Water resources and potential effects of surface coal mining in the area of the Woodson Preference Right Lease Application, Montana

Federal coal lands of the Woodson Preference Right Lease Application are located in Dawson and Richland Counties, northeastern Montana. A probable mine area, comprised of the lease area and adjacent coal lands, contains about 220 million tons of recoverable lignite coal in the 12-37 ft thick Pust coal bed. A hydrologic study has been conducted in the area to describe the water resources and to evaluate potential effects of coal mining on the water resources. Geohydrologic data collected from wells and springs indicate that several aquifers exist in the area. Sandstone beds in the Tongue River Member of the Fort Union Formation (Paleocene age) are the most common aquifers and probably underlie the entire area. The Pust coal bed in the Tongue River Member is water saturated in part of the probable mine area and is dry in other parts of the probable mine area. Other aquifers, located mostly outside of the probable mine area, exist in gravel of the Flaxville Formation (Miocene of Pliocene age) and valley alluvium (Pleistocene and Holocene age). Chemical analyses of groundwater indicate a range in dissolved solids concentration of 240-2,280 mg/L. Surface water resources are limited. Most streams in the area are ephemeral and flow only in response to rainfall or snowmelt. Small reaches of the North and Middle Forks of Burns Creek have intermittent flow. Water sampled from a small perennial reach of the Middle Fork had a dissolved solids concentration of 700 mg/L. Mining of the Pust coal bed would destroy one spring and four stock wells, dewater areas of the Pust coal and sandstone aquifers, and probably lower water levels in seven stock and domestic wells. Mining in the valley of Middle Fork Burns Creek would intercept streamflow and alter flow characteristics of a small perennial reach of stream. Leaching of soluble minerals from mine spoils may cause a long-term degradation of the quality of water in the spoils and in aquifers downgradient from the spoils. Some of the effects on local water supplies could be mitigated by development of new wells in deeper sandstones of the Tongue River Member. Effects of mining on water resources would be minimized if only areas of dry coal were mined. (Author 's abstract)