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ERI 205513 OSR15 Tosco Service Station 5886 Stockton California Janun 25 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 (MW4S/D through MW7S/D), one on-site UST cavity well, and nine off-site groundwater monitoring wells (MWBSII/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 duringongoing g oin g 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 ' FOURTH 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 fourth quarter 2000 • Gettler-Ryan, Inc (GRI) of Dublin, California performed groundwater monitoring and sampling at the site on November 16 and 17, 2000 The GRI Fourth Quarter 2000 Groundwater Monitoring and Sampling Report is provided as Attachment A The results of the fourth quarter monitoring and sampling event indicate the following • Groundwater was measured at approximately 6 1 to 9 1 feet below ground surface (bgs) in the groundwater monitoring wells (an elevation increase of approximately 0 4 to 0 8 feet 1 when compared to the third quarter 2000 groundwater monitoring and sampling event) During November 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 Flow 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, laboratory detection limits were elevated for groundwater samples collected from wells MW 1 through MW3 • Diesel-range hydrocarbons were not detected at or above laboratory method detection Iimlts 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 260 parts per billion (ppb) However, diesel-range hydrocarbons were not detected at or above laboratory method detection limits in this 3