the samplecollected from 20 feet bgs near TK2, indicating that contamination is limited to shallower depths. No hydrocarbon constituents were detected in the samples collected from borings C-1 through C-4, indicating that no contamination in association with the underground piping beneath the Heritage Square building is present. The detected concentrations of hydrocarbon constituents in the groundwater sample collected from boring B-5 near TK2 were below values requiring regulatory action. In addition, though the groundwater sample collected from boring B-3 near TKI had detectable concentrations of TPH-MO, Condor is not aware of any established regulatory guidelines for heavy oil contamination of groundwater. Furthermore, it is Condor's opinion that the laboratory analytical results for the groundwater samples collected from borings B-3 and B-5 represent cross -contamination of the samples during advancement of the borings for the following reasons: • The depth to groundwater at the site is greater than 30 feet bgs. • The laboratory analytical results indicate that no soil samples collected at the site from deeper than 15.5 feet bgs were impacted by hydrocarbon contamination. • The local soil types at the site are tight clays, restricting the movement of contaminants through the subsurface. • The soil contamination is characterized by heavy hydrocarbon constituents and direct push boring activities have the potential to smear such contaminants during boring advancement. Even if the heavy hydrocarbon constituent groundwater contamination is not the result of cross - contamination, it is reasonable to expect the contaminants to be immobile in water and to adhere to the soil rather than migrate with groundwater. The total lead concentrations detected in the soil samples collected from borings C-1 through C-4 and for the soil samples collected at 15.5 feet bgs in borings B-3 and B-5 are within background levels for the site location and are below the 400 mg/kg PRG value for residential soils. Additionally, the detected lead concentrations were compared to typical lead concentrations of California soils, as identified in the Kearney Foundation Special Report "Background Concentrations of Trace and Major Elements in California Soils" (Bradford, et al., 1996). The report details the results of soil sampling performed on 50 benchmark California soils to determine typical background concentrations of 46 elements and found a lead concentration range from 12.4 mg/kg to 97.1 mg/kg. The lead concentrations detected at the site fall below and within this range of typical California soil background lead concentrations.Tbe detected CAM 17 metals concentrations in the soil samples collected at 15.5 feet bgs in borings B-3 and B-5 were all below the PRG values for residential soils, except arsenic. Arsenic was detected in these samples above the PRG cancer endpoint value, but below the PRG noncancer endpoint value. Arsenic is commonly detected in soil samples collected from locations in the San Joaquin Valley at concentrations in this range. IW %46� CONDOR