2014-05-05_HYDROLOGY - M1977348 - Laserfiche WebLink

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April 2014 4 10381640A 2.4 Previous Groundwater Monitoring Secor collected groundwater samples from the four monitoring wells (MW -1, MW -2, MW -3 and MW -4) for water quality analysis from April 1999 through July 2000. During this period, a total of eight groundwater samples were collected from each well. Golder conducted one groundwater sampling event in September 2010 and quarterly sampling of the monitoring wells beginning in March 2011. The water quality data for these wells are summarized in Table 3 through Table 6. To help evaluate whether the CKD disposal in area A2 is impacting groundwater quality, benchmark concentrations of the chemical constituents sampled were established (Golder 2011). Benchmark concentrations were based on the analytical results of the groundwater samples collected from monitoring wells MW -2, MW -3 and MW -4 during the quarterly sampling events that occurred from April 1999 through July 2000. Benchmark concentrations were determined with a step -based statistical approach outlined in the Sampling and Analysis Plan (SAP; Golder 2010). 2.5 Hydrogeologic Conditions 2.5.1 Geology The Boettcher Quarry was mined for Niobrara Formation limestone, which is of Cretaceous age and marine origin. The Niobrara Formation consists of alternating bands of limestone, argillaceous (i.e., clayey) limestone, and calcareous shale (Ideal 1977). The argillaceous limestone bands are typically fossiliferous and contain trace amounts of pyrite. The Niobrara Formation is bounded by the older Codell sandstone (of the Carlile Formation) and the younger Pierre shale (Ideal 1962). The Niobrara Formation limestone is subdivided into members (bands), identified as A through G from older to younger aged rock. The E -band is further subdivided into bands E1 through E3 based on calcium oxide (CaO) content. The boundaries within the E -band are gradational, rather than discrete, based on physical or chemical characteristics (Ideal 1977). The A band was mined at the Site because of its high limestone grade. The A through G band names and approximate thicknesses are provided on Figure 2. The Niobrara Formation strikes north -south and dips approximately 16 degrees to the east (Ideal 1977). The A -band forms a prominent north -south trending ridge or hogback along the west side of the Site and each of the overlying limestone bands form less prominent north -south hogbacks paralleling the A -band outcrop. These ridges are not present where the high lime content bands have been quarried. Changes in strike represent zones of flexure and are seen as a bend or curve in the hogback. There are some minor changes in dip and minor faulting (with small displacements) within these flexure areas (Ideal 1977). Such a curve /band is located in the area of investigation (CKD disposal area A2). Figure 3 provides a generalized cross section through the area of investigation. Golder is 110\ 81640a\ 04001gwcharaellinstallfielGrep _fnl 30apr14 \10381640a gwcharrpt_holcim boettcher_fnl 28apr14 vnth holcim e0itsAOCx Associates