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presence of moderate to high concentrations of gasoline. The report recommends that EPA <br /> Method 8260 when monitoring low concentrations of oxygenates.The California Regional <br /> Water Quality Control Board,San Francisco Bay Region,has provided a guidance <br /> document: Guidance on Analytical Methods for Oxygenates and Additives at Gasoline UST Sites, <br /> May 15, 1998. The RWQCB states that"false positives are common in samples with high <br /> TPH concentration". The guidance document states that EPA Method 8020 may be <br /> appropriate in areas with low concentrations of TPH,but recommends that detections be <br /> confirmed by EPA Method 8260. <br /> Conclusions <br /> Since 1991,groundwater levels have risen approximately 30 feet(from 77 ft bgs to 47 ft bgs), <br /> and the direction of groundwater flow has changed directions by almost 180 degrees. <br /> Between 1991 and July 1994,the groundwater flow direction was predominantly towards <br /> the northwest at an average groundwater flow direction and magnitude of 312°and 0.0006 <br /> ft/ft. From October 1995, through today,the groundwater flow direction has been <br /> 1 consistently towards the southeast,at an average direction and magnitude of 143°and 0.001 <br /> AM. The groundwater flow direction has been toward the southeast for the past 11 <br /> sampling events,indicating a stable flow regime. Due to the switch in flow directions, <br /> previous downgradient wells are now upgradient wells. <br /> Prior to the switch in groundwater flow directions,.the extent of the plume was defined <br /> horizontally by monitoring wells MW-DIS-3,MW-DIS-BS,MW-DIS-9,and MW-DIS-10. The <br /> vertical extent downgradient of the source area was defined by monitoring well <br /> MW-DIS-BD. Currently, the extent of the plume in the southeast direction is unknown. <br /> Eleven sampling events have occurred since the air sparging/soil vapor extraction <br /> (AS/SVE)system was turned off. Analytical results from all wells at the site have been <br /> extremely consistent,with the exception of monitoring wells MW-DIS-1 and-2,which have <br /> had significant reductions in concentrations of petroleum hydrocarbons. <br /> There have been no confirmed detections of gasoline additives or oxygenates at this site. <br /> Tert-butanol(TBA)has been detected a single time in five different wells. TBA was detected <br /> in monitoring wells NP-1-70',NP-1-90',and SVE-North on December 23, 1997. TBA has not <br /> detected in any of these wells during the past four consecutive sampling events. TBA was <br /> detected in monitoring wells MW-DIS-1,and SVE-South on May 21, 1998. TBA has not been <br /> detected in these wells during the past three consecutive sampling events. The Disco tanks <br /> were in operation between 1966 and 1976. MTBE was first used as an octane booster in the <br /> mid-1980's,and widespread use began in the 1990-1992 period in order to meet air quality <br /> requirements. Therefore,it is highly unlikely that the gasoline released from the Disco site <br /> contained any additives or oxygenates. <br /> Recommendations <br /> Extent of plume <br /> Monitoring points are needed downgradient of the site,and the vertical extent of thep lume <br /> directly beneath the source area needs to be determined.A work plan is currently being <br /> I <br /> drafted to address these issues. <br /> f <br /> 4 <br /> i <br /> I <br />