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-43- 1) by increasing infiltration which would, in turn decrease runoff, 2) by lowering the piezometric surface and causing springs to dry up, and 3) by direct infiltration from streams and their alluvium, or terraces. In areas not covered by soils, subsidence fractures would directly divert runoff into the ground water system. In such an area, numerous open fractures could significantly reduce surface runoff. In areas covered with soils (which would not maintain an open fracture), any reduction in runoff would be less severe. Subsidence fractures would increase deep percolation of soil water, but the infiltration characteristics of the soil would still control the amount of runoff. Infiltration might still increase slightly, because deep percolation might more rapidly deplete soil moisture and decrease anticedent moisture. A lowering of the piezometric surface might cause intermittent and perennial springs to go completely dry. Exhibit 2.8.1.U identifies numerous springs in or adjacent to the area to be mined. The application (2-253-6) indicates that springs contribute 11 percent of the flow in Lower Dry Fork, 4 percent of the flow in Lick Creek, 12.6 percent of the flow in South Prong, and 0.9 percent of the flow in Horse Creek. Excluding the Sylvester Gulch data, springs could contribute 6.2 percent of the low flow runoff. In reality, it is probable that not all the springs would be affected and, therefore, reduction of surface flow would be less than the worst case scenario. Subsidence could reduce stream flow by directly diverting surface flow and alluvial ground water into the bedrock ground water system. This effect is undesirable both because of the hydrologic effects and the mine operation problems it could produce. The applicant has proposed a subsidence protection plan for the Dry Fork of Minnesota Creek to protect streamflow. The adequacy of this protection plan was not assessed as a part of this permit review, The Division prefers that the site specific monitoring data concerning subsidence and its hydrologic effects betaken into account in the design of any protection plan. Stipulation No. 9 WFIQI THE APPLICANT SUBMITS A PERMIT APPLICATION ~ UNDERMINE THE DRY FORK OF MINNESOTA CREEK, THE SUBSIDENCE PROTECTION PLAN SHOULD BE BASED UPON MONITORING INFORMATION COLLECTED DURING THE EARLIER PHASES OF MINING. Three perennial streams cross the southern portion of the life-of-mine area, Lick Creek, South Prong, and Horse Creek. Subsidence effects could be significant along these stream courses as well, unless the applicant undertakes to design subsidence protection plans for these drainages as well as Dry Fork.