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The Cook Creek area of the Ashland coal field contains large reserves of Federally owned coal that have been identified for potential lease sale. A hydrologic study has been conducted in the potential lease area to describe existing hydrologic systems and to assess potential impacts of surface coal mining on local water resources. Hydrogeologic data collected from wells, springs, and drill holes indicate that shallow aquifers exist within the Tongue River Member of the Fort Union Formation (Paleocene age) and within valley alluvium (Pleistocene and Holocene age). Shallow aquifers within the Tongue River Member include coal beds, clinker, and lenses of sandstone and siltstone. The Knobloch coal bed, a principal shallow aquifer used for stockwatering in the area, averages about 55 feet in thickness and is completely saturated throughout most of its extent. Coarse alluvial deposits are the most productive aquifers and are a major source of stock water in the Cook Creek basin. Surface-water resources are limited to the upstream reach of Cook Creek, which flows intermittently. The downstream reach of Cook Creek, plus all other small drainages that originate in the study area, are ephemeral. Mining of the Knobloch and Sawyer coal beds would remove two alluvial springs, one bedrock spring, and two wells, which are all used for watering of livestock. The potentiometric surface within the Knobloch coal aquifer and the alluvial aquifer in the downstream part of the Cook Creek basin would be lowered during mining. Lowered water levels in these aquifers might substantially affect water levels in five wells outside the mine boundary. After mining, water in the alluvial aquifer downgradient from the mine area might show a long-term degradation in quality as a result of leaching of soluble salts from overburden materials used to backfill mine pits. Although mining would alter the existing hydrologic systems and remove several springs and shallow wells, alternative groundwater supplies are available that could be developed to replace those lost by mining. INTRODUCTION Development of western coal to meet national energy needs has recently received increased emphasis. A large part of the western coal is under Federal ownership; therefore, considerable demand exists for the leasing and development of Federal coal lands. To ensure orderly leasing and development of Federal coal, a Federal Coal Management Program was developed, which requires the U.S. Bureau of Land Management to identify tracts of coal for potential lease, analyze the tracts for potential environmental impacts, and schedule selected tracts of coal for lease sale. One of the primary considerations in the selection of tracts for lease is potential adverse impacts to the water resources of the area during mining and reclamation operations, and after abandonment. To determine potential impacts and reclamation potential of coal tracts, the U.S. Geological Survey is cooperating with the Bureau of Land Management under the EMRIA (Energy Minerals Rehabilitation Inventory and Analysis) program. As part of this program, the U.S. Geological Survey is conducting hydrologic studies on several potential coal-lease tracts in the Powder River structural basin of southeastern Montana. The Cook Creek area of the Ashland coal field is one of these tracts. Purpose and scope The purpose of the study was to describe existing hydrologic systems, to obtain data on the water quality in the area, and to assess potential impacts of surface coal mining on local water resources. Specific objectives of the study were to: (1) Identify ground-water resources; (2) identify surface-water resources and runoff characteristics; (3) determine chemical quality of the water resources; (4) determine probable impacts on existing water resources from mining operations, including changes in the quantity and quality of water; and (5) evaluate the potential for reclamation of local water resources. To accomplish these objectives, hydrogeologic data were collected from existing wells, springs, and drill holes. Eleven additional test holes and observation wells were drilled and completed where data were lacking. Aquifer tests were made at all suitable wells and a network of observation wells was established to measure long-term fluctuations of ground-water levels. Water samples were collected from ground-water and surface-water sources and analyzed for chemical quality. Channelgeometry measurements were made to estimate runoff characteristics in small watersheds. A maj or use of this report will be to provide information that can be used to rank various tracts of Federal coal for potential mining. The information for all tracts will be presented in a similar format, emphasizing the potential effects of mining and the potential for reclamation of the hydrologic systems. Supporting technical information on geology, water resources, and water quality also is given for the interested reader. Location and description of area The Cook Creek study area encompasses about 22 mi.2 in Rosebud and Powder River Counties, southeastern Montana. The town of Ashland, Mont., is about 1 mile southwest of the study area, and the Northern Cheyenne Indian Reservation is adjacent to the western boundary of the study area (fig. 1). 46°00' I06°30' I06°00' Boundary of Ashland coal field 45°30' Boundary of Ashland coal field CUSTER/// / / X X / / / :NORTHERN:::CHEYENNE

Potential effects of surface coal mining on the hydrology of the Cook Creek area, Ashland coal field, southeastern Montana