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URS RIECUVED OCT 18 2002 ENVIRONMEN F HEALTH October 9, 2002 PERMIT/SERVICES Mr Harlin Knoll 1119106 a4 Senior Environmental Health Specialist San Joaquin County Environmental Health Department 304 East Weber Avenue Third Floor Stockton, CA 95202 Re Supplemental Investigation Workplan Addendum BP Service Station#11191 1469 East Hammer Lane Stockton,California Dear Mr Knoll URS Corporation Americas (URS), on behalf of Group Environmental Management, is pleased to present this addendum to the June 21, 2002, Supplemental Investigation Work Plan prepared by Cambria Environmental This addendum proposes changes in the well locations and screen depths of the three proposed additional groundwater monitoring wells at the former British Petroleum (BP) service station located at 1469 East Hammer Lane in Stockton, California (site) Based on the August 5, 2002, telephone conversation between URS and your office, URS has revised the scope of work to include the mstallation of two shallow off-site groundwater monitoring wells and one deep on-site groundwater monitoring well The shallow wells are being installed to allow for the lateral assessment of the groundwater conditions south of the site The deep monitoring well will be installed dust west of the current UST locations in the area expected to contain the highest groundwater petroleum concentrations to assess the vertical extent of petroleum impacted groundwater SITE LOCATION AND STATUS The site is an active 76-branded gasoline retail outlet located at the northwest corner of the intersection of Hammer Lane and West Lane in Stockton, California (see Figure 1) BP acquired the site as a gasoline retail outlet from Mobil Oil Corporation in 1989 In 1994, BP transferred the gasoline retail outlet to TOSCO Marketing Company (TOSCO) and has not operated the facility since that time SUBSURFACE CONDITIONS The sail types at the site consist primarily of silty clays and clayey and sandy silts to approximately 25-35 feet bgs, which are underlain by sandy gravels, silty and gravelly sands, and sands to the total depth explored of 80 feet bgs In December 2001, groundwater was first observed at depths ranging from 40 to 45 feet bgs and static water levels range from 44 to 49 feet bgs Based on first and second quarter 2002 monitoring data, groundwater beneath the site flows westward at a horizontal hydraulic gradient of 0 001 to 0 003 URS Corporation Crown Corporate Center 2870 Gateway Oaks Drive, Suite300 Sacramento CA 95833 tel 916 679 2000 K V5 BP ARCO%11 191/11191 WP Addendum doe Fax 916 679 2900