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3.0 SUBSURFACE CONDITIONS 3.1 Geology Based on available historic soil boring logs, soil beneath the site and the surrounding area is generally fine-grained (predominantly silty clays, clayey silts, and clays, and minor intervals of silts, sandy silts, and material classified by cone penetration test [CPT] as "stiff fine grained") to the total depth explored of 100 feet bgs. Three laterally extensive sand strata (identified as Sands A, B, and C herein) have been encountered within the fine-grained sequence, along with several thin laterally restricted sand bodies. Sand A is encountered approximately 5 to 8 feet bgs, and is approximately 3 to 10 feet thick. Sand B is encountered at depths between 25 and 35 feet bgs; and is approximately 7 to 18 feet thick. Sand B appears to extend north to south across the entire site vicinity, but may be limited laterally to the southwest. Sand C is encountered at depths between 50 and 65 feet bgs, and is approximately 15 to greater than 25 feet thick. Geologic cross-sections illustrating interpreted stratigraphic relationships beneath the site vicinity are included as Figures 3, 4, and 5. The surface traces of the sections are shown on Figure 2b. 3.2 Hydrogeology Groundwater was first encountered during drilling at depths of about 7 to 10 feet bgs. A total of fifteen groundwater monitoring wells (MW-1 through MW-7, MW-9, MW-10, MW-11, MW-14A, MW-15A, MWO-1, MWO-2, MWO-3) have been installed across the water table and screened to depths of about 20 feet bgs to monitor groundwater occurrence and quality in Sand A. Four groundwater monitoring wells (MW-8, MWO-lx, MW-14B, and MW-1513) have been discretely screened to depths of up to approximately 40 feet bgs to monitor groundwater occurrence and quality in Sand B. In addition, one well (MW-13A/B) is screened from approximately 11 to 31 feet across both Sand A and underlying Sand B. Two groundwater monitoring wells (MW-12C and MW-13C) have been discretely screened from 60 to 70 and 65 to 75 feet bgs, respectively, to monitor groundwater occurrence and quality in Sand C. Well construction details are summarized in Table 1. 3.2.1 Sand A Monitoring Wells Historically, in the Sand A monitoring wells (MW-1 through MW-7, MW-9, MW-10, MW-11, MW-14A, MW-15A, MWO-1, MWO-2, MWO-3), depth to water levels have been as shallow as 6.2 feet bgs to as deep as 12.4 feet bgs; the average depth to water over the historic monitoring period is 9.7 feet bgs. Seasonal fluctuations of water table levels in beneath the site in Sand A wells on the order of one to three feet per year are typical (Figure 6). In general, an overall decreasing trend in water levels has been observed during the nine year historic monitoring period. Groundwater flow beneath the site, based on wells screened across the water table in Sand A, has been predominately northeast; however, in early 2006, an apparent shift of the flow direction towards the northwest was noted. L Page 11 5TKA TU5