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' = 04 August 2000 AGE-NC Pioject No 98-0521 - Page 3 of 11 r - north,and the Sierra Nevada to the east This trough has been filled with sediments derived from both ' - marine and continental sources Thickness of the sedimentary fill ranges from thin veneers along the — valley edges to gi eater than 20,000 feet in the south central portion of the valley The sedimentary formations i ange in age with the older deposits being primarily marine in origin and the younger ' deposits being primarily continental Continental-derived sediments were primarily deposited in lacustrinc, fluvial, and alluvial environments with sources being the mountain ranges surrounding the valley (Olmsted and Davis, 1961) The site is located on unconsolidated and semi-consolidated . - - alluvium, lake, playa and-tefi ace deposits of Quaternary age (California Division of Mines and Geology, 1977) -- ' _ The Modesto, Riverbank and Turlock Lake Formations and oveilying Recent alluvium are the piincipal sources of domestic ground water in the 13,500-square mile San Joaquin Valley Ground - Water Basin (Basin 5-22) ' 2 4 GROUND WATER DEPTH AND FLOW DIRECTION - Based on the San Joaquin County Flood Control District and Water Conservation District Lines of ' Equal Depth to Ground Water Fall 1998 map, the estimated depth to ground water in the vicinity of the site is between 10 and-20 feet below sur face grade (bsg) During the UST removal, ground - water was encounter ed at a depth of approximately 10 feet bsg T - _ - Based upon data obtained from nearby sites,the prevailing ground water flow direction in the vicinity of the site is estimated to be north However, flow direction may be both locally and seasonably ' variable The Stockton Deep Water Channel is the nearest surface water feature to the property, - located approximately 750 feet west of the site Water from the channel drains into the San Joaquin 1 River and is primarily used for commercial and recreational boating, with agricultural usage- as a secondary use The channel contains-water continually and has a potential tidal effect upon ground water depth of flow direction in the area 3.0 PROCEDURES - - The-field activities wei e petformed in actor dance with the PHS-EHD-approved Preliminary Site Assessment Work Plan pi epared by AGE,dated 29 November 1999 and Preliminary Site Assessment Work Plan Addendum for monitoring well installation prepared by AGE, dated 17 May 2000 Three gi ound water-monitoring wells have been installed at the site as part of the site assessment,however, additional gi ound water monitoring wells were proposed in the-initial scope of work The additional ' wells will be installed after a vale ftradient has Been determined for the site The remaining wells will • need to be established either down-gradient and/or outside of the known dissolved hydrocarbon plume - _ 4dvanced GeoEnvlronmental,Inc