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u in ERI 205Q3 OSR11 Tosca Service Station 5886. Stockton.California January 21. ZM • 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, Tosco performed the following tasks during the fourth quarter 1999 • ERI submitted the report entitled Air Sparge/Soil Vapor Extraction Test, dated October 18, 1999, presenting the data collected during ERI's May 1999, AS/SVE feasibility test • ERI submitted the report entitled Corrective Action Plan dated December 28, 1999 The report provided recommendations for a proposed groundwater treatment system consisting of a 4-inch diameter extraction well constructed in the UST cavity The extraction well will extend to a depth of approximately 12 feet into the UST cavity backfill Groundwater (up to 5,000 gallons) will be periodically purged from the UST cavity utilizing a VAC truck The groundwater will be transported to the Tosco refinery in Rodeo, California for disposal The groundwater will initially be purged once per week for a period of four weeks, after which purging will continue once per month Quarterly groundwater monitoring and sampling data will be used to evaluate the success of purging for remediation of dissolved hydrocarbons in groundwater and plume stability • Gettler-Ryan, Inc (GRI) of Dublin,California performed groundwater monitoring and sampling at the site on November 29 and 30, 1999 The GRI Fourth Quarter 1999 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 6 to 9 7 feet bgs ul the groundwater monitoring wells (an elevation increase of approximately 0 01 to 0 26 feet since the third quarter 1999 groundwater monitoring and sampling event) During November 1999, groundwater flow direction was towards the west-southwest at a calculated hydraulic gradient ranging from approximately 0 003 to 0 004 Historical groundwater flow directions and gradients are shown on the Groundwater Flow Direction Rose Diagram (Plate 3) • Total purgeable petroleum hydrocarbons (TPPHg) and benzene were not detected at or above laboratory method detection limits in groundwater samples collected from the groundwater monitoring wells However, detection limits were elevated for groundwater samples collected from wells MW2 and MW3 • Diesel-range hydrocarbons were not detected at or above laboratory method detection limits in groundwater samples collected from wells MW4S, MW5S, MW6D, MW7D, MW7S, MW8S through MW8D, MW9S through MW91) and MW101 Diesel-range hydrocarbons were detected in the remaining wells at concentrations ranging from 50 ppb to 98 ppb However, diesel-range hydrocarbons were not detected at or above laboratory method detection limits in the samples after the silica gel clean-up was performed (with the exception of wells MW 1 and MW2) Unidentified diesel-range hydrocarbons were • 3 I I