2022-09-26_PERMIT FILE - M2022042 (6)

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Windsor East Mine, Exhibit G – Water Information September 2022 Page 8 | 11 percent of gross precipitation; thus, the average annual effective precipitation was determined to be 10.56 inches. The resulting evaporative loss rate is therefore 27.92 inches. The maximum total annual evaporative consumptive use at the site is estimated at 12-17 acre-feet, which is primarily a function of the water used for dust control (10-15 ac-ft/yr). 2.0 MONITORING AND MITIGATION PLAN This Groundwater Monitoring and Mitigation Plan is prepared as part of Martin Marietta’s application to the Colorado Division of Reclamation, Mining and Safety (DRMS) for a permit for the Windsor East Mine in Weld County, Colorado. This plan presents the methods and locations for monitoring of groundwater during gravel mining and site reclamation activities. Although adverse impacts to other local users of groundwater are not expected as a result of activities at the mine, this plan addresses how any adverse effects to groundwater would be mitigated, should they occur. Martin Marietta will submit a Temporary Substitute Water Supply Plan to the State Engineer’s Office for approval. The temporary substitute supply plan is designed to protect senior vested water rights and mitigate potential depletions of flows in adjacent waterways. 2.1 MINING PLAN Except for Cell A, the mining plan has been designed to allow for up to five quarters worth of groundwater monitoring to occur before excavation below the water table occurs. To allow for sufficient time for groundwater characterization to occur, mining is only planned to occur in the unsaturated zone until one year’s worth of monitoring and groundwater sample collection has been conducted. An exception will be made with regard to mine Cell A. This cell is the eastern-most cell in the mining plan and is located within several hundred feet of the Parsons mine. As a result, water levels are already lowered in the area from Parsons dewatering. Since changes to the groundwater flow regime have already been substantially implemented, trenching and mining below the water table at Cell A with associated dewatering will begin before the five quarters of monitoring is complete. Following the five quarters of monitoring, dewatering trenches will be excavated around the perimeter of each remaining mine cell on a schedule determined by the mining plan. Dewatering will occur initially adjacent to the area on the east where dewatering associated with Parsons mine has already reduced water levels (Cell A). The bottom of the trench will be maintained at or deeper than the deepest point in the excavated mine cell, thereby intercepting all groundwater before it reaches the mine cell. After collection of five quarters of groundwater monitoring, mining will gradually progress westward, with perimeter dewatering drains preceding excavation below the water table. Groundwater flow into each dewatering trench will be accumulated in connected sumps and discharged directly into the Cache la Poudre River. Following completion of mining activities, each mine cell will be finished with a compacted embankment liner from material located on-site, keyed into the bedrock at the base of the mine cell, thus forming a low-permeability bathtub in the mine cell. Once finished, dewatering of the perimeter trench will cease, and the trench will be backfilled, allowing groundwater to return to a state of natural flow around the now-lined mine cell. It is expected that some minor hydraulic mounding may occur upgradient of the lined mine cell, with some “shadow effect” (decline in groundwater level) downgradient of the mine cell. Since no existing water wells have been identified downgradient between the mine and the river, the shadow effect is not anticipated to impact other users. Figure G-6 depicts the anticipated groundwater flow directions resulting from the installation of the compacted liners during reclamation. 2.2 MONITORING The monitoring plan will consist of regular data collection from the set of five monitoring wells installed around the perimeter of the Windsor East property (Figure G-3). Data collection activities will include measurement of water levels in wells and sampling of water quality from wells and surface discharge locations.