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COLORADO LEGACY LAND <br /> $CHWARTZWALDER MINE <br /> E.5.2. Physical and Chemical Stabilization Conclusion <br /> It is important to note that the regulatory limit of dewatering the mine pool to 150 ft below the Steve level was <br /> specifically chosen by the DRMS to(i) re-establish a hydraulic gradient away from Ralston Creek in the permit <br /> area, and (ii) reduce the exposure of wall rock to oxygen in order to minimize uranium oxidation in the <br /> workings (MLRB, 2012). The hydraulic gradient away from Ralston Creek means there is no connection <br /> between the mine workings and Ralston Creek,i.e.,uranium concentrations and other analytes in the mine pool <br /> are not adversely affecting Ralston Creek.As discussed above,physical stability of the mine pool over the last <br /> three consecutive years has been demonstrated and physical stability will continue for the following reasons: <br /> • The mine pool elevations over the past three consecutive years shown on Figure E-4 are below the <br /> regulatory limit.The noted`exception'on the chart during the winter of 2019 had been verified to be <br /> the results of the transducer below the mine pool and therefore not accurately measuring the depth <br /> of the mine pool. <br /> • Actual groundwater inflow into the mine during the periods that active dewatering of the mine pool <br /> was not occurring did not raise the mine pool elevation above the regulatory limit. <br /> • The natural recovery rate allows for approximately six months before the regulatory limit is <br /> approached,based on a starting mine pool elevation of approximately 345 ft below the Steve Level. <br /> • Water level measurements in bedrock wells MW-15 and MW-18 at the site demonstrate an inward <br /> gradient toward the mine pool and away from Ralston Creek. <br /> Chemical stabilization of the mine pool has been demonstrated to occur as a result of in-situ treatments that <br /> have occurred since 2013. The stabilization is most notable in the decrease in uranium and molybdenum <br /> concentrations and the overall decrease in concentrations for most of the analytes,as shown in Table E-2,for <br /> the last three consecutive years as compared to the concentrations in the pre-2017 samples. <br /> A summary of the WTP operations for the three consecutive years from 2018 through 2020 is provided in <br /> Table E-3. As shown, the mine pool has been below the regulatory limit when operating the WTP for <br /> approximately 50%of the time.In addition,the mean concentration of dissolved uranium in the mine pool has <br /> been consistently below 14 mg/L. <br /> JUNE 2021 24 AMENDMENT 6 <br />