2012-06-20_PERMIT FILE - C2010089 (87)

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Montana and concluded that:, "Water in spoils was found to be significantly more highly min zed than natural ground water in terms of total dissolved solids, a magnesium, and sulfate. Spoils water exceeds the recommended r imits in these and other ions (manganese and cadmium), an d 1 t the water could be used for long -term irrigation." These higher concentrations will probably decreas time as material is leached, and chemical forms change, but the rate of decrease is uncertain. McWhorter et al. (1977) have estimated, for northwestern Colorado, that if 20 cm of water in- filtrates into the spoils annually it would take 680 years to reduce conductivities by 95 percent if weathering is not considered, and longer if weathering is consi- dered. Mining of an area upstream or "upgradient "* of an alluvial valley floor also affects the water resource and the character of the drainage system in terms of flow, carrying— capacity of the channel, and channel stability (or resistance to erosion). ,Specific impacts are dependent upon the local and regional hydrology. Surface mining in, as well as outside, an alluvial valley floor can, for example, involve the diver- sion of streams. Often such diversions have been done without adequate regard for the slope of the channel (i.e., stream gradient). Though diversions could be gen- erally considered as lengthening stream channels (since streams are being diverted away from their channels) and, therefore, as reductions in stream gradients, past diversions include the straightening of meandering streams such as the Belle Fourche River (Wyoming) resulting in a steeper gradient and diversions causing both lower and steeper gradients of streams such as Little Youngs Creek (Wyoming). If the gradient is significantly steepened, erosive capabilities of the drainage system are increased in the area of steepening. Concurrently, increased eedimentatioa will probably occur downstream. The post - mining grade of the stream is equally critical. Significant steepening of portion of a disturbed stream channel will result in progressive up- stream channel and bank erosion and downstream deposition until, at some future time, equilibrium is again reached. Channel depths and widths will change as a re-' sult of the disturbance of equilibrium. Surface mining of shallow coals in the western United States is now occurring in areas which have been identified as alluvial valley floors, in this report, and by others. These mines include the Belle Ayr, Eagle Butte, Cordero, Wyodak, PSO #1, and Big Born mines in Wyoming (see Appendix 1 for locations). Mining at these sites has not been reported as adversely affecting alluvial valley floors other than where alluvial valley floors ars actually removed during mining or where spoil is placed on the surface of the valley floor. Nor have pre - mining investigations of hydrology, vegetation, or operational hydrolic monitoring been comprehensively analyzed tn•pre- dict the presence or absence of longer -term impacts of mining an these alluvial valley floors. There is no strong evidence of chronic water pollution caused by the current coal mining operations in the interior western United States. There is limited evidence of surface water quality degradation in same streams such as Little Youngs Creek in northern Wyoming (due to intrusion of spoiled overburden into the alluvial valley floor). Water quality data collected along Little Youngs Creek are presented in 'Cable 2. Samples collected in 1976 indicate a significant increase in total sul- fates and dissolved solids across the disturbed area. The data also show a seasonal increase in total dissolved solids unrelated to the mine (upstream). Measurements of impacts an water by surface mining in the western United States are limited. One must utilize sophisticated water sampling and analysis devices suited to allow sampling during infrequent runoff events, from remota locations, and * 'Up gradient" and "downgradient" refer to locations higher or lower along the potentiometric surface of the water table relative to the point of reference.