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Jonas & Associates Inc <br /> ' 6.0 REMEDIATION EFFECTIVENESS AND COUNTY CLOSURE <br /> The source of contamination, a 1,500-gallon UST, associated piping, and the fuel <br /> ' dispenser, were removed from the facility in 1987 A mass balance calculation of <br /> contaminants based upon 1987 and 1988 data supported an estimate of <br /> ' approximately 129 pounds, or 22 gallons of gasoline in the subsurface at that time <br /> (PG&E, 1996) <br /> ' Groundwater sampling and analysis performed from 1987 through 1995, indicates <br /> a disappearing plume of groundwater contamination Monitoring well OW-2 has <br /> been found to be consistently downgradient, and is within 10 feet of the former UST, <br /> ' and is the only well which has ever shown any level of gasoline contamination Wells <br /> OW-4, OW-5, and OW-6, located approximately 60 feet from the former UST, have <br /> also been found to be consistently downgradient of the former UST <br /> rDuring the last four consecutive quarterly sampling events, TPH-G and TEX were <br /> not detected in any of the wells Benzene (0 72 ppb) was detected only in OW-2 on <br /> August 8, 1995 The detected benzene concentration is below the drinking water <br /> standard (MCL) of 1 ppb MTBE (29 ppb) was also detected in OW-2 during the <br /> April 16, 1995 sampling event <br /> Analytical results for the last four quarters of groundwater data collected at the site <br /> indicates that groundwater is no longer being affected by tank leakage On May 28, <br /> ' 1996, San Joaquin County's Public Health Services Department issued a letter to <br /> PG&E accepting closure of the former UST <br /> 1 <br /> 1 <br /> 14 <br />