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• 1.0 INTRODUCTION Stratus Environmental, Inc (Stratus), on behalf of Atlantic Richfield Company (ARCO, a BP- affiliated company) has prepared this Additional Subsurface Assessment Report and Site Conceptual Model Update 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 and vertical extent of petroleum hydrocarbon impact to soil appears to be adequately characterized 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 to 67 feet bgs in the • upper sandy interval, and 89 to 107 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 two off-site wells screened across the lower sandy interval As of March 28, 2006, approximately 287 gallons of free product/impacted groundwater mixture have been manually removed from wells at the site An on-site water supply well is located in the southwestern portion of the property This well is screened from approximately 396 to 416 feet bgs and 496 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 August 11, 2005, San Joaquin County Environmental Health Department (SJCEHD) requested that a work plan be prepared proposing activities to further characterize the extent of subsurface petroleum hydrocarbon impact in areas adjacent to the service station property Stratus submitted Additional Site Assessment Work Plan (September 21, 2005), SJCEHD subsequently approved the proposed scope of work in a letter dated October 3, 2005 Advancement of 5 exploratory soil borings, and installation of 5 additional groundwater monitoring wells, has recently been completed at the site Details associated with completing this work, and findings of the additional subsurface assessment, are described in this document Soil vapor extraction (SVE) is currently being performed at the site Between March 2004 and November 2004, two 200 cubic feet per minute (cfm) electric catalytic oxidizers were utilized at the site The electric catalytic oxidizers were subsequently removed, and a 500 cfm thermal oxidizer was installed SVE using the thermal oxidizer was initiated in May 2005 As of K\Hp Arco for URS161005Rcports%100 assessment rpt&scm update doc Page 1 5MATU5