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-41- Cumulative Hydrologic Impact Assessment - Gro~;rd Water lCont~d) Subsidence in the area has been studicri or observed at the Somerset Mine, the Hawk's Nest Mine, the Bear Mine, and h!rlbtheses forwarded concerning the inactive and abandoned Oliver No. 2 Mine. Dunz'ud (1976) discussed numerous subsidence causes and effects, and while no specific and consistent effects were delineated, it is evident that the physical parameters mentioned above are important topics. For instance, the presence of strong rock strata (such as sandstone) at the ground surface will permit the opening of tension cracks which could extend through the affected rock unit. Such a condition could significantly affect the interception of surface water and the recharge of ground water. The existence of weak rocks (such as shale) or soils at the ground surface could result in much less pronounced surface cracking. The dimensions of surface cracks appear to relate to the type of rocks in which they occur, the thickness of coal removed, the depth of the overburden, and the location of crack occurrence. Cracks in competent rocks tend to remain open while cracks in incompetent rocks and soils are likely to fill. 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 sub-surface strata, could migrate towards the underground workings, eventually permeating the strata and gradually migrating downdip beneath the Grand Pfesa. Impacts on Surface Waters Review of the permit applications for the mining operations considered in this assessment and Division experience with other mining operations throughout the State has identified several possible mining effects to surface water quality and quantity (Table 7). Surface water quality might be affected l:y 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 f.Zow, reversing hydraulic gradients, and through subsidence effects, may deplete springs and seeps or storm flow. In addition, the withdrawal of water of 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. 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. 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. Rs a result, the increased sediment load carried by disturbed flows would not be expected to reach receiving streams.