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0..` '.,� Ew C.KRO JR. /Mn�w flmcouEz Water Boards Cerrtral Valley Regional Water Quality Control Board 1 September 2015 Ms. Georgia Rooks STAND Affordable Housing 1209 east 81'Street Stockton, CA 95206 SOIL GAS INVESTIGATION, CASA DE OASIS, 1700 SOUTH EL DORADO STREET, STOCKON, SAN JOAQUIN COUNTY California Regional Water Quality Control Board, Central Valley Region (Central Valley Water Board) staff reviewed the Soil-Vapor Sampling Report(SVS Report), dated 30 July 2015, submitted by Advanced GeoEnviron mental Inc. (AGEI) on behalf of STAND Affordable Housing (STAND). We understand STAND is currently considering purchasing the property at 1700 South EI Dorado Street in Stockton (Casa De Oasis), for use as an adult(senior) apartment complex with a community center. In January 2015 a Phase I Environmental Assessment was conducted by AGEI on behalf of STAND and reported five underground storage tanks (USTs), identified in 1991, in the City of Stockton sidewalk adjacent to the proposed community center. Evidence of a fuel dispenser was found inside the community center. The USTs and associated piping were removed on 11 December 1991. Petroleum hydrocarbons or volatile organic compounds were not detected in the soil samples collected at the time of the tank removal activities. On 16 September 1992, the San Joaquin County Department of Public Health Services issued a closure letter to the City of Stockton for the USTs case. In March 2015 a Phase II Site Assessment Report was prepared by AGEI. AGEI reported the results of seven soil vapor samples collected at five and ten feet below ground surface (' bgs). Naphthalene was detected in the sample collected in the sidewalk near the former USTs. Naphthalene concentrations of 1.2 micrograms per Liter(Ng/L) in SV3-5 exceed the California Human Health Screening Level (CHHSLs) of 0.032 pg/L for residential exposures, developed by the Office of Environmental Health Hazard Assessment. Toluene and xylenes were detected in SV3-5 and also in SV4-5 (150' from the USTs) but results did not exceed the CHHSLs for residential exposures. AGEI recommended additional soil vapor sampling in the Phase 11 Site Assessment Report and submitted the 7 May 2015 Soil Vapor Survey Work Plan proposing installation of additional soil borings for the purpose of collecting additional soil vapor samples. The Soil Vapor Survey Work Plan was conditionally approved on 26 May 2015. On 1 July 2015 AGEI installed seven (7) vapor wells in soil borings along EI Dorado Street and 3rd Avenue and within the adult senior apartment complex. Soil vapor sampling occurred on 3 July 2015 and 7 July 2015 to investigate potential for petroleum vapors to migrate into indoor air. AGEI did not include oxygen sampling as requested by Central Valley Water Board staff in the 26 May 2015 letter. However, results of the soil vapor wells sampling did not exceed the CHHSLs for the following petroleum hydrocarbons: benzene, ethylbenzene, toluene, xylenes, methyl tert butyl ether(MTBE), tert amyl methyl ether, di-isopropyl ether, ethyl tert butyl ether, tertiary butyl alcohol, 1,2 dichloroethane, ethylene dibromide, and naphthalene by TO-14 and naphthalene by TO-17. In the SVS Report, AGEI recommends against K,wa E. Lewem SCD,RE„cw I Nweu C.CIRIDOw P.E„scEE,mornrE arc-+cee I1Gt70urt Ocntr 0lrc liao,licncho CaMeMa CA YE770 I www.wttvbwroseaper/oenbclrNar u nccrum ww