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ERI 205505 R02 Tosco Service Station 5886,Stockton,California September 25,2001 16 • July 1997 - Tosco submitted a Work Plan for Supplemental Environmental Investigation (ERI, July 14, 1997) to the County • August 1997 - Tosco submitted the Addendum to Work Plan (ERI, August 8, 1997) to the County • July 1997 through December 1998 - Tosco performed an underground utility search (ERI, October 6, 1997), continued quarterly groundwater monitoring and sampling, obtained access agreements to drill soil borings at select properties located south and southwest of the site, and requested that Sequoia Analytical Laboratories (Sequoia) review chromatograms of groundwater samples collected during the September 1995 Geoprobe® investigation (Delta, February 6, 1996) and provide an estimate of methyl tertiary butyl ether (MTBE) concentrations in groundwater (ERI, March 2, 1999) • February 1999 - Tosco advanced three off-site cone penetrometers (CPT1 through CPT3) and drilled three off-site soil borings (B8 through B10) (ERI, March 2, 1999) L • April 1999 - Tosco installed nine off-site groundwater monitoring wells, consisting of three nested wells (shallow, intermediate, and deep) at each of three well locations (ERI, June 9, 1999) • April 1999 - Tosco submitted the Work Plan for Supplemental Evaluation of Groundwater (ERI, April 16, 1999) to the County • May 1999 - ERI submitted the report entitled Air Sparge/Soil Vapor(AS/SVE) Extraction Test, dated October 18, 1999, presenting the data collected during ERI's May 1999 AS/SVE feasibility test • December 1999 - ERI submitted a CAP for the site (ERI, December 28, 1999) Tosco received a letter from the County dated February 22, 2000, approving the remediation recommendations provided in ERI's December 28, 1999 CAP as an interim remedial action(IRAP) • Second quarter 2000 - ERI installed one tank cavity well in general accordance with ERI's December 28, 1999 CAP and Tosca initiated periodic purging of groundwater from the UST cavity ' well and from groundwater monitoring well MW3 To date, Tosco has purged approxiinately 201,000 gallons of hydrocarbon-impacted groundwater from the UST cavity well and well MW3 • June 2001 - Tracer Research Corporation (TRC)performed an enhanced leak detection test on underground storage systems to evaluate the system integrity The results of the enhanced leak detection test indicated that the underground storage system integrity is acceptable Currently, there are three on-site, single-casing groundwater monitoring wells (MW1 through MW3), four on-site, dual-casing groundwater monitoring wells (MW4S/D through MW7S/D), one on-site UST cavity well, and nine off-site groundwater monitoring wells (MW8S/1/D through MW10S/I/D) consisting of three wells at each nested well location, as shown on Plate 2 The boring logs are provided in Appendix B The purpose of these wells is to evaluate the lateral and vertical extent of idissolved hydrocarbons detected in groundwater beneath the site ( 3