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April 18, 2022 Z054000033 Mr. Alan Buehler Central Valley Regional Water Quality Control Board 11020 Sun Valley Drive, Suite 200 Rancho Cordova, California 95370 Subject: Workplan for Additional Off -Site Groundwater Delineation and On -Site Soil Vapor Sampling, Farmer Property/Former Toyota Town Inc., 610 North Hunter Street, Stockton, California Dear Mr. Buehler: ATC Group Services LLC (ATC) doing business as Atlas Technical (Atlas) has prepared this workplan on behalf of Ms. Linda Wilson, to perform additional off-site groundwater delineation and on-site soil vapor investigation activities for the site located at 610 North Hunter Street in Stockton, California (Figure 1). The purpose of the additional groundwater delineation is to evaluate the potential presence of dissolved petroleum hydrocarbons off site and downgradient of the source area at selected depths below the potentiometric surface. The purpose of the soil vapor investigation is to evaluate the potential presence of petroleum hydrocarbon vapors beneath the southeastern portion of the footprint of the former building that was located in the western portion of the site. The scope of work described by this workplan includes advancing one CPT boring with a companion hydropunch boring, installing two temporary soil vapor points, collecting groundwater and vapor samples for analyses, and preparing a report describing the investigation activities and making recommendations for additional investigation activities, if warranted. The scope of work is described in detail in the following paragraphs. PREVIOUS INVESTIGATIONS On October 61, 19871 Al Williamson, General Contractor, removed three underground storage tanks (USTs) from the site; two of the USTs were 1,000 -gallon capacity used to store leaded gasoline and one the third UST was 550 -gallon capacity used to store waste oil. Soil samples collected after the tank removals contained elevated concentrations of petroleum hydrocarbons and volatile organic compounds (VOCs). On January 21, 1988, Falcon Energy, attempted to remove contaminated soil by over -excavating soil in the vicinity of the former UST locations. The former tank locations were excavated to approximately 22 feet below ground surface (bgs) and were limited to the north by the on-site building. The confirmation samples collected from the base of the over -excavation contained elevated concentrations of VOCs. The excavated soil was transported to the Forward Landfill near Stockton, California for disposal. On June 30 and July 1, 1997, Smith Technology personnel supervised the advancement of seven soil borings (SB1 through SB7) to evaluate the lateral and vertical extent of impacted soil and groundwater 1117 Lone Palm Ave, Suite 201 B, Modesto, CA 95351 209.579.2221 I oneatlas.com