2012-06-20_PERMIT FILE - C2010089 (88)

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62. species develop root systems that extend perhaps ten meters deep to obtain sufficient water, but many grass species favored for hay are more shallow rooted (i.e., on the order of two meters in depth). Examples of these grasses include such native species as canada wildrye (Elymus canadensis), little bluestem (Andropogon scoparigs), and blue grama (Bouteloua gracilis). The relatively shallow, tibrous nature root systems characteristic of these grasses is documented by Heath et al. (1973) and Weaver (1968). The deeper- rooted species include forbs and woody plants which serve a useful role in supporting game animals and afford cover and forage for domesticated animals such as cattle or sheep. Usually the vegetative growth of an alluvial valley includes native grasses. At times the alluvial valley floor may in- clude alfalfa; however, alfalfa is generally considered to be a deep - rooted phreato- phyte and may also be grown outside an alluvial valley. The alluvial valley can be important for agriculture in that it not only provides forage during the growing season, but also can provide sufficient vegetative growth for forage to be cut and stored for winter use. It is often possible to obtain three cuttings of hay in sub - irrigated alluvial valley floors. The agricultural and wildlife uses of lands in the interior western United States* have been prominent since they provide the principal source of income. Though oil and gas production and now coal mining have provided some development and economic support to the region, they are activities which remove a non - renewable re- source and which might be considered temporary compared to farming, ranching, fish- ing, hunting and enjoyment of scenery. It is important to insure that the agricul- tural and recreational economy is as strong, if not stronger, after mining as it was prior to mining. Thus, if the alluvial valley floor is critical to the success of a ranch or to the maintenance of local wildlife populations, these vital functions must be protected - at least until such time as it is demonstrated that the alluvial valley floor's usefulness may be efficiently and economically replaced by an alter- native source of domestic fodder and /or wildlife habitat. Surface mining of coal has the potential, in the western United States, to temporarily, and even permanently, change the ground water flow system. If the mine involves disturbances of saturated strata or aquifers, it is likely that while mining, the local water table(s) will drop somewhat and ground water flow will be intercepted by, or will be directed toward the mine pit. But this is a temporary situation, since in most cases the water table will be reestablished after mining is completed and the mine pits are backfilled and graded. However, if the mining dis- turbs confining strata that serve to isolate aquifers ** which have significantly different potentiometric heads, such as that which appears to occur in the Sarpy Creek watershed in southeastern Montana, there may be a significant change in the depth to water after mining and reclamation. Changes in the water table will also affect the postmining use of subirrigated alluvial valley floors. If changes in the depth to the water table are more than a few meters, it appears reasonable to expect some effects on vegetation in lowland areas. Phreatophytes (alfalfa, for example) are especially sensitive to changes in the depth to available water. Grasses with fibrous root systems are not able to penetrate deeply into the substrate. Such vegetation may be adversely affected if the post- mining water table is deeper or if the post - mining moisture holding capa- city of the growth medium is significantly reduced. Effects on vegetation could involve a shift toward deeper- rooted and more drought - resistant vegetation, or a shift to shallow- rooted vegetation (the latter if the depth to water is decreased). Thus strata functioning as relatively impermeable lower boundaries for water tables in the alluvial valley floors might have to be recreated after mining to prevent undesirable lowering of the water table. * The term "interior western United States" is loosely used to include North Dakota, South Dakota, Montana, Wyoming, Colorado, Utah, New Mexico, and Arizona. ** Aquifer: relatively permeable rock containing and conducting sufficient ground water to yield significant quantities of water to wells and springs. 63. Extensive mining of an alluvial valley floor, even when followed by reclamation, has the potential to produce impacts on uses of that water, both at the mine site and wherever the affected water flows. Overburden material (or spoil) returned to the mined area is in many cases more susceptible to oxidation and leaching of the elements than was the undisturbed overburden. Salts concentrations often increase after mining and are noteworthy since high salt concentrations in the soil solution impede the uptake of water by plant roots. High sodium concentrations (relative to calcium and magnesium) can also detrimentally affect the soil structure and conse- quent uptake of water and oxygen by plants. Table I contains representative results of ground water quality measurements collected by the Montana Bureau of Mines and Geology from two surface coal mining areas in southeastern Montana. Examination of this limited amount of water quality data first suggests that concentrations are highly variable and conclusions cannot be drawn,. However, the analyses of ground waters near Decker, Montana, do suggest that spoiled overburden contains water with higher concentrations of most elements than water obtained from undisturbed overburden and coal (the strong exception is the carbonate ion). Spoils waters at the Decker mine are higher in specific con- ductance than waters collected from the shallow alluvium, undisturbed overburden, and coal. The increase is due, principally, to calcium, sodium, bicarbonate, and sulfate concentrations. However, indicative of data variability is the fact that the second - highest specific conductance at Decker mine was found in a sample col- lected from undisturbed overburden, and that concentration is higher than one of the spoil water samples collected at the same mine. Water quality data from ground water samples collected near Colstrip, Montana, show trends similar to those collected near Decker (Table I). Spoils water is some- what higher in specific conductance than waters from undisturbed overburden and coal. The ions contributing chiefly to this increase are calcium, magnesium, bicarbonate, and sulfate. Although sodium content.was higher in Decker spoils water, it is lower in Colstrip soils water than in undisturbed overburden and coal. On the other hand, magnesium levels are higher in Colstrip spoils water, but about the same in spoils and undisturbed overburden at Decker. It should be noted that the data in Table I are from samples which do not neces- sarily represent a long term, "before and after mining" analysis (i.e., no samples were collected in the same area before and after mining a particular location.) Thus comparison of the quality of water in undisturbed overburden and spoils does not necessarily indicate expected water quality following complete "reclamation ". However, we do conclude that spoiled overburden contains ground water of poorer quality than in nearby undisturbed overburden and water- bearing coal seams, and thus that newly spoiled overburden has the potential, at least in selected circumstances, to produce ground water of relatively poorer quality. Whether deterioration in water quality will significantly affect overlying vegetation is not known. However, initial stands of vegetation on regraded lands at the Decker mine appear to be healthy one and two years following seeding. In the case of coal mining near Decker, Montana *, it has been projected that spoils water will have dissolved solids content ranging from 4,000 to 7,000 mg /1, principally sodium and sulfate, (USDI and State of Montana, 1976), It is also concluded that in spite of these high levels of dis- solved solids in ground water, the cumulative impacts of the Decker operations ad- jacent to the Tongue River Reservoir on reservoir water quality, and thus on water uses, will be insignificant and difficult to detect. Rahn (1976) has statistically analyzed twenty -eight ground and surface water samples collected from eight mines ** in northeastern Wyoming and southeastern * Current mining at the Decker and Rosebud mines at Colstrip does not involve direct disturbance of subirrigated alluvial valley floors. * *Wyodak Mine, Big Horn Mine, Decker Mine, Rosebud Mine (Colstrip), Big Sky Mine, Hidden Water Mine (abandoned), Antelope Mine, and Bell Ayr Mine. Appendix 1 pro- vides a more complete listing of surface coal mines examined for alluvial valley floors in this report.