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S E C 0 R 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(138 through B10) (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—Tosco submitted the Work Plan for Supplemental Evaluation (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, ASISVE 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 Groundwater purging was temporarily discontinued during third quarter 2000 pending clarification from the County regarding water sampling and data collection requirements Purging was resumed during January 2001, and continued through January 2002, when it was terminated following installation and operation of the microsparge remediation system Approximately 285,000 gallons of groundwater were purged from the UST cavity well and well MW3 and transported to the Tosco refinery for recycling during interim groundwater remediation efforts • During June 2001, Tracer Research Corporation (TRC) performed an enhanced leak detection test on underground storage systems to evaluate the system integrity According to Tosco, the results of the enhanced detection test indicated that the underground storage systems are functioning within acceptable parameters • During third quarter 2001, ERI submitted a CAP, dated September 25, 2001 The CAP included recommendations to install a microsparge system to remediate MTBE impacted groundwater beneath the site • During first quarter 2002, the microsparge remediation system was completed and ERI performed system startup and initiated monthly operation and maintenance (O&M)of the system Currently, there are ten on-site AS wells (AS1 through ASS, AS7, ASB, and ASiO, and nested wells AS6S1D and AS9SID), three on-site, single-casing groundwater monitoring wells (MW1 through MW3), four on-site, dual-casing groundwater monitoring wells (MW4SID through MW7SID), one on- site UST cavity well, and nine off-site groundwater monitoring wells (MWBSIIID through MW10SIIID) consisting of three wells at each nested well location as shown on Plate 2 and the On-Site Well Locations (Plate 3)