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111 205513 0SR13Tosco Service-Station al f r 2 000 groundwater monitoring wells(MW$S/I/D through MWIOS/I/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 SECOND 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 second quarter 2000 • Upon authorization from the County to install a UST cavity well in general accordance with the CAP (ERI, December 28, 1999) and without a County well installation permit, ERI installed the UST cavity well at the site The well location is shown on Plate 2 • Tosco purged approximately 15,000 gallons of groundwater from the UST cavity during June 2000 The extracted groundwater was transported to the Tosco Refinery in Rodeo, California for disposal • Gettler-Ryan,Inc (GRI) of Dublin, California performed groundwater monitoring and sampling at the site on May 10 and 11, 2000 The GRI Second Quarter 2000 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 5 S to 8 9 feet below ground surface (bgs) in the groundwater monitoring wells (an elevation decrease of approximately 0 9 to 2 2 feet when compared to the first quarter 2000 groundwater monitoring and sampling event) During May 2000, groundwater flow direction was towards the southwest at a calculated hydraulic gradient of approximately 0 004 Historical groundwater flow directions are shown on the Groundwater FIow Direction Rose Diagram(Plate 3) • Total purgeable petroleum hydrocarbons as gasoline (TPPHg) and benzene were not detected at or above laboratory method detection limits in groundwater samples collected from the groundwater monitoring wells However, detection lirnits were elevated for groundwater samples collected from wells MW2, MW3, and MW6D • Diesel-range hydrocarbons were not detected at or above laboratory method detection limits in groundwater samples collected from the groundwater monitoring wells with the exception of well MW3 Diesel range hydrocarbons were detected in the groundwater sample collected from well MW3 at 223 parts per billion (ppb) and SO 2 ppb following the silica gel clean-up • 3