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7-Eleven/Stockton 7 <br /> August 1990 <br /> investigate a potential upgradient source possibly emanating <br /> from the former underground-storage tank pit at the Thornton <br /> - Automotive Repair Shop, located approximately 300-feet north of <br /> the site. <br /> In November, 1939, Vapor Extraction Technology (VET) was <br /> retained. by Southland to expand the soil-vent system. <br /> Groundwater Technology continues to oversee operation of the <br /> groundwater-treatment system. <br /> In February, 1990, an on-site monitoring well-cluster <br /> __. (MW-17) was installed in the vicinity of the inactive undergrourd <br /> -storage tanks to further define the lateral and vertical extent <br /> of groundwater impacted by gasoline hydrocarbons. soil samples <br /> were analyzed to monitor the progress of the expanded soil-vent <br /> remediation system. During the same month, all wellheads were <br /> upgraded with water-tight expansion caps and new locks, and were <br /> professionally surveyed. <br /> SITE SUBSURFACE CONDITIONS <br /> The application of continuous coring during the drilling of <br /> monitoring wells MW-15, MW-15, and MW-17, provided for a more- <br /> accurate characterization of the subsurface soil profile at the <br /> site. These additional details have enhanced the interpretation <br /> of the subsurface geology and hydrogeology and have allowed <br /> Groundwater Technology to formulate a cor.cept'ual model for the <br /> geology underlying the site. <br /> Subsurface materials encountered during drilling suggest <br /> that the subsurface stratigraphy is composed of two distinct <br /> sequences. The upper sequence is marked by a basal gravelly <br /> coarse sand, overlain by fine sands and silts, and capped by a <br />