2024-01-31_REVISION - M1977344 (28)

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Red Creek Quarry, Groundwater Modeling Analysis 3 DRAFT for review purposes only. Use of contents on this sheet is subject to the limitations specified at the beginning of this document. RedCreekQuarry_Modeling_TM_DRAFT_20230911 1.3 Site Conceptual Leapfrog Model The site conceptual model was built on available data in the area and information obtained from the existing Holcim mine plan Leapfrog model. The site-specific Leapfrog geologic model includes the base Codell Sand- stone, overlying Fort Hays Limestone, and additional overlying sediments. The contact between the Codell and Fort Hays dips toward the north across the site. Groundwater occurs primarily within the Codell Sand- stone and within the lower portion of the Fort Hays limestone in localized areas. Sediments overlying the Fort Hays are generally unsaturated. 1.4 Mine Plan Operations The MPO created by Holcim includes 100 years of mining broken into 10, 10-year mine blocks consisting of a continuously developing open pit with the addition of each mine block (Figure 2). The mine blocks begin in the western portion of the site and transition to the east during mining. The mine blocks in the simplistic groundwater model are represented as flat bottom pits with a single bottom elevation. The mine blocks in the groundwater model therefore do not account for elevation change of the contact between the Fort Hays and Codell, resulting in the modeled mine blocks extending into the Codell Sandstone in the southern por- tion of the site. In reality, mining will cease when the Fort Hays/Codell contact is encountered. The flat mine block bottom assumption likely results in a conservative (over) estimate of mining impacts to groundwater and Red Creek. An exception to mining into the Codell Sandstone occurs in a planned additional smaller, deeper pit 15-feet below the top of the Codell Sandstone in Mine Block 0-10 and Mine Block 10-20 (Figure 2). Although the extent of a smaller pit into the Codell has not been explicitly defined in later Mine Plan years, it will likely oc- cur in the southern portion of the site in all subsequent mine plan phases. Due to the flat bottom representa- tion of the mine blocks, the model inadvertently simulates the sandstone pits since the mine block bottom elevations in subsequent MPO mine blocks following Mine Block 10-20 are at an elevation at least 15-feet below the top of the Codell Sandstone in the southern area of each mine block. Therefore, following Mine Block 10-20, the mine blocks do not require an additional smaller, deeper pit into the Codell Sandstone to be explicitly represented in the groundwater model. As the MPO progresses, the mine blocks will be backfilled with overburden primarily consisting of the Smokey Hill Shale member of the Niobrara Formation. Figure 2 shows the location of the MPO pit as individ- ual mine blocks as well as the two additional smaller, deeper Codell pits in the first two MPO mine blocks.