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-38- Evaluation of the impacts of subsidence upon hydrologic systems requires the collection of extensive monitoring data consisting of surface water and ground water monitoring (quantity and quality), precipitation gaging, evapotranspiration rates, geological mapping, topography surveying and recording of mine inflow discharge and consumption rates. The monitoring data should be updated continuously as mining progresses to improve engineering control design parameters or to mitigate any unexpected subsidence occurrences. (Such preceoures are necessary at each mine within the Somerset Coal Field in order to coordinate a comprehensive subsidence control plan.) This monitoring is now being required of all operators through the Colorado Permanent Regulatory Program permitting process. Subsidence in the area has been studied or observed at the Somerset Mine, the Hawk's Nest Niine, the Bear Mine, and hypotheses have been forwarded concerning the inactive and abondoned Oliver No. 2 Mine. Dunrud (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 subsurface strata, could migrate towards the underyround 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.