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Extensive mining of an alluvial valley floor, even when followed by reclamation,
<br />has the potential to produce impacts on uses of that water, both at the mine site
<br />and wherever the affected water flows. Que burden- eateriel far soil) returned to
<br />the mined area is in many cases more susceptible to oxidation and leaching of the
<br />elements than was the undisturbed overburden.
<br />is (relative to
<br />re and cones-
<br />'" upssie +vatee h �-« rno s High sodium co
<br />calcium and magnesium) can also detrimentally affe,,
<br />quent uptake of water and oxygen by plants.
<br />Table I contains representative results of gro Lsi,quality measurements
<br />collected by the Montana Bureau of Mines and Geology am two surface coal mining
<br />areas in southeastern Montana. Examination of this limited amount of water quality
<br />data first suggests that concentrations are highly variable and conclusions cannot
<br />be drawn. However, the analyses of ground waters near Decker, Montana, do suggest
<br />that spoiled overburden contains water with higher concentrations of most elements
<br />than water obtained from undisturbed overburden and coal (the strong exception is
<br />the carbonate ion). Spoils waters at the Decker mine are higher in specific
<br />ductance than waters collected from the shallow alluvium, undisturbed overburden,
<br />and coal. The increase is due, principally, to calcium, sodium, bicarbonate, and
<br />sulfate concentrations. However, indicative of data variability is the fact that
<br />the second - highest specific conductance at Decker mine was found in a sample col-
<br />lected from undisturbed overburden, and that concentration is higher than one of the
<br />spoil water samples collected at the same mine.
<br />Water quality data from ground water samples collected near Colstrip, Montana,
<br />show trends similar to those collected near Decker (Table I). Spoils water is some-
<br />what higher in specific conductance than waters from undisturbed overburden and coal.
<br />The ions contributing chiefly to this increase are calcium, magnesium, bicarbonate,
<br />and sulfate. Although sodium content was higher in Decker spoils water, it is lower
<br />in Colstrip soils water than in undisturbed overburden and coal. On the other hand,
<br />magnesium levels are higher in Colstrip spoils water, but about the same in spoils
<br />and undisturbed overburden at Decker.
<br />It should be noted that the data in Table I are from samples which do not neces-
<br />sarily represent a long term, "before and after - mining" analysis (i.e., no samples
<br />were collected in the same area before and after mining a particular location.)
<br />Thus comparison of the quality of water in undisturbed overburden and spoils does
<br />not necessarily indicate expected water quality following complete "reclamation ".
<br />However, we do conclude that spoiled overburden contains ground water of poorer
<br />quality than in nearby undisturbed overburden and water - bearing coal seams, and thus
<br />that newly spoiled overburden has the potential, at least in selected circumstances,
<br />to produce ground water of relatively poorer quality. Whether deterioration in
<br />water quality will significantly affect overlying vegetation is not known. However,
<br />initial stands of vegetation on regraded lands at the Decker mine appear to be
<br />healthy one and two years following seeding. In the case of coal mining near Decker,
<br />Montana *, it has been projected that spoils water will have dissolved solids content
<br />ranging from 4,000 to 7,000 mg /1, principally sodium and sulfate, (OSDI and State of
<br />Montana, 1976). It is also concluded that in spite of these high levels of dis-
<br />solved solids in ground water, the cumulative impacts of the Decker operations ad-
<br />jacent to the Tongue ..aver Reservoir on reservoir water quality, and thus on water
<br />uses, will be insignificant and difficult to detect.
<br />Rahn (1976) has statistically analyzed twenty -eight ground and surface water
<br />samples collected from eight mines ** in northeastern Wyoming and southeastern
<br />* Current mining the Decker and Rosebud mines at Colstrip does not involve direct
<br />disturbance of subirrigated alluvial valley floors.
<br />**Wyodak Mine, Big Horn Mine, Decker Mine, Rosebud Mine (Colstrip), Big Sky Mine,
<br />Hidden Water Mine (abandoned), Antelope Mine, and Bell Ayr Mine. Appendix 1 pro-
<br />vides a more complete listing of surface coal mines examined for alluvial valley
<br />floors in this report.
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