2023-10-03_REVISION - M1977344 (32)

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Red Creek Quarry, Groundwater Modeling Analysis 7 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 2.2 Model Layering The RCQ groundwater model consists of 6 layers. Model layering was exported from the Holcim mine plan Leapfrog model. Since groundwater generally only occurs within the Codell Sandstone, all sediments overly- ing the Codell Sandstone were combined into one layer for input to the groundwater flow model. Layer 1 rep- resents the combined sediments above the sandstone, determined from the Leapfrog model. Layers 2 through 5 represent the upper portion of the Codell Sandstone impacted by the MPO. Layer 6 represents the remaining, lower Codell Sandstone unaffected by the MPO. The total depth of layer 6 was determined from the Leapfrog model. Layers 2 through 5 allow for the change in hydraulic properties of the backfilled mine blocks as the MPO pro- gresses. Each layer in the upper portion of the Codell Sandstone is approximately 25-feet thick with the bot- tom of layer 5 reaching approximately 100-feet below the top of the Codell Sandstone. This depth was deter- mined from the deepest mine block, Mine Plan 60-70, in relation to the top of the Codell Sandstone in the Leapfrog model. 2.3 Hydraulic Boundary Conditions General Head boundary conditions were assigned along the model edges to simulate groundwater levels based on the May 2023 water table generated from static groundwater measurements at the five monitor well locations (Figure 1). The May 2023 groundwater contours were extrapolated to the model boundary to form the interpolated water table for the site. There is a measure of uncertainty in the May 2023 interpo- lated water table due to lack of data in the central, eastern, and southern portions of the site. Figure 1 shows the five monitor wells used for water table generation in the northwest portion of the site. 2.4 Hydraulic Properties Slug Testing was performed in May 2023 at groundwater monitor wells 1 through 4 completed in the Codell Sandstone (Figure 1). The hydraulic conductivity for the Codell Sandstone (layers 2 through 6) was deter- mined from these tests to be approximately 0.011 feet per day (BC, 2023). The hydraulic properties for Layer 1 were assigned based on existing hydraulic information for the Fort Hayes Limestone. Layer 1 repre- sents the Fort Hayes and all the overlying units, however, the overlying sediments are unsaturated and therefore not accounted for in the groundwater model. Hydraulic properties for layer 1 were estimated based on previous groundwater modeling efforts in the area to be 0.0054 feet per day (RGI, 1999). As the MPO progresses, the mined out areas are backfilled as discussed in Section 1.4. Due to a lack of available data regarding hydraulic properties of the overburden consisting primarily of the Smokey Hill Shale, the backfill material was estimated to have a hydraulic conductivity of 0.5 feet per day, two orders of magnitude larger than the sediments in Layer 1. Due to uncertainty of hydraulic properties of the backfill material, a model sensitivity analysis on backfill hydraulic conductivity was conducted and was not found to be a sensi- tive parameter during the analysis. 2.5 Groundwater Recharge and Storage Recharge from precipitation is assumed to be negligible due to laterally extensive shale units in the unsatu- rated zone within the model domain. Aerial recharge may occur in fault zones within the limestone; however, due to lack of available data, aerial recharge was estimated based on historical reports for the area (USGS, 1985; USGS, 1987). Specific storage was estimated from slug testing preformed in May 2023 as discussed in Section 2.4 (BC, 2023).