1992-07-22_GENERAL DOCUMENTS - C1981017

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-40- Evaluation of the impacts of subsidence upon hydrologic systems requires the collection and analysis of extensive monitoring data consisting of surface water and ground water monitoring (quantity and quality), precipitation gaging, geological mapping, topographic surveying and monitoring 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 hydrologic impacts of subsidence. Such procedures are necessary at each mine within the Roaring Fork Drainage in order to coordinate a comprehensive subsidence control plan. This monitoring is now being required of operators through the Colorado Permanent Regulatory Program permitting process. Subsidence in the adjacent North Fork of the Gunnison River Basin has been studied or observed at the Somerset Mine, the Hawk's Nest Mine, the Bear Mine, and hypotheses have been forwarded concerning the inactive and abandoned 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, and communication of surface and ground waters with the underground mines. 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 (sandstones and coals) tend to remain open, while cracks in incompetent rocks (shales, mudstone, and siltstones) and soils are likely to fill . In light of these observations a range of subsidence impact scenarios are possible. Subsidence related cracking could increase infiltration rates from surface and ground waters, diverting water from streams and impoundments and aquifers above the affected areas (area within the angle of draw of subsidence over underground mine workings) . Related changes in local piezometric surfaces could result in the changing of flow rate of springs and seeps. Waters, having entered the sub-surface strata, could migrate towards the underground workings, and appear as mine inflows. During mining these inflows will be discharged to the surface, and thus impacts to the quantity of surface water will be mitigated. After closure of the mines, mine inflows would presist until there is an equilibrium established with the overlying affected strata. None of the three mining operations will affect the use of ground water from bedrock aquifers either separately or cumulatively, since there are no bedrock water supply wells completed in the region hydrologically adjacent to these mining operations. Also, the potential for impacting future use of bedrock ground water is low due to the depth of drilling required and to the problems of accessing steep slopes on private and U.S. Forest Service land.