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• 1.0 INTRODUCTION AND EXECUTIVE SUMMARY Stratus Environmental, Inc (Stratus), on behalf of Atlantic Richfield Company (ARCO, a BP- affiliated company) has prepared this Subsurface Assessment Report and Updated Site Conceptual Model for ARCO Service Station 6100, located at 25775 Patterson Pass Road, Tracy, California (the site, see Figure 1) The site is an active service station, situated on the eastern slope of the coast range mountains, at an elevation of approximately 290 feet above sea level The areas surrounding the property are undeveloped and utilized for cattle grazing Subsurface petroleum hydrocarbon impact to soil was discovered in June 1990, and the former underground storage tanks (USTs) were replaced in November 1990 Based on historical soil analytical data, petroleum hydrocarbon impact is primarily centered near the fuel dispenser islands, product line trenches, and former UST cavity Petroleum hydrocarbon impact to soil generally extends to approximately 105 feet below ground surface (bgs) on-site and 115 feet bgs off-site The lateral extent of petroleum hydrocarbon impact to soil does not appear to be fully characterized southeast of the site Regional groundwater appears to be deeper than 200 feet bgs in the site vicinity Perched groundwater has been encountered in monitoring wells screened across two separate sandy intervals Groundwater has been measured at depths of approximately 63 and 67 feet bgs in the upper sandy interval and 93 to 105 feet bgs in the lower sandy interval, although several wells . screened across these sandy intervals do not contain groundwater Free product has historically been reported in an off-site well screened across the lower sandy interval Approximately 190 gallons of free product/impacted groundwater have been manually removed from wells at the site Free product in the off-site monitoring well has decreased from a maximum thickness of 189 feet in December 2000, to thicknesses of a sheen to 0 03 feet during 2004 monitoring An on-site water supply well is located in the southwestern portion of the property This well is screened from approximately 396 to 536 feet bgs, and has a static water level of approximately 40 to 45 feet bgs Petroleum hydrocarbons have not been detected in quarterly groundwater samples collected from this water supply well In a letter dated July 17, 2002, San Joaquin County Public Health Services, Environmental Health Division (PHS/EHD) requested that a Site Conceptual Model (SCM), and a work plan for additional assessment based on the SCM, be prepared The July 17, 2002 letter also requested a plan to reduce petroleum hydrocarbon mass present in the subsurface beneath the site Stratus subsequently submitted the SCM (October 3, 2002), Work Plan for Additional Site Assessment (Work Plan, October 15, 2002), and Remedial Action Plan (RAP, July 1, 2003), San Joaquin County PHS/EHD subsequently approved the Work Plan in a letter dated December 16, 2002, and the RAP in a letter dated August 27, 2003 This report documents the findings of the work activities associated with implementing the Work Plan The RAP proposed the installation of a soil vapor extraction (SVE) system at the site SVE was • initiated in March 2004, using two electric catalytic oxidizers As of June 7, 2004, the SVE P\Bp-Arco for ISRS\61061Reponslnrco6100-o sstrent si.t,Vdnte&c Page 1 5 MA W6