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M. Scott Mansholt -2 - 20 September 2006 <br /> Chevron <br /> Vernalis Property <br /> Groundwater samples from om two upgradient borings (SB-30 and 36) were ND for TPH <br /> ' compounds; and the downgradient borings (SB-22, 23 and 25) respectively, reported 780, 560 <br /> and 660 pg/I TPH as diesel (TPHd). Low PAH concentrations were reported in the SB-25 <br /> sample. Benzene at 0.5 pg/I and toluene at 2.0 pg/I was reported in SB-22; the remaining <br /> TPH compounds were ND. <br /> Several monitoring wells (not including the onsite monitoring wells) and water supply wells <br /> were identified within the well search area. The monitoring wells are associated with the <br /> Western Farm Services business, north of the site. The water supply wells are situated up <br /> and/or crossgradient and have sanitary seals generally extending below depths where TPH <br /> ' compounds have been identified at the subject properties. <br /> Based on our review, we have the following comments: <br /> • Affected groundwater at the Bellini site has not been delineated downgradient (east) of <br /> borings SB-23, 24, 25 and MW-7. Additional borings and groundwater sampling and <br /> ' analysis are needed; <br /> • The extent of TPH impacted groundwater has not been delineated downgradient (east) <br /> of the Arcos property, east of Highway 33; additional borings and groundwater sampling <br /> and analysis are needed; and, <br /> t • Access at certain areas east of the site has been difficult to obtain; as an alternative, <br /> and as discussed recently with SAIC staff, Chevron plans to proceed and install the <br /> proposed borings within the Highway 33 right-of-way. <br /> ' Regional Water Board staff concurs with theIan to complete additional soil borings <br /> p P <br /> downgradient of the Arcos and Bellini sites as discussed above. Contingency borings and/or <br /> additional downgradient monitoring wells may be required. The revised quarterly groundwater <br /> monitoring should not proceed until the new monitoring wells have been installed and <br /> developed. <br /> Please advise Regional Board staff at least 48ho <br /> urs in advance of conducting the additional <br /> drilling, sampling, and field activities. <br /> ' Ifou have an questions or <br /> Y Y q comments, please contact Dean Hubbard at (559) 445-5179. <br /> SHELTON R. GRAY C. DEAN HUBBARD <br /> Senior Engineering Geologist Engineering Geologist <br /> PG No. 6357 <br />' cc: Michael Sonke, Stanislaus County Department of Environmental Resources, Modesto <br /> Michael Infurna, San Joaquin County Environmental Health Department, Stockton <br />' Luis Mercado, SAIC, Sacramento <br />