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Discussion and Conclusions - Phase 11 Site Investigation between the rows of apricots trees on an annual vertical extent of impacted soil have been basis. Containers of Surflan another pre- adequately delineated (Plates 3, 4, and 5). emergent herbicide were observed on site. It is reasonable to assume that other brands may also 8.1.4 Volume of Impacted Soils have been used. To evaluate potential human health risks from exposure to diuron in the soil The vertical and lateral distribution of TPHd near the mixing areas, the concentration of too diuron detected in the composite soil sample was impacted soil is shown in cross-sectional views in Plates 4 and 5. A conservative estimate of the compared to risk-based soil screening total volume of impacted soil can be made by concentrations developed by Region IX of the �+ U.S. EPA(EPA, 1996). For a residential land use approximating the soil volume as that of a for an cylinder with a radius of 17 feet, centered at scenario, the PRG for DDE is 130 m g1kg' boring SB-2, that extends to a depth of 40 feet industrial scenario, the PRG is 1,400 mg/kg. The detected concentration of 0.37 mg/kg is bgs. Although locally the cross-sectional area of .r substantially below both PRGs, suggesting that impacted soil in Plates 4 and 5 can be seen to human health risks are unlikely to result from extend beyond this distance of 17 feet and exposure to diuron at the three agricultural elsewhere be less than 17 feet radius, the mixing areas onsite. uncertainties of the available soil data warrant a two conservative value for the radius. Therefore, using this assumption, the approximate volume of 8.1.3 Former Underground Storage impacted soil is 1,350 cubic yards. Tanks Location 8.2 Conclusion One 550-gallon and one 1,100-gallon UST, Four RECs were identified during the Phase I reportedly used to store gasoline for fueling farm PSA. The four RECs included the former UST's, equipment and vehicles, were removed 5 to 6 the existing AGT's, the former AGTrand the years ago. The former USTs location constituted agricultural mixing areas. Based on the data one of RECs that was assessed during the initial collected, and evaluation of the analytical data part of the Phase II Investigation with boring SB-2 presented as part of the Phase II investigation, all (Plate 2). As previously discussed, the former of the RECs, with the exception of the former .., UST location is also the area where a diesel fuel UST location, are eliminated from further AST was formerly located. consideration-as an REC. The data collected at Six borings were advanced and 26 soil samples the former UST location, which is coincident were analyzed during the Phase II investigation to with the location of the former AGT, indicated assess the vertical and lateral extent of impacted soil impacted with TPHd constituents. These soil. None of these samples contained detectable impacted soils total approximately 1,350 cubic ..+ concentrations of TPHg,benzene, toluene, or yards of material that require mitigation. ethylbenzene. Concentrations of TPHd, ranging from 1.3 mg/kg to 13,000 mg/kg, were detected in .r 10 of the 26 samples. The maximum concentrations of TPHd detected occurred in the samples from SB-2 and SB-5 at depths of 20 feet bgs. Based on the field and analytical data,TPHd �+ impacted soils at SB-2 begin just below the surface and extend down to a depth of 35 feet but were not detected at 40 feet bgs. TPHd impacted soils encountered in boring SB-5 extend slightly deeper than in boring SB-2 with 2.8 mg/kg detected in the sample from a depth of 40 feet bgs. Concentrations greater than 1,000 mg/kg TPHd appear to be present above 25 to 30 feet bgs. Based on the field and analytical data presented and discussed above, the lateral and V ' G:\WPIGDH\GH36371Z.151 Harding Lawson Associates 17 V