2020-08-20_REVISION - C1980007 (36)

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The potential effects of MCC's water diversion on North Fork flows have been projected for flow rates and flow volumes. With the existing pump capacity, 450 gallons per minute (gpm), the diversion would result in a measurable change in flow during low flows having return periods of about 200 years or greater, and would be less than 5 percent. During extreme low flow periods, MCC's water rights would be out of priority and diversion would not be undertaken. Although subsidence fractures that develop over and next to mine workings have the potential to dewater streams and ponds, Section 2.05.6(6) of the permit application explains that dewatering is unlikely to occur because subsidence cracks that develop at the surface will not extend deep enough to intersect fractures propagating upward from the caved and fractured zones. Also, the stratigraphic sequence in stream valleys is resistant to fracturing due to the presence of interbedded fine- grained units. These beds would more likely undergo ductile deformation(bed bending), rather than brittle deformation(bed fracturing). Stream flows and local channel elevations will continue to be monitored in the permit area, as set forth in the permit application. Subsidence magnitudes of stock ponds will also be monitored. Specific measures MCC will employ for assessing and protecting the Minnesota Creek water supply are described in Exhibit 58 of the permit application titled, "Protection of Minnesota Creek Water Supply." Possible subsidence impacts to streams and ponds are discussed in the following Subsidence Control section. Underground mine workings will come within 800 ft. of the Monument Dam and Minnesota Reservoir. MCC explains in Section 2.05.6(6) of the permit application that the reservoir is unlikely to be affected by mining because the dam and reservoir are outside the predicted angle of draw and a subsidence control plan will be implemented for mitigating possible impacts from mining-induced seismicity. (Possible subsidence impacts to the dam and reservoir and mitigation measures are described in the following discussion of the subsidence control plan.). MCC's surface water monitoring plan is described in Exhibit 71 for the South of Divide and Dry Fork lease areas, in Exhibit 71 a for the Sunset Trails area, and in Section 2.04 of the permit application for other areas. As of late 2008, MCC monitors surface water quantity and quality at 22 stream stations, comprised of 10 stations with continuous recording and 12 stations with instantaneous recording. A minimum of one year of monthly baseline data has been collected on all streams prior to mining in a watershed 3. Summary and Findings The Division has reviewed the Probable Hydrologic Consequences due to 48