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4.0 SITE CHARACTERIZATION 4.1 Hydrogeology 4.1.2 Stratigraphy Three relatively continuous stratigraphic units can be traced from boring to boring across the site. They vary slightly in thickness and drill depth and also in color and grain size, but where penetrated by samples the contacts are generally quite sharp and the units are distinctive. The upper unit is fine grained, but varies from clay at the ground surface to silty clay or silt below a depth of 6 or 7 feet. Near the surface, the color is black, blue, or brownish gray, but in the lower part it is generally brown or grayish to reddish brown. The upper unit is between 17 and 21 feet thick. One sample of this unit was submitted to the laboratory to determine its hydraulic conductivity and porosity. This sample, collected from boring GP-4 at a depth of 16 feet, had a hydraulic conductivity of 3.7 x 10-6 cm/sec, which is typical of fine-grained sediment that is generally non- water-bearing. The middle unit consists of gray sand that coarsens downward from fine to very coarse grained. The sand is generally moderately sorted to well sorted, but thin lenses of silt are interbedded with the sand near the contact with the upper unit. In GP-2, the middle unit is only about 3 feet thick and is present between 24 and 27 feet below grade, but elsewhere the base of the unit is at about 30 feet and it is nearly 10 feet thick. Two samples of the middle unit were tested for hydraulic conductivity. Both are typical of reasonably good aquifers, with hydraulic conductivities of 1.5 x 10-3 cm/sec and 5.1 x 10-4 cm/sec. Hence,the middle unit is the first water-bearing unit beneath the site. A few borings have penetrated,the base of the middle unit and encountered blue-gray clay below. This unit is poorly known, but it is more massive than the upper unit and is relatively dry in comparison to the middle unit. One sample of the lower unit had a hydraulic conductivity of 1.1 x 10-6 cm/sec, implying that it is somewhat of an aquitard. A second sample, collected from a sandy lens within this unit at a depth of 36 feet in GP-3, had a hydraulic conductivity of 1.1 x 104 cm/sec, similar to that of the sand in the middle unit. 4.1.2 Depth to Groundwater Groundwater was present at a depth of about 19 feet when this investigation began in 1994, but began rising in 1995, reaching a depth of approximately 12 feet in 1997 (Table 1). A dramatic rise occurred in early 1998, and in May of that year the depth was less than 6 feet. Since then, the level has declined somewhat, but has remained between 7 and 11 feet(Figure 4). �. 5