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Geosyntec" r,on„(Jllaln, 2, BACKGROUND This section summarizes relevant background information including the Site location,use,history, Site hydrostratigraphy, and previous remedial activities.A detailed description of Site background has been presented extensively in previous Site documents, including the Remedial Investigation (RI) Report (LFR, 1999), the Final Feasibility Study Report (FS; Geosyntec, 2015a), Remedial Action Plan(RAP;Geosyntec,2015b),and the Remedial Design/Remedial Action(RD/RA)Work Plan (Geosyntec,2015c). 2.1 Site History The Site is a retail shopping center encompassing approximately 30 acres (Figure 2) in Stockton, California. In the late 1950s, several dry-cleaning businesses began operations at the Site. Due to disposal practices at the time, wastewater containing dissolved phase dry-cleaning fluids was discharged into the sanitary sewer lines that serviced the property abutting Benjamin Holt Avenue. Following entry into the sanitary sewer, dry-cleaning fluids subsequently migrated downwards to groundwater beneath the dry-cleaning facilities through leaks and cracks in the sanitary sewer lines (LFR, 1999). The primary constituents of concern (COC) at the Site are tetrachloroethene (PCE) and its degradation byproducts including tichloroethylene (TCE), cis-1,2-dichloroethylene (cis-DCE), trans-1,2-dichloroethylene (trans-DCE)and vinyl chloride. 2.2 Site Hydrostratigraphy The aquifers beneath the Site are delineated into three hydrostratigraphic zones, the A-zone, B- zone, and C-zone. The A-zone extends from approximately 35 to 75 feet below ground surface (bgs)and appears to be partially confined. The upper A-zone interval extends from approximately 35 to 50 feet bgs and the lower A-zone interval extends from approximately 65 to 75 feet bgs. The A- and B-zone aquifers appear to be relatively distinct in the former source area. The B-zone aquifer extends from approximately 80 to 140 feet bgs. There appears to be a laterally continuous aquitard beneath the B-zone that consists of hard silty clay. The C-zone extends from approximately 150 to 250 feet bgs. The lack of COCs in C-zone groundwater suggests that the aquitard between the B-zone and C-zone is competent and has impeded the migration of PCE-impacted groundwater from the B-zone aquifer to the C-zone aquifer. 2.3 Water Levels and Gradient Sitewide groundwater levels in the A-zone and B-zone typically range from approximately 20 to 35 feet bgs (approximately -10 to -25 feet mean sea level [mslj). C-zone groundwater levels historically range from approximately 23 to 55 feet bgs (approximately -12 to -44 feet msl). The groundwater level varies seasonally with higher groundwater levels generally occurring after precipitation during the winter months. This is specifically pronounced in the A-zone, where groundwater elevations increased by approximately 3 feet after substantial precipitation events in Fortner Source Area Injection Work Plan 3 August 03,2019