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MEMORANDUM CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARD • CENTRAL VALLEY REGION 3443 Routier Road, Suite A Phone: (916} 255-3000 i. Sacramento, CA 95827-3098 CALNET' 8-494-3000 TO: Gordon L. Boggs FROM: Elizabeth A. Thayer ` UST Program Manager Assoc. Engineer . DATE 19 July 1996 - - SgqCGNATURE: SUBJECT: NO FURTHER ACTION RECOMMENDATION, SoUTHLAIVD CORPORATION, 7-ELEVEN STORE NO. ?0680, 9110 THORNTON ROAD, STOCKTON, SAN- JOAQUIN COUNTY The 7-EIeven store is located in a semi-rural Iisvbdivision of Stockton, where each lot has at least one domestic water supply well. An underground tank°precision test in March 1986; indicated that there was a leak in the unleaded fuel system. It was estimated that 3000 to 5000 gallons of product were lost since November 1985. An initial investigation, indicated that the tanks were in good condition, however, the store stopped selling gasoline'iii 1986 and the tanks were emptied. On 6 May 1986, four monitoring wells were installed and free product 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 I� 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'io the households and businesses with impacted wells, instituted periodic sampling of impacted domestic weils, submitted quarterly reports on monitoring well sampling and site activities, and continued to investigate the extent of soil and ground water contamination. Dec Well Sang Some households and businesses were provided with City water and others with bottled water. Various domestic wells were sampled periodically from'June 1986 to September 1995. On 2 October 1995, after evaluating sampling results and determining that the wells were no Ionger contaminated, San Joaquin County Environmental Health Division notified the propery owners that their wells would no longer be sampled and bottled. water was'`no longer'necessary. �" r 1,wesdga&wvMKber±of Sohl and Grmmd Warr ConWninafm The extent of ground water contamination was'invesugated and defined, between June 1986 and 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 1 soil boring. At its maximum, the ground water plume covered approximately 13 acres with dissolved contamination in all but 5 of the monitoring and ground water extraction wells and with up to 4 feet: of free product in MWI, MW3, MW4, and PW3. The direction of ground waiter flow was generally 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 ground water contamination at lower-levels. Only very low-concentrations of contamination were detected initially and none was detected after August 1991 in thexdeep wells.. Soil samples were not collected for analysis from 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 sand.