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1 1 ERI 205505 R01 Tosco Service Station 5886, Stockton California December 28, 1999 • 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 soli 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 1 • August 1997 - Tosca submitted the Addendum to Work Plan (ERI, August 8, 1997) to the County • JuIy 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 Geoprobelm 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 CPTl through CPT3 and • Y p ( 8 ) drilled three off-site soil borings (B8 through Bla) (ERI, March 2, 1999) • April 1999 - Tosco installed nine off-site groundwater monitoring wells, each consisting of three wells (shallow, intermediate, and deep) at each well location (ERI, June 9, 1999) ' April 1999 - Tosco submitted the Work Plan for Supplemental Evaluation (ERI, April 16, 1999) to ' the County • May 1999 - Tosco performed an extended air sparge/soil vapor extraction test at the site (ERI, October 18, 1999) 3