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�. ERI 205513 OSR18 Tosco Service Station 5886 Stockton California October 25 2001 During August 2001, groundwater flow direction was towards the southwest with a calculated hydraulic gradient of approximately 0 004 Historical groundwater flow directions are shown on the Groundwater Flow Direction Rose Diagram (Plate 3) • Total petroleum hydrocarbons as diesel (TPHd), total petroleum hydrocarbons as gasoline (TPHg), and benzene were not detected at concentrations at or above laboratory method detection limits in groundwater samples collected from the groundwater monitoring wells However, groundwater samples collected from wells MW8S/MW8I/MW8D through MW10S/MW10I/MW10D were analyzed beyond the recommended holding time for TPHd analysis • MTBE was not detected at concentrations at or above laboratory method detection limits in groundwater samples collected from wells MWSD, MWSS, MW7D/MW7S, MW8S/MW8I, MW9S/MW9I/MW9D, and MW IOS/MW l0I/MW10D (Environmental Protection Agency [EPA] Method 8260) MTBE was detected in the remaining wells at concentrations ranging from 2 4 ppb to 5,200 ppb using EPA Method 8260 (ranging from 5 1 ppb to 8,300 ppb using EPA Method 8020) MTBE was not detected in wells MW4S, MW6D, MW6S, and MW8D using EPA Method 8020 • Oxygenated compounds ethanol, tertiary butyl alcohol (TBA), dl-isopropyl ether (DIPS), ethyl tertiary butyl ether (ETBE), and tertiary amyl methyl ether (TAME), and lead scavengers 1,2-dichloroethane (1,2-DCA) and ethylene dibromide (EDB) were not detected at concentrations at or above laboratory method detection limits in the groundwater samples collected from the wells FOURTH QUARTER 2001 ACTIVITIES The following activities are proposed to be completed during fourth quarter 2001 • ERI will prepare and submit to the County the remedial design package for the microsparge groundwater remediation system proposed in ERI's September 25, 2001 CAP • ERI will prepare and submit an electrical permit application to the City of Stockton (the City) to construct the proposed microsparge groundwater remediation system • Tosco will continue weekly purging activities at the site utilizing the UST cavity well and groundwater monitoring well MW3 as extraction wells until the proposed microsparge groundwater remediation system is constructed and started The extracted groundwater will be transported to the Tosco refinery for disposal • Tosco will continue to collect monthly groundwater samples from wells MW 1 through MW4S/D and wells MW6S/D for a period of 12 consecutive months to evaluate dissolved MTBE concentrations in groundwater beneath the site • Tosca will continue to perform quarterly groundwater monitoring and sampling at the subject site Groundwater samples will be collected from all wells (MW 1 through MW 10S/I/D) during the • 4