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2. SITE INVESTIGATIONS <br /> 2- 1 PREVIOUS INVESTIGATIONS <br /> When the underground storage tanks Were removed, soil samples <br /> were taken from the excavations for the three fuel tanks and the <br /> waste oil tank (BTS 1987 ) . The site was investigated in March <br /> 1987 by Groundwater Technology, Inc. (GTI 1987), who installed <br /> three monitoring wells (MW1-MW3, Figure 3 ) around the location of <br /> the former tank field. The boreholes were drilled, and soil <br /> samples taken, on 18 March; the wells were sampled on 19 March. <br /> In August 1987, EA ( 1987 ) sampled water in nearby ►rater supply <br /> wells for analys. s, and evaluated those and the earlier results. <br /> 2. 1 . 1 Site_Hydrogeology <br /> t� GTI reported a depth to the water table of 40 feet and a slope of <br /> 0 . 004, down to the north (GTI 1987 ) . Although it is nearly an <br /> order of magnitude larger than the regional value (FCWCD 1986) , <br /> this value for the gradient is not unreasonable, considering the <br /> highly heterogeneous nature of local aquifers and the proximity <br /> of domestic water wells. <br /> Although representative measured values of hydraulic parameters <br /> for the Stocktqn area (conductivity, transmissivity) were not <br /> found in the literature, driller's logs were found for the wells <br /> shown in Figure 1 . These indicate that lenses of sand or of sand <br /> and gravel, commonly from to 20 feet thick, are found at <br /> various depths between 50 and 200 feet. Ine axiller's report for <br /> the well nearest the site (3246 E. F--.cont) indicates sand layers <br /> whose depths-to-top are 25, 69, 109, and 130 feet. The thickest <br /> extends from 130 to 180 feet. Based on representative data, the <br /> averege hydraulic conductivity (R) of these highly permeable <br /> sand-and-gravel lenses may be about 10-3 meters per second <br /> (m/sac) (Freeze and Cherry 1379) . Given a gradient (i ) of 0.404 <br /> c23/72g2 3 <br />