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5513 OSR11 Toso Smice Station 5886 Stm n California _ _ January 21.20W . • 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 - 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) (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 (CPT1 through CPT3) and drilled three off-site soil borings (B8 through B10) (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 Currently there are three on-site, single-casing groundwater monitoring wells(MW i through MW3), four on-site,dual-casing groundwater monitoring wells(MW4S/D through MW7S/D), and nine off-site groundwater monitoring wells(MWBS/I/D through MWIOS/UD)consisting of three wells at each nested well location as shown on Plate 2 The purpose of these wells is to evaluate the lateral and vertical extent of dissolved hydrocarbons detected in groundwater beneath the site Based upon data collected during ongoing quarterly groundwater monitoring and sampling, dissolved hydrocarbons are delineated both laterally and vertically in the vicinity of the site However, dissolved 2