ARCHIVED REPORTS XR0006135 - Laserfiche WebLink

Home Browse Search

within this unit at a depth of approximately 10 feet, particularly beneath the Chevron property (cross sections A-A' and B-B'). The middle unit is saturated and is the first water-bearing zone below the surface. It 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 top, indicating that it is in gradational contact with the upper unit. Beneath much of the Kwikee Foods site, the middle unit is more than 10 feet thick, but it thins westward and is only about 3-4 feet thick in GP-2 and less than 1 foot thick in MW-6, although it apparently thickens again toward MW-3 (cross section B-B'). It thins rapidly west of MW-1 and is apparently absent in MW-4 (cross section A- A'). Figure 4 illustrates these thickness variations in map view. Although thickness measurements are precise only in the Kwikee Geoprobe® borings, which were continuously cored, it is evident that the sand bed is a curvilinear body that trends and thickens from southwest to east across the two sites. The maximum thickness is greater than 10 feet on the eastern side of the Kwikee site. Figure 4 also reveals that the middle unit widens in an eastward direction, and beneath the Kwikee site is more than twice its width beneath the Chevron site. Hence, in comparison to the Kwikee site, the aquifer is laterally relatively confined beneath the Chevron site. Presumably, this would create some degree of flow convergence beneath the Chevron site and flow divergence beneath the Kwikee site, and would have important implications for migration of dissolved-phase hydrocarbon contaminants. Several borings have penetrated the base of the'middle unit and encountered blue-gray clay below. In GP-5 (cross section A-A') this contact is sharp and erosional. The lower unit is more uniform and massive than the upper unit and is relatively dry in comparison to the middle unit: The characteristics of this unit indicate that it is a significant aquitard (see below). Figure 5 is a related figure that shows the approximate depth to the top of the middle unit. This map has not been corrected for differences in elevation of the ground surface, which could introduce some errors, but they would be very minor because both sites are basically flat. It is evident that the depth of this bed varies by more than S feet across the area, which is much greater than could be accounted for by differences in ground elevations. To some extent, the sand body is thickest in those borings where the top of the body is highest(e. g. GP-5, KF-2, MW-1), and the top of the bed is lower in areas where the bed is also thin(e. g. MW-6). This implies that the thinning of the bed is due in part to loss of the upper portion of the unit, as shown between KF-5 and MW-6 in cross section B-B'. However, thinning also occurs due to differences in the depth of erosion at the base of the sand body, as shown between MW-1 and S134 in cross section A-A'. 2.1.2 Hydraulic Conductivity .Table 1 presents data on hydraulic conductivity determined from selected core samples from the Kwikee Foods soil borings. Samples were collected from each of the three lithologic units. Although the data represent vertical conductivity, and horizontal conductivity may be greater (or 6