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i amec 4.2 SITE HYDROGEOLOGY Soils beneath the site consist mostly of clay and silt that contain varying minor amounts of sand (Geomatrix, 2007b). Prior assessments, described in the CAP, indicate a relatively continuous, thick layer of clay upgradient, beneath, and immediately downgradient of the site. Data collected off site indicate clay from approximately 10 to 20 feet bgs down to about 60 to 70 feet bgs. On-site data indicate continuous clay from ground surface to 45 feet bgs, with a silt layer at about 35 feet of variable thickness. These data also indicate that sediments become less fine grained and more highly interbedded farther downgradient of the site to the northeast. Very few sand layers are present beneath the site and vicinity. Data presented in the CAP from cone penetrometer measurements indicated thin layers of stiff, fine-grained material, possibly sand, occasionally within the first 90 feet and a silty sand layer at a depth from about 100 to 110 feet bgs and a fine to medium-grained sand layer a depth of about 120 to 125 feet bgs in one boring (Geomatrix, 2007b). Cross-sections of the site and vicinity are shown on Figures 3 and 4. 4.3 GROUNDWATER OCCURRENCE Ground occurrence at the site is measured by a network of on-site and off-site monitoring wells. The construction details of these wells are summarized in Table 1. Depth to groundwater beneath the site and vicinity varies from approximately 25 to 35 feet bgs. Groundwater occurs in this area under unconfined to semi-confined conditions. Groundwater elevations measured in March 2010 are shown on Figure 5. 4.4 GROUNDWATER GRADIENTS The horizontal gradient of groundwater at the water table on March 16, 2010, was approximately 0.0038 (19.7 feet per mile)to the northeast (AMEC, 2010b). This gradient direction and magnitude are consistent with those measured since the on-site wells were installed in May 2005. The horizontal gradient of groundwater measured between depths of 60 to 80 feet bgs on March 16, 2010, was approximately 0.0023 (11.9 feet per mile) to the north-northeast. The potentiometric surface contours for these two sets of groundwater levels are shown on Figure 5. The gradient along a straight line between the deepest wells (MW-5C and MW-9, completed at depths greater than 80 feet bgs)was 0.0029 (15.0 feet per mile) on March 16, 2010. The vertical gradient of groundwater in the vicinity of the site was calculated using the groundwater level measurements made in the MW-5 cluster wells. The vertical gradient of groundwater at the location of these wells on March 16, 2010, was 0.006 upward between AMEC Geomatrix, Inc. 1:113000s\132901Archive113290-49.DOC 4