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=t ERI 205513 0SR9 Tosco 76 Service Station,Stockton.California _ _ lulu 22 1924 • Based upon the results of the soil and groundwater investigation, ERI proposed the installation of nine off-site groundwater monitoring wells (three 3-well clusters) (ERI, March 2, 1999) The County approved the well installation and locations in a letter that was not dated but was received (via facsimile) on March 26, 1999 SECOND 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 second quarter 1999 • Gettler-Ryan, Inc (GRI) of Dublin, California performed groundwater monitoring and sampling at the site on June 1 and 2, 1999 The GRI Second 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 3 to 9 9 feet bgs in the groundwater monitoring wells (an elevation decrease of approximately 0 4 to 0 8 feet since the first quarter 1999 groundwater monitoring and sampling event and installation of wells MWBS/I/D through MW 10S/1/D) During June 1999, groundwater flow direction was towards the southwest at a calculated hydraulic gradient ranging from approximately 0 002 to 0 003 Historical groundwater flow directions and gradients are shown on the Groundwater Flow Direction and Gradient Diagram (Plate 3) S - TPPHg and benzene were not detected at or above laboratory method detection linuts 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 the groundwater monitoring wells MTBE was not detected in samples collected from wells MWSD, MWSS, MW7D, MW7S, MWBS, MW81, MW9S through MW9D, and MWIOD MTBE was detected in the remaining wells at concentrations ranging from 4 0 ppb to 36,000 ppb (4 7 ppb to 36,000 ppb using Environmental Protection Agency (EPA) Method 8260) Oxygenated compounds ethanol, tertiary butanol, di-isopropyl ether (DIPE), ethyl tertiary butyl ether (ETBE), and tertiary amyl methyl ether (TAME) were not detected at or above laboratory method detection limits • During April 1999, ERI performed a groundwater evaluation downgradient of the site (ERI, June 9, 1999) The scope of work included installation, development, and sampling of nine off-site groundwater monitoring wells (MWBS/IID through MWIOS/IID) at three cluster locations (three 3-well clusters) The well locations are shown on Plate 2 The data obtained during the investigation indicated the following 4