2023-10-03_REVISION - M1977344 (22)

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Red Creek Quarry Baseline Water Investigation Section 3 3-8 DRAFT for review purposes only. Use of contents on this sheet is subject to the limitations specified at the end of this docu ment. 3.5 Groundwater Modeling Groundwater modeling was conducted in support of mine permitting activities to evaluate potential groundwater impacts from planned mining operations at the RCQ (BC 2023b, Appendix 3.7). The objective of the RCQ groundwater modeling was to simulate a flow field that is representative of groundwater flow conditions in the area to provide a tool that will aid in evaluation of impacts to groundwater from planned mining activities. Specifically, the RCQ model was used to estimate the volume of groundwater which accumulates in the mine pits during each 10-year mining period over 100 years, and potential effects from mining to groundwater contributions to flow in Red Creek, and ephemeral streams. A transient numerical groundwater flow model was developed using the U.S. Geological Survey (USGS) code MODFLOW. Groundwater modeling was conducted in a sequential order as described below and was conducted during June and July 2023. The predictive groundwater model was developed to assist in the assessment of potential groundwater impacts from mining operations. The groundwater modeling included: 1) developing a numerical groundwater flow model to represent Existing Conditions (EC); 2) calibration and sensitivity analysis of the EC model; and 3) simulating the mine plan operations (MPO) using the calibrated model. The EC model development included: • Model domain and grid spacing development was selected to allow representation of site features and MPO. • Model layering was developed using the Holcim mine plan Leap Frog model. • Hydraulic properties including horizontal and vertical hydraulic conductivity, and storage were estimated based on site specific hydraulic conductivity estimates from slug testing and historical reports for the area. • Boundary conditions included a combination of drains and general head boundaries. • Groundwater recharge due to precipitation was assumed to be negligible due to laterally extensive shale units within the limestone. Aerial recharge may occur in fault zones within the limestone; however, definition of individual fault zones was not currently available for the Site. Aerial recharge was estimated based on historical reports for the area (U.S. Geological Survey [USGS] 1985; USGS 1987). • EC Model calibration was conducted to benchmark the model against measured groundwater conditions at the Site. Hydraulic properties were allowed to vary within reasonable ranges during model calibration. Sensitivity of the model to changes in hydraulic properties was also evaluated. • Simulation of MPO to represent 10-year mining periods were conducted after the calibration of the EC model was established. An assessment of groundwater impacts for each 10-year period of mining operations was conducted. 3.6 Surface Water Reconnaissance BC personnel conducted a field reconnaissance at the Site on May 28, 2019, to observe and document conditions of the property, specifically related the surface water drainages on the Site. The location of the sites visited are shown on Figure 3-3. A second field reconnaissance was conducted by BC on October 1, 2019, accompanied by U.S. Army Corps of Engineers (USACE) Southern Colorado Regulatory Branch Senior Project Manager, Joshua Carpenter. The purpose of this field reconnaissance was to observe water resources of the site and evaluate likely jurisdictional determination for these water resources. In addition, a third site visit was conducted on December 3, 2020, to assess the Red Creek and Arkansas River tributaries that were not previously field assessed. Results of the surface water reconnaissance are documented in the Preliminary Jurisdictional Approval to USACE (BC 2021, Appendix 3.3).