Laserfiche WebLink
NO FURTHERgACTION RECOMMENDATION 2 19'July 1996 <br /> 7-ELEVEN, 9110 THORNTON ROAD "' •'' �" <br /> STOCKTON, SAN'JOAQUIN COUNTY <br /> Ground Water Exracdon and Remediation <br /> The ground water extraction and treatment system operated between May 1986 and August 1990. <br /> Hand bailing from MW4 began in May:1986 two days after the well was installed. Pumping of MW4 <br /> started'in July 1986. In December 1986, pumping from MW12, PW1, and PW3 was added. By early <br /> 1988, ground water levels had dropped and ground water extraction from the monitoring wells and <br /> pumping wells had become inefficient. In April 1988, a larger ground water recovery well was <br /> installed and operated until 1990:, Separate phase.-hydrocarbons were recovered-and extracted <br /> ground water was treated .by air stripping and discharged to the City sewer. Approximately 1.6 <br /> million gallons of contaminated ground water were extracted and treated with the recovery of 780 <br /> gallons of-free, product. - - <br /> Saul Vapor Extraction and Remediation . <br /> The soil uapo`r extraction'and treatment'system operated^froin February'1987'to January 1991. Soil <br /> vapors were initially extracted from.the four vapor extraction wells. In 1990, after ground water <br /> extraction became impractical due to the'diop in groundwater elevation, the vapor extraction system <br /> was expanded to include"three,monitoring wells and'the ground water recovery well. Extracted air <br /> and vapors were'treated with carbonto remove the vapors and the clean air was discharged to the' <br /> atmosphere. An estimated 576 gallons of petroleum.hydrocarbons were recovered.` <br /> UandTank Excavation <br /> In 1991, the three underground'tanks and the associated'piping were excavated. Soil samples from <br /> the tank pits contained low to nondetectable levels of hydrocarbons. One soil sample from under the <br /> product lines'contained.up'to 2100 ppm gasoline at a depth of 2.5 feet. a, y ' <br /> w <br /> Coi11'fini�ation Sam � t .. <br /> Periodic ground water sampling from June 1986 to March 1996 showed reduction in contamination <br /> from free product in several wells and dissolved contamination in numerous wells to no detectable <br /> concentration in.any wells except for.0.8 ppb benzene in MWL In March 1996, ground water samples <br /> were`also`analyzed for'Methyl Tert-Butyl Ether (MTBE); however, none was detected. Sixteen, <br /> confirmation borings were'ddrilled in 1993 and .1995.,to-determine the extent of soil contamination w <br /> remaining. . Soil"contamination was found in one boring qt 600 ppb gasoline.at a depth of 20 feet, <br /> .160 ppb gasoline at 25 feet, and 180 ppb at 35 feet. A soil vapor extraction"test.was conducted in <br /> 1994 to evaluate residual contamination. The test revealed the continued presence of hydrocarbon' <br /> contamination at a removal rate of 0.29.pounds.of gasoline per day_ <br /> Resldtral, ri <br /> .Based on.the continued'.presence`of dissolved gasoline in groundwater, soil contamination, and <br /> hydrocarbon vapors, the remaining volume of'contaminated soil was estimated to be 450 cubic yards <br /> ately 10 gallonsof gasoline: " ' <br /> containing approxim ' n. <br /> 11 1 <br /> Site Ucmik e <br /> ,-Because the cleanup at this site was`aggressive and'comprehensive, no{contamination-is found in the <br /> domestic wells,'only low levels of dissolved,ground water contamination.remain on site,-and the soil <br /> contamination remaining is no longer;.a significant threat to water quality, we recommend concurrence <br /> �` with San Joaquin County's letter of 12 March,1996,requiring h6 further action.'` "' <br />