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14 (1 ERI 205513 QSR17 Tosco Service Station 5m, Stockton Califorrua 1u] 23 2001 • During second quarter 2000, ERT 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 gallons of groundwater during second and third quarters 2000 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 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 MW7SID), one on-site UST cavity well, and nine off-site groundwater monitoring wells (MW8S/I/D through MW10S/IID) 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 hydrocarbons continue to be detected in groundwater samples from beneath the site and are concentrated in the vicinity of the UST cavity SECOND QUARTER ACTIVITIES Based upon previous work completed at the site, and in accordance with the correspondence and • request trom the County, the following tasks were performed during the second quarter 2001 • Approximately 63,000 gallons of MTBE impacted groundwater were purged from beneath the site and transported to the Tosco refinery for disposal Approximately 156,500 gallons of groundwater have been purged from beneath the site since IR activities were initiated in June 2000 • During June 2001, Tracer Research Corporation (TRC) performed an enhanced leak detection test on underground storage systems to evaluate the system integrity • Tosco arranged to have surface cracks on the site seated to reduce the possibility of surface hydrocarbon spills impacting soil • Gettler-Ryan, Inc (GRI) of Dublin, California collected monthly groundwater samples from on-site wells MW I through MW4D/MW4S and MW6DIMW6S for MTBE analysis and performed quarterly groundwater monitoring and sampling at the site on May 7 and 8, 2001 The GRI Second Quarter 2001 Groundwater Monitoring and Sampling Report is provided as Attachment A The results of the second quarter monitoring and sampling event indicate the following • Groundwater was measured at approximately 6 0 to 9 8 feet below ground surface (bgs) in the groundwater monitoring wells (an elevation variation of approximately 0 1 to 0 4 feet when compared to the first quarter 2001 groundwater monitoring and sampling event) During May 2001, groundwater flow direction was towards the southwest with a calculated hydraulic gradient of approximately 0 002 to 0 006 Historical groundwater flow directions are shown on the Groundwater Flow Direction Rose Diagram (Plate 3) 3