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August 2013 2 103- 81640A <br />2.0 BACKGROUND INFORMATION <br />The Site is located on the northwestern fringe of the Colorado Piedmont subprovince of the Great Plains <br />Physiographic Province (Topper and others 2003). This subprovince is characterized by sedimentary <br />rock formed from Pennsylvanian through Tertiary deposits, including the Late Cretaceous Niobrara <br />Formation and Carlile Shale (Topper and others 2003), both of which crop out at the Site. <br />2.1 Geology <br />The Site was mined for limestone in the Niobrara Formation, which is Late Cretaceous age and of marine <br />origin. The Niobrara Formation consists of alternating bands of limestone, argillaceous (i.e., clayey) <br />limestone, and calcareous shale (Ideal 1977). The argillaceous limestone bands are typically fossiliferous <br />and contain trace amounts of pyrite. The Niobrara Formation is bounded at its base by the older Codell <br />sandstone (the top unit of the Carlile Formation) and at its top by the younger Pierre shale, as shown on <br />the geologic stratigraphic column of the Boettcher Quarry area (Ideal 1962) in Figure 2. The Niobrara <br />Formation is subdivided into members (bands), identified as A through G from older to younger aged <br />rock. The boundaries within the E -band are gradational, rather than physically or chemically distinct <br />(Ideal 1977). The A through G bands names and approximate thicknesses are provided in Figure 2. <br />The Niobrara Formation strikes north -south and dips approximately 16 degrees east (Ideal 1977). The A <br />band forms a prominent north -south trending hogback along the west side of the quarry. Each of the <br />overlying limestone bands form less prominent north -south ridges or hogbacks paralleling the A -band <br />outcrop. These ridges are removed where the high lime content bands have been quarried. Changes in <br />strike represent zones of flexure and are seen as a bend or curve in the hogback. There are some minor <br />changes in dip and minor faulting (small displacement) within these flexure areas (Ideal 1977). Such a <br />curve /band is located in the area of interest (the CKD disposal areas A2 and A2 -A). Figure 3 is a <br />generalized geologic cross section of the Site <br />2.2 Hydrogeology <br />The Dakota - Cheyenne aquifer is the principal aquifer of the Colorado Piedmont subprovince and is <br />comprised of the Early Cretaceous Dakota Sandstone and the Cheyenne Sandstone Member of the <br />Purgatoire Formation (Topper and others 2003). The Niobrara Formation and Carlile Formation are not <br />considered to be part of a principal aquifer system, but portions of these units can yield some water to a <br />few stock wells in parts of the Colorado Piedmont (Topper and others 2003). Previous mining activities <br />and the existing monitoring wells at the Site are limited to the Niobrara Formation and the top of the <br />Carlile Formation (the Codell sandstone) and do not penetrate into the Dakota - Cheyenne aquifer. <br />The groundwater flow direction has been observed to generally flow southeasterly using the July 30, 2010 <br />groundwater level data from monitoring wells MW -1 through MW -4 (Golder 2011). Monitoring wells MW -1 <br />through MW -3 were inferred to be installed within or at the interface of the Codell sandstone and <br />i \10 \81640a\ 0400\ gwcharwellinstalifieldrep_ 09aug13\ gwcharwelhnstallfieldrep _09aug13 docx <br />t Golder <br />Associates <br />