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SUBSURFACE INVESTIGATION WORK PLAN SJC Records Retention Center 630 N.California Street Stockton,CA 95202 1.0 INTRODUCTION This Subsurface Investigation Work Plan (Work Plan) was prepared by Condor Earth (Condor) at the request of the County of San Joaquin Facilities Management Division(Client). In a letter dated August 29, 2017,Ms.Vera Fischer,P.G.,of the Central Valley Regional Water Quality Control Board(Central Valley Water Board)directed the submittal of a work plan for further investigation at the City of Stockton Records and Retention Center at 630 N. California Street, Stockton, California (Site). The Central Valley Water Board requested the investigation to 1) define the lateral and vertical extent of petroleum impacts to groundwater and to collect sufficient data to generate a Conceptual Site Model that assesses the nature, extent,and mobility of the release,and 2)collect shallow data for comparison to low risk screening criteria detailed in the State Water Resources Control Board's Low Threat Closure Policy (LTCP). Figure 1 (Appendix A)shows the location of the Site and Figure 2 presents a Site Plan that shows Site features and the locations of historical soil and groundwater samples. 2.0 SITE BACKGROUND Prior to 1917,the Site was the location of the Stockton Electric Railroad Company car house and blacksmith repair shop. The existing buildings (current County Records Center and adjoining warehouse) were originally constructed in 1917, occupied by the Stockton Transfer Company. By 1940, the Site buildings were occupied by the Stockton Transfer Company and H.S. Dawson Van and Storage, possibly one company or two companies sharing the buildings. The warehouse contained vans and automobiles according to the 1950 Sanborn®Map. From at least 1970 until 1984,the warehouse was used for parking, and the main building was occupied by Morris Brothers Office Supplies and Equipment. The County of San Joaquin reportedly purchased the Site in the late 1980s. A vent pipe consistent with those typically associated with historic underground storage tanks(USTs)was located immediately south of the west doorway to the warehouse along the exterior wall (Figure 2, Appendix A). 2.1 INITIAL SUBSURFACE INVESTIGATION On May 5,2011,Condor conducted a subsurface investigation to evaluate potential soil contamination,and to determine whether a UST was present. At approximately 3.4 feet below ground surface (bgs), a metal object was confirmed to be present near the vent pipe,measuring approximately four feet in diameter(based on magnetic response). Four continuous core borings were advanced in the vicinity of the UST to 30 feet bgs. Contaminated soil was observed in three of the four borings. Fifteen soil samples were collected for laboratory analysis and submitted under chain-of-custody procedures to Argon Laboratories in Ceres, California. The analytical results indicated that subsurface soil was impacted by total petroleum hydrocarbons as gasoline (TPH-g), and to a lesser extent, by toluene, ethyl benzene, and xylenes. Metal analyses results were typical of background metals concentrations for the Stockton area. Condor recommended that the UST be removed. 2.2 UST CLOSURE Condor prepared and submitted a UST Removal Work Plan (dated January 11, 2012) and removal application to the City of Stockton Fire Department Fire Prevention Division (FPD) and the San Joaquin 0% *t�- IL4f CONDOR