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<br />Montana and concluded that:
<br />65.
<br />"Water in spoils was found to be significantly more highly mineralized than
<br />natural ground water in terms of total dissolved solids, calcium, magnesium,
<br />and sulfate. Spoils water -exceeds the recommended drinking water limits in
<br />these and other ions (manganese and cadmium), and it is doubtful that the water
<br />could be used for long -term irrigation."
<br />These higher concentrations will probably decrease with time as material is
<br />leached, and chemical forms change, but the rate of decrease is uncertain. McWhorter
<br />et al. (1977) have estimated, for northwestern Colorado, that if 20 cm of water in-
<br />filtrates into the spoils annually it would take 680 years to reduce conductivities
<br />by 95 percent if weathering is not considered, and longer if weathering is consi-
<br />dered.
<br />Mining of an area upstream or "upgradient "* of an alluvial valley floor also
<br />affects the water resource and the character of the drainage system in terms of flow,
<br />carrying capacity of the channel, and channel stability (or resistance to erosion).
<br />Specific impacts are dependent upon the local and regional hydrology. Surface mining
<br />in, as well as outside, an alluvial valley floor can, for example, involve the diver -
<br />felon of streams. Often such diversions have been done without adequate regard for
<br />the slope of the channel (i.e., stream gradient). Though diversions could be gen-
<br />erally considered as lengthening stream channels (since streams are being diverted
<br />away from their channels) and, therefore, as reductions in stream gradients, past
<br />diversions include the straightening of meandering streams such as the Belle Fourche
<br />River (Wyoming) resulting in a steeper gradient and diversions causing both lower and
<br />steeper gradients of streams such as Little Youngs Creek (Wyoming). If the gradient
<br />is significantly steepened, erosive capabilities of the drainage system are increased
<br />in the area of steepening. Concurrently, increased sedimentation will probably occur
<br />downstream. The post - mining grade of the stream is equally critical. Significant
<br />steepening of portions of a disturbed stream channel will result in progressive up-
<br />stream channel and bank erosion and downstream deposition until, at some future
<br />time, equilibrium is again reached. Channel depths and widths will change as a re-
<br />sult of the disturbance of equilibrium.
<br />Surface mining of shallow coals in the western United States is now occurring
<br />in areas which have been identified as alluvial valley floors, in this report, and
<br />by others. These mines include the Belle Ayr, Eagle Butte, Cordaro, Wyodak, PSO #1,
<br />and Big Horn mines in Wyoming (see Appendix 1 for locations). Mining at these sites
<br />has not been reported as adversely affecting alluvial valley floors other than where
<br />alluvial valley floors are actually removed during mining or where spoil is placed
<br />on the surface of the valley floor. Nor have pre mining investigations of hydrology,
<br />vegetation, or operational hydrolic monitoring been comprehensively analyzed to pre-
<br />dict the presence or absence of longer -term impacts of mining on these alluvial
<br />valley floors.
<br />There is no strong evidence of chronic water pollution caused by the current
<br />coal mining operations in the interior western United States. There is limited
<br />evidence of surface water quality degradation in some streams such as Little Youngs
<br />Creek in northern Wyoming (due to intrusion of spoiled overburden into the alluvial
<br />Valley floor). Water quality data collected along Little Youngs Creek are presented
<br />in Table 2. Samples collected in 1976 indicate a significant increase in total sul-
<br />fates and dissolved solids across the disturbed area. The data also show a seassnal
<br />increaae_in_total dissolved solids unrelated_tm the mine (upstream).
<br />
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