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SOIL VAPOR MONITORING WELLS INSTALLATION AND SAMPLING WORK PLAN San Joaquin County - Public Works 1810 E. Hazelton Avenue Stockton, CA 95205 1.0 INTRODUCTION This Soil Vapor Monitoring Wells Installation and Sampling Work Plan (Work Plan) was prepared by Condor Earth (Condor) at the request of San Joaquin County Public Works, Capital Projects (Client) for the Hazelton Complex facility located at 1810 East Hazelton Avenue, in Stockton, California (Site). The Site location and key Site features are present on Figures 1 and 2, respectively (Appendix A). In a letter dated September 9, 2022, the Central Valley Regional Water Quality Control Board (Central Valley Water Board) staff directed the submittal of a Work Plan to evaluate the vapor intrusion risk to the office and maintenance buildings located immediately north and south of the Site contamination. 2.0 SITE BACKGROUND The Site serves as a motor pool facility and office complex for the San Joaquin County Facilities Management Division, The Site currently houses two underground storage tanks (USTs), associated underground piping, and dispensers. Currently, the fueling system consists of one gasoline UST and one diesel UST, and two dispenser islands. Five former USTs were removed,and one former UST was abandoned in place. A municipal irrigation well is located in Stribley Park, approximately 550 feet north of the fueling system. Residual petroleum hydrocarbon constituents have been detected in soil and groundwater samples collected at the Site during UST removal activities and during subsequent subsurface investigations conducted at the Site. Notably, elevated gasoline constituents were detected in soil and groundwater samples collected in June 1995 beneath the former 12,000-gallon gasoline UST that was located just west of the existing dispensers. After removal, the former UST basin was over-excavated to a depth of approximately 27 feet and additional soil samples were collected. The additional soil samples collected at the base of the excavation contained elevated concentrations of gasoline constituents. Subsequent investigations conducted by various consultants included the installation and sampling of multiple soil borings, eleven groundwater monitoring wells (MW-I through MW-11), one soil vapor extraction test well (SVE-1), and one combination soil vapor extraction test well/air sparge test well (AS-l/SVE-2). Periodic groundwater monitoring has been ongoing since April 1999. Remediation efforts at the Site have included the following: • Free product removal through the installation of a passive skimmer and in well MW-1 and manual removal (March 2003 through December 2006); • Air sparge and soil vapor extraction pilot testing, including short-term soil vapor extraction from wells SV-lA, SV-1 B, and SVE-2B, and a longer term test at well MW-1 (June 2011); • Interim remediation consisting of soil vapor extraction from well MW-1 (April 2012 to November 2012); • Interim remediation consisting of soil vapor extraction from well SVE-213 (November 2012 to February 2013), with the additional of air sparging through AS-1 (January 2013 to February 2013); • Installation of seven ozone injection wells (OW-1 through OW-7) and ozone injection (April 2019 to May 2022). 4ft Ilk. %** CONDOR