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• ERI 2Q5513 QSR14 Tosco S,.crvgQStation 5 6 t ckt n alt or a cto r 27 2 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 MW7S/D), one on-site UST cavity well, and nine off-site groundwater monitoring wells (MW8S/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 THIRD 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 third quarter 2000 • Gettler-Ryan, Inc (GRI) of Dublin, California performed groundwater monitoring and sampling at the site on August 3 and 4, 2000 The GRI Third Quarter 2000 Groundwater Monitoring and Sampling Report is provided as Attachment A The results of the third quarter monitoring and sampling event indicate the following • Groundwater was measured at approximately 6 6 to 9 5 feet below ground surface (bgs) in the groundwater monitoring wells (an elevation decrease of approximately 0 6 to 0 9 feet when compared to the second quarter 2000 groundwater monitoring and sampling event) During August 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 • 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 wells MW2 and MW3 Diesel-range hydrocarbons were detected in the groundwater samples collected from wells MW2 and MW3 up to 56 3 parts per billion (ppb) However, diesel-range hydrocarbons were not detected at or above laboratory method detection Iimits in the samples after the silica gel clean-up was performed • MTBE was not detected at or above laboratory method detection limits in groundwater samples collected from wells MW5D, MW5S, MW7D/MW7S through MW9S/MW9I/MW9D, MWlOS, and MW1OD MTBE was detected in the remaining wells ranging from 2 61 ppb to 20,900 ppb using Environmental Protection Agency (EPA) Method 8260, (ranging from 6 01 ppb to 28,700 ppb using EPA Method 8020) MTBE was not detected in well MW4S using EPA Method 8020 • 3