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Page 7 September 11,2023 Brusca File No. 260-004 Anderson Barngrover Ranch significantly to the north of this well. The data from TH8 and the North Water Well provide reasonable definition of the northerly plume conditions. The groundwater sample collected from Boring TH9 situated to the south of the former UST location contained elevated concentrations of petroleum hydrocarbons and non-detect to very low concentrations of fuel-related VOCs. We note that the TPH as diesel concentration detected in this groundwater sample (6,300,000 ug/L) is higher than the concentration detected in the sample collected from Boring TH7 situated to the north of TH9, suggesting significant southerly migration of the diesel groundwater plume to the south, and thus the southerly extent of the diesel groundwater plume is undetermined. The groundwater sample from Boring TH10 situated to the west of the former UST contained a much lower concentration of TPH as diesel (3,200 ug/L) and mostly low concentrations of VOCs. Notable VOCs detected in groundwater sample from Boring TH10 include naphthalene(46 ug/L), benzene (1.8 ug/L), and MTBE (1.7 ug/L). The diesel,benzene, and naphthalene groundwater data from the recent borings,previous borings, and the water supply wells are plotted site maps along with inferred isoconcentration contours and are presented on Plates 7, 8, and 9 respectively. As shown, groundwater impacts to the north and east of the former diesel UST are fairly well defined. Although groundwater impacts were detected at Boring TH10 to the west of the former UST, the concentrations at that location are substantially lower than were detected near the former UST and extrapolation of the groundwater data to the west provide reasonable westerly plume delineation. As shown on Plates 8 and 9, the southerly extent of benzene and naphthalene in groundwater appear to be constrained by non-detect results from the TH9 groundwater sample. However, as shown on Plate 7, the southerly limit of diesel groundwater impact is undetermined. As indicated above,a very low concentration of MTBE(1.7 ug/L)was detected in the groundwater sample from TH10 (and as summarized earlier in this report, revised laboratory reporting for the groundwater sample collected from Boring TH5 in May 2023 also revealed a low MTBE concentration [1.2 ug/L]). The source of the MTBE detected in groundwater at these locations is uncertain, as it is unlikely that the diesel release from the form UST contained MTBE. Nonetheless, the limited detected concentrations of MTBE in groundwater are well below the drinking water standard of 5.0 ug/L and, as such, are not considered a significant concern. Soil Gas Results As shown on Table III,the soil gas sample collected from the temporary probe SG3 contained low concentrations of some of the tested VOCs (predominantly fuel-related VOCs that are considered attributable to the release from the removed UST). LTCP soil gas criteria (for naphthalene, benzene, and ethylbenzene) are included on Table III; as shown, soil gas sample SG3 did not contain VOCs above these criteria concentrations. Soil gas sample SG3 contained oxygen at 11 percent, indicating that bioattenuation conditions exist at the site.