2021-09-07_REVISION - M1973021 (4)

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Aggregate Industries, Morrison Quarry Amendment No.7 to Permit M-1973-021 FN Morrison,CO identified a bottom elevation of El. 6,500, which roughly correlates to the average groundwater level of these two monitoring wells (6,461 and 6,588 feet, respectively). Based on these observations, it is possible that groundwater may be encountered at the base of the Phase I I I benches, particularly in the southern and southwestern corner of the South Quarry. When the monitoring wells support a determination that groundwater has been encountered, the excavation requires a well permit and approved substitute water supply plan or decreed plan for augmentation to continue mining activities. Well permit applications are in process, and the development of a substitute water supply plan is underway. If groundwater is encountered before these applications and plans are completed, the excavation will be backfilled or sealed, and mining in that area will cease. Once all permitting and planning activities have been approved, mining may continue if groundwater is encountered and managed. Since the Phase III groundwater is anticipated to be minimal, groundwater will be managed in a similar manner to stormwater in this area. The groundwater will be allowed to infiltrate the operation. If it remains after 72 hours, temporary sumps, pumping equipment, and piping will be used to dewater the area. Water will be conveyed to Outfall 003 for discharge to surface water bodies. Local dewatering from the mine pit has proven effective in the Central and North Quarries. This observation-based and temporary approach will continue in this manner until Phase III mining activities are completed. Once Phase III is completed, if Phase IV is delayed or postponed, more permanent dewatering structures will be constructed (refer to Exhibit E— Reclamation Plan, Dewatering). Planned Hydrogeologic Investigations Investigations are necessary before Phase IV mining activities can begin. This is, in part, due to all of Phase IV mining being conducted within the anticipated water table as well as data gaps precluding the development of a comprehensive water management approach. Additionally, the end use for the South Quarry has not been determined, and additional investigations are required to evaluate feasibility of potential reservoir uses and reclamation bonding requirements to satisfy State Engineering Office standards if this use is pursued. Hydrogeological investigations are in the planning phases; however, it is anticipated that this investigation will include exploratory drilling and testing to evaluate rock characteristics and measure groundwater flow in selected areas throughout the South Quarry. Where appropriate, additional temporary or permanent wells or piezometers will be installed in select locations to better measure groundwater levels, inform dewatering volumes and hydraulic conductivity, and understand potential infiltration rates within the proposed Phase IV excavation. Once completed, this information will be used to determine a Phase IV water management approach that will include technical considerations (evaporation, infiltration, mechanical dewatering and saturation impacts to slope stability), locations for water conveyance to outfall discharge points, potential permit modification requirements, and water augmentation plan needs. Additionally, this information will be critical in determining the potential end use for the South Quarry, which would be reflected in a future amendment to Exhibit E—Reclamation Plan. September 2021 1 61