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-42- <br />Evaluation of the impacts of subsidence upon hydrologic systems <br />requires the collection of extensive baseline data consisting of <br />surface water and ground water monitoring (quantity and quality), <br />precipitation gaging, evapotranspiration rates, geological mapping, <br />topographic surveying and recording of mine inflow rates. The baseline <br />data should be updated continuously as mining progresses to improve <br />engineering control design parameters or to mitigate any unexpected <br />subsidence occurrences. Such procedures are necessary at each mine <br />within the district in order to coordinate a district wide subsidence <br />control plan. These procedures are now being required of all operators <br />through the Permanent Colorado Coal Mine Regulatory Program permitting <br />process. <br />Subsidence in the area has been studied or observed at the Somerset <br />Mine, the Hawk's Nest Mine, the Bear Mine, and hypotheses forwarded <br />concerning the Oliver No. 2 Mine. Dunrud, in a study regarding the <br />Somerset Mine (USGS Prof. Paper 969), discussed numerous subsidence <br />causes and effects, and while no specific and consistent effects were <br />delineated, it is evident that the physical parameters mentioned above <br />are important topics. For instance, the presence of strong rock strata <br />(such as sandstone) at the ground surface will permit the opening of <br />tension cracks which could extend through the affected rock unit. Such <br />a condition could increase the interception of surface water. However, <br />since the sandstones in the area above the coal seams tend to be <br />lenticular and isolated, the underlying clays should tend to inhibit <br />deep percolation of water. <br />The dimensions of surface cracks appear to relate to the type of rocks <br />in which they occur, the thickness of coal removed, the depth of the <br />location of crack occurrence. Cracks in competent rocks tend to remain <br />open while cracks in incompetent rocks and soils are likely to fill. <br />In light of these observations a range of subsidence impact scenarios <br />are plausible. Subsidence related cracking could increase surface and <br />over-infiltration rates, diverting water from streams and impoundments <br />above the affected areas. Related changes in local piezometric <br />surfaces could result in the modification of springs and seeps. <br />Waters, having entered the subsurface strata could reemerge downstream <br />in the same drainages or migrate towards the underground workings. It1 <br />some cases this water may reemerge as a discharge from the portal or as <br />springs along the subcrop or outcrop. <br />Possible Effects of Subsidence upon Minnesota Creek Drainage in future <br />permit terms if mining does occur. <br />Over the life of the mine, subsidence from Mt. Gunnison's underground <br />mining operations could affect surface water hydrology in the North <br />Fork of the Gunnison and the Minnesota Creek watersheds. Subsidence <br />effects would reduce the surface flow from an affected watershed in <br />several ways: <br />