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r too ERI 205513 OSR10 Tosco 76 Service Station 5886,Stockton.Califortna _ October,27. 1999 Based upon the results of the soil and groundwater investigation, during April 1999, ERI installed, developed, and sampled nine off-site groundwater monitoring wells (MW8S/I/D through MW lOS1I1D) at three cluster locations (three 3-well clusters) (ERI, June 9, 1999) The well locations are shown on Plate 2 The analytical laboratory results of groundwater samples collected from cluster wells MW8SII/D through MW IOS1I/D indicated that dissolved gasoline and diesel hydrocarbons and oxygenated compounds (MTBE, methanol, and ethanol) were not detected at or above laboratory method detection limits in groundwater samples collected from groundwater monitoring well MW9S However, concentrations of dissolved hydrocarbons and select oxygenated compounds were detected in groundwater samples collected from the remaining wells The groundwater samples collected from wells MW8S/IID through MWIOS/I/D were not subjected to a silica get cleanup process prior to Environmental Protection Agency (EPA) Method 8015 modified laboratory analysis for TEPHd Laboratory report notes indicate that diesel range hydrocarbons detected in groundwater samples collected from well MW8S through MW8D, MW9I, and MW 10D were classified as unidentified hydrocarbons or were reported as diesel based upon a single analyte peak because a fuel hydrocarbon pattern was not present On April 16, 1999, in response to a letter from the County that was not dated but was received (via facsimile) on March 26, 1999, ERI submitted a Work Plan to the County for a supplemental evaluation of groundwater downgradient of the site The Work Plan included a recommendation to perform four quarters of groundwater monitoring and sampling of wells MW i through MW l0S/11D prior to the implementation of the Work Plan Tosco has not received a response letter from the County regarding the scope or implementation of the Work Plan During May 1999, ERI performed an extended vapor extraction test (VET) at the site The results of VET will be reported under a separate cover THIRD 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 third quarter 1999 • Gettler-Ryan, Inc (GRI) of Dublin, California performed groundwater monitoring and sampling at the site on August 16 and 17, 1999 The GRI Third Quarter 1999 Groundwater Monitoring and Sampling Report is provided as Attachment A The results of the quarterly monitoring and sampling event indicate the following • Groundwater was measured at approximately 6 7 to 9 8 feet bgs in the groundwater monitoring wells (an elevation decrease of approximately 0 1 to 0 5 feet since the second quarter 1999 groundwater monitoring and sampling event During August 1999, groundwater flow direction was towards the west-southwest at a calculated hydraulic gradient ranging from approximately 0 002 to 0 004 Historical groundwater flow directions and gradients are shown on the Groundwater Flow Direction and Gradient Diagram (Plate 3) i 4