Potential effects of surface coal mining on the hydrology of the Horse Creek area, Sheridan and Moorhead coal fields, southeastern Montana

The Horse Creek area of the Sheridan and Moorhead coal fields, 16 miles east of the Decker Coal Mines near the Tongue River, contains large reserves of Federally owned coal that have been identified for potential lease sale. A hydrologic study was conducted in the area to describe existing hydrologic systems and to assess potential impacts of surface coal mining on local water resources. Hydrologic data collected from private wells, observation wells, test holes, and springs indicate that the aquifers are primarily coal and sandstone beds in the upper part of the Tongue River Member of the Fort Union Formation (Paleocene age) and sand and gravel of valley alluvium (Pleistocene and Holocene age). Surface-water resources are mostly limited to a few stock ponds receiving discharge from springs in the higher ports of the area. Two stock wells, one spring, and three stock ponds receiving discharge from springs supply most of the water used within the Horse Creek basin; the only use is watering of livestock. Mining of the Anderson and Dietz coal beds would destroy one stock well and two ponds receiving discharge from springs, and would lower the potentiometric surface within the coal and sandstone aquifers. Although mining would alter existing hydrologic systems, alternative deeper water supplies are available. (USGS)