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SUPPLEMENTAL SITE INVESTIGATION WORK PLAN South Pointe Property Stockton, California Site Background The conceptual site model developed by WKA postulated that elevated concentrations of OCPs and lead are generally associated with historical materials handling processes and incidental spills along the railroad spur historically present in the central portion of the site. Excavation during soil assessment activities identified fill materials (indicated by the presence of wood, metal, brick, concrete, and asphalt debris) believed to be associated with demolition of historical structures at the site. Test pits advanced by Stantec encountered evidence of fill materials from ground surface to depths ranging from approximately 2 to 8 ft bgs. Fill materials consisted of native soils (generally sandy silt and gravel) mixed with wood, metal, brick, concrete, and asphalt debris. The pesticide reported most frequently in soil samples was dieldrin and the compounds aldrin, DDD, DDE, DDT, endrin, chlordane, and toxaphene were detected in one or more soil samples. Only concentrations of dieldrin and toxaphene exceeded their respective screening levels in one or more samples. Locations of samples with OCPs exceeding screening criteria are illustrated on Figure 3 in Appendix B. Concentrations of lead reported in 13 samples exceeded the unrestricted screening level of 80 milligrams per kilogram (mg/kg). The locations of these samples, with concentrations ranging from 80.1 to 219 mg/kg, are illustrated on Figure 3 in Appendix B. The 2021 investigation provided additional vertical delineation of OCPs and lead in soil beneath portions of the conceptual remediation area identified by WKA. This investigation confirmed previous findings regarding the presence of fill materials along the former rail line and confirmed that elevated concentrations of OCPs and lead are generally associated with these materials. The depth of fill materials ranges from approximately 2 to 11 ft bgs, and accumulations of fill are thickest in the central portion of the site. Based on current and historical data, soil chemical impacts exceeding screening criteria are limited to the upper ten feet. In evaluating historical data, Stantec compared historical and current chemical data to updated soil screening criteria for unrestricted use established by California DTSC (historical data, compared against current screening levels, are included in Appendix A with the historical data collected by WKA). Using this comparison approach, Stantec developed a conceptual soil remediation area generally conforming to the remediation area identified by WKA in 2007.This area, depicted on Figure b in Appendix B, illustrates the extent of soil/fill at the site with concentrations of OCPs and/or lead exceeding unrestricted site use screening criteria. Soil Vapor Conditions The passive soil vapor survey identified several VOCs and total petroleum hydrocarbons (TPH) in the C10 to C15 carbon chain range in one or more of the passive vapor modules. Petroleum hydrocarbons reported as TPH C10-C15 was the most commonly-detected compound, followed by benzene with nine detections. Stantec requested that Beacon (supplier of the passive vapor modules) produce isoconcentration maps for these compounds. As illustrated on Figure 4 in Appendix B, two 'hot spots' of relatively elevated TPH mass were identified in the northern portion of the site. These were generally not accompanied by elevated concentrations of benzene, where the highest concentrations were observed in the southern portion of the site. 2.6