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MEMORANDUM CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARD-* CENTRAL VALLEY REGION 3443 Routier Road, Suite A Phone: (916) 255-3000 Sacramento, CA 95827-3098 CALNRT: 8-494-3000 f TO: 'Gordon L. Boggs FROM: Elizabeth A. Thayer UST Program Manager Assoc. Engineer k DATE: 19 July 1996 ci" ' SIGNATURE: � s Ir SUBJECT: NO FURTHER ACTION RECOMMENDATION, SOUTHLAND CORPORATION, 7-ELEVEN STORE NO. 20580, 91I0.THORNTON ROAD, STOCKTON, SAN JOA QUIN COUNTY' t; { . r, f,, �:: , The 7-Eleven store is located in a semi-rural subdivision of Stockton'; where each lot has at least one domestic water supply well.',An underground tank precision test in March 1986,:indicated that there y was a leak in the unleaded fuel system. It was estimated.that 3000",toP5000 gallons of roduct were r •� lost since November 1985.' An initial investigation,.indicated that the tanks were in good condition, however, the store stopped sellin asolinein 1986 and the tanks were emptied. On' 6 Ma 1986 four Pp g g p May monitorin wells were installed and free roduct'was detected in two of the wells. In May and June 1986, five additional monitoring wells were installed. Sampling in June and July 1986 indicated that 14 of the 19 tested domestic.supply wells were contaminated with gasoline and\or BTEX constituents. Subsequent sampling revealed two additional domestic wells were contaminated. .- Southland provided alternative drinking water supplies to the households and businesses with y impacted wells, instituted periodic sampling of impacted domestic wells, submitted quarterly reports. on monitoring well sampling and site activities, and continued to investigate_the extent of soil And, �I ground water contamination' 4 Donne511C Wel SaMPEW s- Some households and businesses were provided with! City,water and'' others with bottled-water. li Various domestic wells were lsampled periodically from June 1986 to September 1995. On 2 October 1995, after evaluating sampling results and determining that the wells were no longer contaminated, I -San Joaquin Country Enviroriirierltal.r€edit;�' �1, .Ii hon ecl: lhe;-prop y� oivnersAhate-:thei wells.�..rbu1d; •- .. no longer be sampled and bottled.water was no longer necessary. It of Soil c'lllLl G1mmd.W The extent of ground water contamination was investigated and defined, between June.1986 and i September 1991, with a total of 18 ground.water'monitoring wells, 2 three-well clusters;:3 pumping wells, 1 ground water recovery well, 4 vapor 'extraction wells, and 11'soil boring. At its maximum, the. ground water plume covered approximately l3 acres with dissolved contamination in all but 5.of the monitoring and.ground water extraction wells and with up to 4 feet4,of free product-in MW1, MW3; MW4, and.PW3. The direction of ground water-flowgenerally easterly or southeasterly. Depth,to ground water varied between 30 and 50 feet bsg Several wells were drilled and screened below-60 feet in order-,"to investigate g ound 'water contamination at lower-levels. .Only.very-low concentrations of contamination-were detected,initially and none was detected aftei-August 1991-in the*deep;wells. Soil samples were not collected for analysisWnrri�most of these wells and borings, therefore, the extent of soil contamination was not well defined initially...'Soils to a depth of 50 feet-were predominantly sands with layers of clay, silty clay,'and silty san i.-