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GAVIN NEWSOM <br /> C nVFRNR <br /> CA LfFORRIR FEN �RETARVF R <br /> Water Boards ENVIRONMENTAL PROTECTION <br /> Central Valley Regional Water Quality Control Board <br /> 29 August 2023 <br /> Steve Azevedo <br /> West Clay Properties/Knife River Corporation <br /> P.O. Box 6099 <br /> Stockton, CA 95206 <br /> REQUEST FOR ADDITIONAL SOIL AND GROUNDWATER SAMPLING, <br /> WEST CLAY PROPERTY(T0607700323), 639 WEST CLAY STREET, STOCKTON, <br /> SAN JOAQUIN COUNTY <br /> Central Valley Water Board staff (Staff) has reviewed the 3 August 2023 No Further <br /> Action Report (NFA Report) submitted on your behalf by AdvancedGeo Inc. (AGI). In <br /> the NFA Report, AGI presented a summary of site conditions and provided an <br /> evaluation of those conditions against the State Water Board's Underground Storage <br /> Tank (UST) Low-Threat Case Closure Policy (Low-Threat Closure Policy) criteria. <br /> Based on that evaluation, AGI concluded the case meets the criteria for closure. Staff <br /> does not concur the case is suitable for closure. <br /> During a 28 August 2023 phone conversation, Staff and AGI discussed the contents of <br /> the site's former 1,000-gallon UST that was removed from the site in May 1990. AGI <br /> reviewed available San Joaquin County Environmental Health Department records and <br /> determined the UST was used for waste oil storage. Based on this discovery, the case <br /> does not meet the criteria for closure because soils in the 0 to 5 feet below ground <br /> surface (bgs) and 5 to 10 feet bgs interval near the former 1,000-gallon UST have not <br /> been sampled for poly-aromatic hydrocarbons to determine if Low-Threat Closure Policy <br /> Direct Contact and Outdoor Air Exposure criteria have been met. <br /> Additionally, in October 2019 and February 2020, AGI collected soil vapor samples from <br /> soil vapor wells SV-1 through SV-6. Tetrachloroethene (PCE) or other volatile organic <br /> compounds (VOCs) were detected in all samples except for the February 2020 SV-6 <br /> sample. The October 2019 SV-4 sample contained PCE at a maximum concentration of <br /> 99 micrograms per cubic meter (pg/m3); which exceeds the San Francisco Bay Water <br /> Board's subslab/soil gas commercial/industrial vapor intrusion human health risk level of <br /> 67 pg/m3. PCE was additionally detected at concentrations ranging from 3.6 pg/m3 (SV- <br /> 6, 2019) to 44 pg/m3 (SV-2, 2020). AGI documented these results in the 5 November <br /> 2019 Soil-Vapor Investigation Report— Fourth Quarter 2019 and the 27 March 2020 <br /> Soil-Vapor Investigation Report— First Quarter 2020 reports. In these reports, AGI <br /> concluded that the VOCs detected in soil vapor did not appear to be associated with the <br /> UST release. However, considering that the former 1,000-gallon UST was used for <br /> MARK BRADFORD, CHAIR I PATRICK PULUPA, ESQ., EXECUTIVE OFFICER <br /> 11020 Sun Center Drive#200, Rancho Cordova, CA 95670 1 www.waterboards.ca.gov/centralvalley <br />