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Page 10 September 11,2023 Brusca File No. 260-004 Anderson Barngrover Ranch 2. Petroleum vapor intrusion to indoor air The referenced State Water Resources Control Board's LTCP describes conditions, including bioattenuation zones, which if met will assure that exposure to petroleum vapors in indoor air will not pose unacceptable health risks. The policy describes four potential exposure scenarios and the criteria associated with each scenario. Our review of soil analytical data from the subject property indicates that the site does not satisfy the bioattenuation zone criteria for Scenario 1 through 3 outlined in the LTCP as concentrations of total petroleum hydrocarbons in vadose zone soils exceed 100 milligrams per kilogram (mg/kg). However, the direct measurement of soil gas VOC concentrations at the site (including the recent testing at the SG3 location very near the former UST) indicates that site conditions meet Scenario 4 criteria. 3. Direct contact and outdoor air exposure To evaluate whether the conditions at the subject site are considered low-threat for the direct contact and outdoor air exposure pathways, we have compared the maximum concentrations of petroleum constituents in site soils to the applicable values presented in within the referenced LTCP (Table 1, Concentrations of Petroleum Constituents in Soil That Will Have No Significant Risk of Adversely Affecting Human Health). None of the soil samples collected within the upper ten feet at the site contained petroleum hydrocarbons at concentrations above the cited LTCP Table 1 values. CONCLUSION AND RECOMMENDATIONS As described herein, we performed additional site investigation to further evaluate the extent of petroleum hydrocarbon impacts in the area of an approximate 1,000-gallon diesel UST that was removed from the subject property in November 2022. We also have compared the results and site information to the referenced LTCP criteria. The results of soil and groundwater sampling at the site generally have defined the extent of petroleum hydrocarbon impacts (either by non-detect results at peripheral locations or extrapolation of contaminant concentrations), except that the southerly extent of diesel groundwater impact remains undefined. To further characterize the southerly extent of diesel groundwater impact, we recommend that an additional boring be advanced to the south for groundwater sampling; the location of the proposed boring is shown on Plate 7. Additionally, as indicated earlier in this report, we recommend that the main water supply well at the site be resampled/retested to confirm that the water supplied by this deep well is unaffected by the shallow diesel release.