The watersheds of the Purgatoire and Apishapa Rivers contain most of the public coal lands in the Raton Basin. The U.S. Geological Survey, in cooperation with the U.S. Bureau of Land Management, investigated the hydrogeology of this area from 1978 to 1982, inventorying 231 wells, 38 springs, and 6 mines, and collecting ground-water samples from 71 sites. The Raton Basin is an asymmetrical trough, containing 10,000 to 25,000 feet of sedimentary rocks that range in age from Pennsylvanian to Eocene. These rocks are intruded by Miocene igneous rocks, covered with Pleistocene and Holocene alluvium on pediments and in stream valleys, and underlain by Precambrian crystalline rocks. Bituminous coal occurs in the Vermejo and Raton Formations of Cretaceous and Paleocene age. Virtually all of the sedimentary rocks transmit water. Stream alluvium is the most productive aquifer. Bedrock aquifers have smaller yields but greater distribution. The principal bedrock aquifers are the Cuchara-Poison Canyon and the Raton-Vermejo-Trinidad. Other formations are nearly impermeable or too deep to be utilized economically. The Cuchara-Poison Canyon aquifer provides small, nonsustainable yields to wells. Sandstone and coal layers in the Raton-Vermejo-Trinidad aquifer provide small, sustainable yields, but many of these beds are lenticular and can be missed easily by wells. Water in alluvium typically is less mineralized than in bedrock but more susceptible to contamination. Sodium and calcium bicarbonate waters predominate in the area, but sodium chloride water commonly occurs in the Cuchara-Poison Canyon aquifer and may occur in the Pierre Shale. Plumes of sulfate-enriched water extend from coal mines into bedrock and alluvial aquifers. Dissolved-solids concentrations range from less than 500 milligrams per liter in calcium bicarbonate water to more than 1,500 milligrams per liter in sulfate and chloride waters. Much of the ground water is hard. Nitrogen is enriched in shallow ground water, and fluoride is enriched in deeper ground water. Levels of iron, manganese, zinc, and selenium locally exceed standards for domestic consumption. The Purgatoire River and its tributaries are predominantly gaining streams, but losing reaches occur. Water quality in streams is affected by tributary inflows, mine discharge, contact with and seepage from tailings, groundwater seepage, diversion ditches, and changes in stage. Ground water flows regionally from west to east and locally from stream divides to valleys. Depths to water vary from 500 feet beneath divides to less than 100 feet in valleys. Springs typically develop where valleys intersect the water table, at or below the contact between the Poison Canyon and Raton Formations, and in stream channels that are crossed by dikes or sills or underlain by shallow bedrock. Most of the water in regional circulation discharges into surface drainages before reaching the east side of the basin. Groundwater supplies probably are insufficient for expanded settlement and coal mining.