Shallow aquifers exist primarily within the Tongue River Member of the Paleocene Fort Union Formation and within valley alluvium. Sandstone beds are the principal aquifers for domestic supply and livestock watering, with the Knobloch coal bed being a secondary source of supply. Surface-water resources consist principally of perennial flow in Otter Creek and intermittent flow in eight small drainage basins. The small streams are generally dry at their mouth, except after intense rainfall or sudden snowmelt. Otter Creek is used for livestock watering and, during spring floods, for irrigating alfalfa fields. The water supplied by wells generally is a sodium bicarbonate type. Dissolved-solids concentrations of water samples ranged from 480 to 3,460 milligrams per liter in sandstone beds and from 910 to 6,260 milligrams per liter in the Knobloch coal bed. Water in Otter Creek contains principally sodium, magnesium, and sulfate ions. The dissolved-solids concentration ranged from 2,050 to 2 ,950 milligrams per liter. Mining of the Knobloch coal bed would remove three private wells and adversely affect the yield of two other wells. After mining, water in the alluvium of Otter Creek might show long-term degradation in water quality as a result of waters leaching the soluble salts from the spoils material used to backfill the mine pits. Although mining would alter the existing hydrologic systems and remove several shallow wells, alternative ground-water supplies are available from deeper aquifers that could be developed to replace those lost by mining.
