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i+ Oxa RI 205513 OSR12T S,co Serum Station 5886,Stoclan. CajifQrn18_ _ Agrjl 2 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, Tosco performed the following tasks during the first quarter 2000 • Tosco received a letter from the County dated February 22, 2000, approving the remediation recommendations provided in ERI's December 28, 1999, CAP as an interim remedial action The County letter is provided in Attachment A In the CAP, ERI proposed a 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 below ground surface Groundwater (up to 5,000 gallons) will be periodically purged from the UST cavity utilizing a vacuum (VAC) truck The groundwater will be transported to the Tosca 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 on a monthly schedule (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 February 28 and 29, 2000 The GRI First Quarter 2000 Groundwater Monitoring and Sampling Report is provided as Attachment B The results of the first quarter monitoring and sampling event indicate the following • Groundwater was measured at approximately 4 1 to 6 9 feet bgs in the groundwater monitoring wells (an elevation increase of approximately 1 6 to 3 1 feet when compared to the fourth quarter 1999 groundwater monitoring and sampling event) During February 2000, groundwater flow direction was towards the southwest at a calculated hydraulic gradient ranging from approximately 0 005 to 0 01 Historical groundwater flow directions and gradients 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, detection limits were elevated for groundwater samples collected from well MW2 Diesel-range hydrocarbons were not detected at or above laboratory method detection limits in groundwater samples collected from wells MW1, MW2, MW4S, MW513, MW5S, MW6S, MW7D, MW7S, MW8D, MW10S, and MW10D Diesel-range hydrocarbons were detected in the remaining wells at concentrations ranging from 51 ppb to 110 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 MW9S, MW91, and MW IOI) Unidentified diesel-range hydrocarbons 3