SITE INFORMATION AND CORRESPONDENCE

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f CHRONOLOGY ARCO Station #4493 205 Center Street Stockton, CA 95202 08/17/98 Unauthorized Release Report#98-U37 filed this date by ARCO. 09/09/98 Laboratory analytical results from sampling of tank pit observation wells were submitted to the San Joaquin County Environmental Health Department (EHD). They reported 52.4 micrograms per liter (µg11) methyl tertiary butyl ether'(MtBE) in the groundwater in one well. See SECOR International Incorporated (SECOR) report Results of Observation Well Sampling, dated August 24, 1998, located in the Unit IV LOP file. 09/10/98 Notification of Hazardous Waste Discharge (Prop 65) #98-136 filed this date. 02/11/99 Four shallow groundwater monitoring wells (MW-1 through MW-4) were installed beginning this date. See SECOR's Monitoring Well Installation Report dated July 7, 1999, available on Questy's. 02/24/99 Three 12,000 gallon underground storage tanks-were removed this date under EHD permit and inspection. Soil samples for analysis were collected from beneath the tanks, lines and dispensers. See SECOR's report Soil Sampling During UST, Product Line and Dispenser Removal Activities, dated July 7, 1999, available on Questy's. 03/10/99 Shallow overexcavation of contaminated soil under the dispenser island areas and towards the tank pit began this date. There was a discontinuous layer of asphalt across the site located -2-2.5 feet below surface grade (bsg). This was the vertical extent of the excavation. A waste oil tank was discovered during this phase of work, located under the former station building' See SECOR's report Soil Sampling During UST, Product Line and Dispenser Removal Activities, dated July 7, 1999, available on Questy's. 04/13/99 One 300 gallon waste oil tank was removed this date under EHD permit and inspection. One soil sample for analysis was collected; reported analytical results were non-detect. See SECOR's report Soil Sampling During UST Product Line and Dispenser Removal Activities, dated July 7, 1999, available on Questy's. 06/02/00 Three shallow groundwater monitoring wells (MW-5, MW-6 and MW-7) were installed downgradient to the site. See SECOR Additional Site Assessment Report dated September 9, 2000, available on Questy's. 05/06/02 Three dual completion groundwater mo riitoring wells (MW-8M/D, MW-9MID and MW-10M/D) were installed, both on- and off-site, for investigation of the vertical _ extent of the contamination. See SECOR's Dual Completion Well Installation Report, dated August 21, 2002, available on Questy's. 11/05/02 One dual completion groundwater monitoring well installed (MW-11 MID) this date. See URS' Monitoring Well Installation Report, dated February 4, 2003, available on Questy's.