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ERI 205513 OSR16 Tosco Service Station 5886 Stockton Caltforma Aril 26 2001 • 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 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 Currently, there are three on-site, single-casing groundwater monitoring wells (MWI through MW3), four on-site, dual-casing groundwater monitoring wells (MW4SID through MW7S/D), one on-site UST cavity well, and nine off-site groundwater monitoring wells (MW8SI1/D through MW10S/1/D) 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 FIRST QUARTER ACTIVITIES Based upon previous work completed at the site, and in accordance with the correspondence and request from the County, the following tasks were performed during the first quarter 2001 • During January 2001, Tosco resumed IR activities at the site following clarification of the sampling and evaluation requirements provided by the County in their December 4, 2000 letter Between January 24 and February 7, 2001, three groundwater purging events were performed at the site, during which approximately 15,000 gallons of groundwater were purged from the UST well and well MW3 The purged water was transported to the Tosco Refinery in Rodeo, California for disposal During each purging event, groundwater samples were collected from the UST cavity well and well MW3 both before and after purging, and a water sample was collected from the purged groundwater Based on the results of laboratory analysis of the groundwater samples, ERI calculated that approximately 14 pounds of MTBE were removed from groundwater during the initial January 24 through February 7, 2001 groundwater purging events (ER1, March 28, 2001) • During first quarter 2001, approximately 47,500 gallons of MTBE impacted groundwater was purged from beneath the site and transported to the Tosco refinery for disposal Approximately 93,500 gallons of groundwater have been purged from beneath the site since IR activities were initiated in June 2000 • Gettler-Ryan, Inc (GRI) of Dublin, California performed groundwater monitoring and sampling at the site on February 5 and 6, 2001 The GRI First Quarter 20401 Groundwater Monitoring and Sampling Report is provided as Attachment A The results of the first quarter monitoring and sampling event indicate the following . 3