ARCHIVED REPORTS_XR0007233 - Laserfiche WebLink

Home Browse Search

4.0 FINDINGS 4.1 Stratigraphy The stratigraphy appears to vary considerably across the site AGS reported clayey,very-fine- grained sand at the surface in MW-1, MW-2, and MW-3 In the northeastern part of the site, however, clayey silt to fine-grained sandy silt is present at the surface, and extends to between 6 and 12 feet in borings B-7, MW-4, and MW-5 Silt was also encountered in B-8 and MW-6 The silt overlies moderately-sorted to well-sorted gray to tan sand,which extends to 25 or 30 feet in all borings In the continuously cored boring (MW-6), the sand fines downward from 16 to 24 feet, but then coarsens to the base of this unit at 27 5 feet Photographs of the core are shown in Figure 3 Brown to gray sandy silt is present from 27 5 to 30 feet in MW-6 Clay is present in this interval in B-8, MW-1, and MW-3, and sand is present in B-7, MW-2, MW-4, and MW-5 A red, oxidized, buried soil was penetrated at 30 feet in MW-4- and MW-5, in the northeastern corner of the site This silty clay contains wood and other plant fragments This soil is probably the paleosol at the top of the Riverbank Formation (Figure 4) It was not recognized in other borings, and may be missing due to erosion A relatively widespread clay or clayey silt bed is present from 35 to 40 feet, but below 30 feet the dominant lrthology is sand in all borings The sand is tan or gi ay in color and coarsens downward from silty and fine grained to gravelly and coarse grained The base of this sand was not penetrated in borings B-7 and B-8, which were terminated at 65 feet, or in MW-1 through MW-4, which were terminated at about 80 feet The base occurs at 78 feet in MW-5 Greenish gray silt is present to the bottom of the boring at 85 feet (Appendix A) 4 2 Relative Groundwater Elevation and Gradient — During drilling on January 17, 1994, most samples above 40 feet were damp From 40 to 60 feet, samples were moist, and from 62 5 to 72 5 feet, samples were wet Below this depth, samples were saturated These data indicate that the capillary fringe zone may be as much as 30 feet thick at this site When groundwater was monitored on January 26, 1994, the depth to groundwater ranged from 7168 feet (MW-4) to 73 00 feet (MW-3) The relative groundwater elevation was approximately 27 feet (Table 1) By February 16, the depth to water had decreased by a few hundredths to several tenths of a foot, in all wells The depth data yield doubtful results for the elevation of the prezometric surface On both January 26 and February 16, the calculated elevation of this surface is anomalously low in MW-5, and on January 26 the elevation is either too low in MW-6 or too high in MW-2 The anomalies do not appear to be related to casing surveys The problem with MW-5 may G.ob l A.41 S,m— Inc (1 L-1wr-d 8-,_.4R,1 93,S5a9 5 I I