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Stantec Site History and Previous Investigations Stockton Charter Way Commingled Plume February 15, 2011 collected from MW-3-79, MW-3-119, or MW-10-81. TPHg and benzene were detected in soil samples collected during the installation activities at concentrations up to 4.5 and 1.2 mg/kg, respectively. TPHg, total petroleum hydrocarbons as diesel (TPHd) and benzene were detected in groundwater samples collected from the wells at concentrations up to 13,000, 720, and 8,600 pg/L, respectively. These activities are summarized in the Cambria Site Investigation and Groundwater Monitoring Report— Fourth Quarter 2005 dated December 16, 2005. In October 2007, WDC Exploration and Wells of Zamora, California installed one A zone (MW- 23) and three B zone (MW-20-83.5, MW-21-80, and MW-22-80) groundwater monitoring wells to further assess the vertical and horizontal extent of petroleum hydrocarbons. TPHg was detected in four soil samples at concentrations up to 250 mg/kg. These activities are summarized in the Cambria Site Investigation and Fourth Quarter 2007 Groundwater Monitoring Report dated February 15, 2008. In a May 6, 2008, Updated Site Conceptual Model, CRA stated that Cambria completed a well survey in June 1999 and identified one water supply well and three municipal wells located greater than 2,000 feet south and northeast of the site. A groundwater monitoring and sampling program has been in place at this property since February 1992. Remediation Activities In October 1988, two dispensers were removed, and soil impacted by petroleum hydrocarbons beneath the eastern dispenser was excavated to an approximate depth of 15 ft below ground surface (bgs). Soil collected from the bottom of the excavation contained total petroleum hydrocarbons as gasoline (TPHg) and benzene at concentrations up to 5,300 and 31 milligrams per kilogram (mg/kg), respectively. These activities are summarized in the GTI Environmental Assessment Status Report dated October 9, 1990. Separate-phase hydrocarbons (SPH) have historically been detected in well MW-1, but no measurable SPH has been observed since March 1993. Between May 1989 and December 1989, approximately 45 gallons of SPH were recovered from well MW-1. In April 1992, GTI installed four soil vapor extraction (SVE) wells (VEW-1 through VEW-4) and conducted an SVE pilot test. These activities are summarized in the GTI Summary Report of Soil Vapor Extraction Test Sampling Activities dated September 16, 1992. An SVE system began operation at the Site in August 1995. These activities are summarized in the SECOR Soil Vapor Extraction System Installation and Start-up dated September 12, 1996. Between January 1996 and March 1998, the SVE system removed approximately 6,250 pounds (Ibs) of TPHg. The SVE system was shut down on March 9, 1998, due to declining influent concentrations and rising groundwater. In June 1999, K.E. Curtis Construction Company removed one 12,000-gallon diesel UST, three 10,000-gallon gasoline USTs, four dispenser pumps, and their associated piping. Approximately 701 tons of soil and pea gravel were excavated during UST removal activities. In Appendix A- Site History and Previous Investigations.doc 8