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5 NO FURTHER ACTION RECOMMENDATION - 2 - 19 July 1996 7-ELEVEN, 9110 THORNTON ROAD STOCKTON, SAN JOAQUIN COUNTY Ground Water Extraction and Remediation The ground water extraction and treatment system operated between May 1986 and August 1990. Hand bailing from 1VW4 began in May 1986 two days after the well was installed. Pumping of MW4 started in July 1986. In December 1986, pumping from MW12, PW1, and PW3 was added. By early 1988, ground water levels had dropped and ground water extraction from the monitoring wells and pumping wells had become inefficient. In April 1988, a larger ground water recovery well was installed and operated until 1990. Separate phase hydrocarbons were recovered, and extracted ground water was treated by air stripping and discharged to the City sewer. Approximately 1.6 million gallons of contaminated ground water were extracted,and treated with the recovery of 780 gallons of free product. Sod vapor E=um ian and Remediation The soil vapor extraction and treatment system operated from February 1987 to January 1991. Soil vapors were initially extracted from the four vapor extraction wells. In 1990, after ground water extraction became impractical due to.the drop in ground water elevation, the vapor extraction system was expanded to include three monitoring wells,and the ground water recovery well. Extracted air and vapors were treated with carbon to remove the vapors and the clean air was discharged to the atmosphere. An estimated 576 gallons of petroleum hydrocarbons were recovered. Urxk ground Tank Excavation In 1991, the three underground tanks and the associated piping were excavated. Soil samples from" the tank pits contained low to nondetectable levels of hydrocarbons. One soil sample from under the product lines contained up to 2100 ppm gasoline.at a depth of 2.5 feet. Cation Same ing Periodic ground water sampling from June 1986 to March 1996 snowed reduction,in contamination from free product in several wells and dissolved contamination in numerous wells to no detectable concentration in any wells except for 0.8 ppb benzene in MW1. In March 1996, ground water samples were also analyzed for Methyl Tert-Butyl_Ether (MTBE), however, none was detected. Sixteen confirmation borings were drilled in 1993 and 1995 to determine the extent of soil contamination remaining: Soil contamination was found in one boring at 2600 ppb gasoline at a-depth of 20 feet, 160.ppb gasoline at 25 feet, and 180 ppb at 35 feet. A soil vapor extraction test was conducted in 1994 to evaluate residual contamination. The test revealed the continued presence of hydrocarbon r contamination at a removal rate of 0.29 pounds of gasoline per day. ResichiaL . Based on the continued presence of dissolved gasoline,in ground water, soil contamination, and hydrocarbon vapors, the remaining volume of contaminated soil was estimated to be 450 cubic yards ° containing approximately 10 gallons of gasoline. she .,h3st._` Because.the cleanup at this site was aggressive and comprehensive, no contamination is found in the domestic wells, only low levels of dissolved ground water contamination remain on site, and the soil contamination.remaining is no.longer a significant threat to water:quality, we recommend concurrence with San Joaquin County's letter of 12:March 1996, requiring no further action. M J