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GAVIN NEWSOM <br /> I" OOV£RNOR <br /> C 0.11FO A>MIA a JARED Bw MENFELD <br /> WSECRETARY FOR <br /> Water Boards <br /> ENVIRONMENTAL PROTECTION <br /> Central Valley Regional Water Quality Control Board <br /> 18 September 2020 <br /> Donna Skobrak <br /> 9947 E. Highway 26 <br /> Stockton, CA 95215 <br /> LARRY'S AUTO, 308 N. GRANT STREET, STOCKTON, SAN JOAQUIN COUNTY, <br /> LUST CASE #390170 <br /> Central Valley Regional Water Quality Control Board (Central Valley Water Board) staff <br /> reviewed the Off-Site Assessment and Soil-Vapor Sampling Report dated <br /> 9 September 2020, submitted on your behalf by Advanced Geo Inc. (AGI) for the Larry's <br /> Auto underground storage tank (UST) petroleum release case at 308 N. Grant Street in <br /> Stockton (Site). In the Report, AGI summarizes the advancement of two off-Site, <br /> downgradient soil and groundwater borings (B-13 and B-14), and the installation and <br /> sampling of five on- and off-Site soil gas wells (VP-1 through VP-5). Based on the <br /> results, AGI concludes that risk of petroleum vapor intrusion on and surrounding the <br /> Site is low, and that the extent of petroleum impact to groundwater downgradient of <br /> Tanks B and C is defined. AGI recommends performing an additional confirmation <br /> round of soil gas sampling from VP-1 through VP-5. Additionally, as two borings <br /> proposed along Lindsay Street, generally downgradient of Tanks A and D, were not <br /> advanced due to underground utility conflicts and property access issues, AGI also <br /> proposes submitting a work plan to define the extent of hydrocarbon impact to <br /> groundwater downgradient of Tanks A and D. <br /> Central Valley Water Board staff concurs with AGI's recommendations, with the <br /> following comments: <br /> 1. In a 11 September 2019 letter, Central Valley Water Board staff requested that <br /> the leak detection concentration under the soil gas sampling shroud be <br /> measured and compared to the concentration in the collected sample, as a <br /> quality assurance measure. While leak detection compound was analyzed in <br /> collected samples and reported, it does not appear that under-shroud <br /> concentrations were measured for comparison. During the next round of soil gas <br /> sampling, please measure leak detection compound concentrations under the <br /> shroud as well as in the sample. Collected samples containing leak detection <br /> compound concentrations greater than 10% of the associated under-shroud <br /> concentration are not representative of subsurface conditions. <br /> KARL E. LONGLEY SCD, P.E., CHAIR I _PATRICK PULUPA, ESQ., EXECUTIVE OFFICER <br /> 11020 Sun Center Drive#200, Rancho Cordova, CA 95670 1 www.waterboards.ca.gov/centralvalley <br />