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ERI 205513 OSR20 Tosco Service Station 5886 Stockton Caltforma A rtl 25 2002 • September 1996 - Unocal drilled four additional soil borings, and constructed groundwater monitoring wells in three of these borings (Delta, December 5, 1996) • March 1997 - Tosco submitted a Work Plan for Supplemental Environmental Investigation (ERI, March 28, 1997) to the County • April 1997 - Tosco submitted an Addendum to Work Plan (ERI, April 3, 1997) to the County • April 1997 - Tosco performed a soil and groundwater investigation at the site The investigation included hand-auguring three soil borings adjacent to catch basins, drilling four soil borings, and constructing dual-completion groundwater monitoring wells (MW4S/D through MW7S/D) at discrete depths in the borings (ERI, June 3, 1997) • July 1997 - Tosco submitted a Work Plan for Supplemental Environmental Investigation (ERI, July 14, 1997) to the County • August 1997 - Tosca 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) (Plate 2), continued to perform 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 (CPTI through CPT3) and drilled three off-site soil borings (138 through 1310) (ERI, March 2, 1999) • 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 - Tosca submitted the Work Plan for Supplemental Evaluation (ERI, April 16, 1999) to the County • May 1999 - ERI submitted the report entitled Air SpargelSotl Vapor(AS/SVE) Extraction Test, dated October 18, 1999, presenting the data collected during ERI's May 1999, AS/SVE feasibility test • December 28, 1999 - ERI submitted the report entitled Corrective Action Plan (CAP) 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 (IR) action • During second quarter 2000, ERI installed one UST cavity well in general accordance with the CAP (ERI, December 28, 1999) and Tosco initiated periodic purging of groundwater from the UST cavity well and from groundwater monitoring well MW3 Tosco purged approximately 46,000 2