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COLORADO LEGACY LAND $CHWARTZWALDER MINE (gpm).The 10-hp pump was replaced with a 25-hp pump in February 2017 to achieve the regulatory limit.The pump was installed in the#2 Shaft behind the Steve bulkhead at 210 ft below the Steve Level and was capable of dewatering the mine pool at 190 gpm. The 25-hp pump dewatered the mine pool to below the regulatory limit by November 2017. The WTP has been operating approximately 50% of the time for the last three consecutive years and as shown in Figure E-1,the mine pool has been consistently below the regulatory limit, with the possible exception of one instance in the Winter of 2019 (January 27-29,2019). In April 2019,a 60-hp pump was installed in the Jeffrey Air Shaft at 400 ft below the Steve Level.The pump is capable of dewatering the mine pool at approximately 300 gpm. In April 2020,there was a malfunction with the 60-hp pump and the 25-hp pump was brought into operation to dewater the mine pool. Operation of just the 25-hp pump was sufficient to keep the mine pool below the regulatory limit. In October 2020,a team entered the mine to verify the mine pool elevation and calibrate the transducer used to measure the mine pool elevation.The team noticed that the mine pool had been dewatered to 22 feet lower than the elevation of the transducer,which caused inaccurate measurements to be recorded.The transducer was lowered from 294 ft to 354 ft below the Steve Level and calibrated to accurately measure the mine pool elevation.The steep drop in October 2020 shown on Figure E-1 reflects when the calibration was performed. The fact that the transducer was providing shallower mine pool elevation measurements indicates that the exceedance of the mine pool elevation above the regulatory limit in the Winter of 2019 may not have been an actual exceedance. Operation of the 60-hp pump and the dewatering/treatment of the mine pool will continue to stabilize the water elevation in the mine pool below the regulatory limit. During the time in which the WTP is not in operation, the mine pool will be allowed to naturally recover as shown in Figure E-1. Projections based on historical mine pool elevation trends allow for at least a 6-month shut-down of the WTP with little risk of exceeding the regulatory limit. The basis for a shut down of at least 6-months is provided in the following section. Maintaining the mine pool elevation below the regulatory limit(150-feet below the Steve Level)has established a hydraulic gradient inward toward the mine pool and away from Ralston Creek.A summary of the observed quarterly groundwater elevations in bedrock wells MW-15 and MW-18 with respect to the mine pool elevation is presented in Table E-1. These data are used to calculate the hydraulic gradient between each well and the mine pool.Depending on the elevation of the mine pool,the gradient from each well ranged from 0.014 to 0.228 feet per foot.The strongest gradient(0.228 ft/ft)was observed in fourth quarter of 2020,when the mine pool was dewatered to its deepest elevation.The data also indicate an anisotropic capture zone from pumping the mine pool. E.5.2.1. Physical Stabilization of the Mine Pool Conclusion The changes in mine pool elevation during natural recovery are historically steady and consistent.On the basis of historical refill trends, once the mine pool is dewatered at the end of the operating season, the natural groundwater recharge will take approximately six months before the mine pool approaches the regulatory limit. Historical mine pool elevations are shown in conjunction with mine pool projections in Figure E-1.The projections made for the natural refill in the spring of 2021 in Figure E-1 are based on previous recovery trends JUNE 2021 15 AMENDMENT 6