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1.0 EXECUTIVE SUMMARY Stratus Environmental, Inc (Stratus), on behalf of Palisades Gas and Wash (Palisades), has prepared the following Site Conceptual Model and Subsurface Investigation Report (SCM) for Palisades Station No 835 (the site), located at 2705 Country Club Boulevard, Stockton, California (see Figure 1 and Figure 2) This SCM was prepared pursuant to a request from San Joaquin County Environmental Health Department (SJCEHD) An Exxon-branded service station operated at the site until November 1995 Subsurface assessment activities were initiated in August 1991 by consultants representing Exxon Total petroleum hydrocarbons as gasoline (TPHG), and benzene, toluene, ethylbenzene, and xylene (BTEX) impact to soil and groundwater was discovered at this time The site underground storage tanks (USTs), fuel dispensers, and product piping were removed in March 1994, new USTs and a new fuel delivery system were installed at a different location in December 1995 Approximately 762 cubic yards of soil generated during the December 1995 site renovation activities were removed from the site at this time The majority of the site assessment activities were completed prior to Palisades' purchase of the site in November 1997 TPHG and benzene were reported at maximum concentrations of 880 milligrams per kilogram (mg/Kg) and 3 5 mg/Kg, respectively, for a soil sample collected beneath the former USTs Petroleum hydrocarbon impacted soil was also reported in the vicinity of the fuel dispenser situated in the eastern portion of the property, parallel to Ryde Avenue (see Figure 2) Groundwater monitoring wells were never installed within the area of highest petroleum hydrocarbon impact to soil The highest concentrations of TPHG (4,000 micrograms per liter [µg/Ll) and benzene (1,700 pg/L) reported during 1990's groundwater monitoring and sampling were from well MW-3 TPHG and benzene were reported at concentrations of 25,000 µg/L and 500 µg/L, respectively, from a grab groundwater sample collected during the excavation of the current UST cavity This grab groundwater sample was collected near the location of well VEW-3 (installed 1997) Depth to groundwater beneath the site has ranged from approximately 6 to 17 feet below ground surface (bgs) (well MW-1) since monitoring was initiated in 1991 Groundwater elevations have generally risen during the monitoring period Northeast and southeast groundwater flow directions, and low groundwater gradients, have predominately been observed Increased groundwater elevations have allowed for the measurement and sampling of groundwater at well VEW-3 since 2001 TPHG, benzene, methyl tertiary butyl ether (MTBE), and tertiary butyl alcohol (TBA) have been reported at maximum concentrations of 16,600 µg/L, 344 µg/L, 54,000 pg/L, and 280,000 pg/L, respectively, for samples collected from well VEW-3 between 2002 and 2005 Although compliance and site investigation soil samples were not chemically analyzed for MTBE and TBA, it is our understanding that MTBE laden gasoline would have been stored and . distributed at the site prior to removal of the former USTs and fuel dispensers in March 1994 K\USA Gaso1me\0835\Reportslusa835-scm-assessment-report doc Page 1 STPATU5