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-36- In light of these observations, a range of subsidence impact scenarios are plausible. Subsidence related cracking could increase surface and overburden infiltration rates, diverting water from streams and impoundments above the affected areas. Related changes in local piezometric surfaces could result in the modification of springs and seeps. Waters, having entered the subsurface strata, could migrate towards the underground workings, eventually permeating the strata and gradually migrating downdip beneath the Grand Mesa. Impacts on Surface Waters The Division, based on review of the permit applications for the mining operations considered in this assessment, has identified several possible effects of mining on surface water quality and quantity (Table 7). Surface water quality might be affected by the discharge of mine water from underground workings or by seepage from waste rock or coal refuse piles to the stream -alluvial aquifer system. The underground mining operations, by altering ground water flow, reversing hydraulic gradients, and through subsidence effects, may deplete springs and seeps or stream flows. In addition, the withdrawal of surface water for mine usage may deplete surface flows. Two other possible surface water effects: increased infiltration in the areas overlying underground workings and increased erosion from disturbed areas, were briefly considered but then deleted from further analysis. Although subsidence might logically increase infiltration rates in an undermined watershed and therefore decrease runoff, the Division is unaware of any literature documenting that this effect actually occurs. However; several of the mines, including Orr ha rd Yalley will examine the affected rangeland and report on the magnatude of surface subsidence cracks, if encountered. Surface flow from disturbed areas would carry higher sediment concentrations than natural runoff. However, Rule 4.05.2 requires that all disturbed flow pass through a properly designed sediment control pond. Effluent from the pond is required to meet NPDES limitations prior to discharge. During operation, monitoring and inspection by the Division ensures that the sediment control system is maintained to function as designed. At the close of operations, surface disturbances would be reclaimed and the sediment control system would be removed following the ensuing liability period. Asa result, the increased sediment load carried by disturbed flows would not be expected to reach receiving streams. Discharge of Mine Water from Underground Workings Underground mines in the North Fork area are generally dry. The only operation to discharge mine water from the underground workings on a continuing basis is the Somerset Mine and the Blue Ribbon Mine. The Somerset Mine discharges this water into Bear Creek at a site approximately 3/4 mile upstream from its confluence with the North Fork of the Gunnison River. The current permit application anticipates a discharge rate of 115 gpm (0.25