SITE INFORMATION AND CORRESPONDENCE

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ik Mr. John Welty April 10, 1990 Page 2 vapor lines under the concrete slab at the gasoline pump island. Subsequently, Southland, 7-Eleven's parent company, contracted GTI to investigate the extent of contamination caused by the leak. It was estimated that between 3,000 and'5,000 gallons of gasoline was leaked. A site assessment report and abatement proposal were prepared in September of 1986. During the period from April to January 1990, 20 monitoring,wells were constructed in the vicinity of the 7-Eleven site. Routine monitoring of groundwater,,quality was begun to determine the extent of the contaminant plume. In December 1986, laboratory testing of groundwater quality revealed that a domestic well at 8968 Thornton Road was contaminated with gasoline constituents. At the time contamination of domestic wells was discovered, property owners were provided with bottled water by Southland. Figure 1 is a site map showing the monitoring well locations. Hydrogeoiogy. The site has an elevation of approximately 11 feet above mean sea level. The regional hydrogeology in the area consists of semi-consolidated saline water bearing units and overlying unconsolidated fresh water bearing units''; The unconsolidated units consist of gravels, sands, silts and clays with intervals of hardpan. Th!e unconsolidated units have a total thickness of 1,100 feet. The site geology, based on borehole logs by GTI, consists of surfzciai silty and sandy clay which varies between 2 to 12 feet in thickness. The clays grade into a sand interval which become coarser with depth and extends to a depth of between 30 and 34 feet. The sand interval is underlain by a dominantly clay interval to the maximum depth explored at 70 feet. An aquitard is estimated to be at a depth of 70 feet below the site. GTI interprets the subsurface as two distinct sequences. The upper sequence is a gravelly, coarse sand, overlain by fine sands and silts and capped by a silty clay. The lower sequence consists of laterally discontinuous sand bodies encased within silt and clay. Figure 2 is a cross-section of the site as developed by GTI. Figure 1 shows the location of the cross-section. Groundwater beneath the site was encountered at a depth of approximately 30 feet in May of 1986 and had dropped to a depth of 35 to 40 feet below grade by August of 1989. The groundwater flow direction since May of 1986 has continued to be toward the east, with peribdic swings to the southeast and northeast. The most recent Project Update Report available in the RWQCB files was prepared by GTI in February 1990. The report summarized that the water table had increased approximately 1.3 feet and that Wells 2, 3, 4, 5, 10, 12, and PW-2 remained dry. Groundwater flow remained in an easterly direction. The gradient decreased from 6.005 feet/feet to 0.002 feettfeet during the previous 3-month period. Remediation. A soil remediation system has been`operating at the site since February 1987 and a groundwater remediation system has been in operation since May of 1986. One well (RW-1) is currently being used to extract groundwater. Brown and Caldwell i