The geohydrology of 12 coal-lease areas in the Powder River structural basin is described in relation to the mining proposed for each area. The description of the hydrology includes identification of recharge and discharge areas, directions of groundwater movement, and possible effects of mining. Understanding the groundwater hydrology of the 12 coal-lease areas will improve the understanding of the shallow groundwater system in the basin. The Paleocene Fort Union Formation, Eocene Wasatch Formation, and Quaternary alluvium comprise most of the surface geology of the Powder River structural basin. The most productive aquifers in the shallow groundwater system are alluvial deposits, sandstone beds, and fractured coal beds. Well yields range from less than 10 gallons per minute in many parts of the basin up to 1,000 gallons per minute from clean, coarse-grained material along some of the rivers. Water in the northern part of the basin tends to be more mineralized than water in the southern part of the basin. Median dissolved-solids concentrations for water from wells in Campbell County (northern part of the basin) was more than 1,200 milligrams per liter compared to a median value of about 400 milligrams per liter for water from wells in Converse County (southern part of the basin). The aquifers that would be affected by mining include alluvium and water-yielding coals and sandstones in the Wasatch and Fort Union Formations. Water-level declines in coal aquifers generally will extend less than ^ or 5 miles from the mines, and water-level declines in overburden sandstone aquifers generally will extend less than 2 or 3 miles from the mines. Mining of lease areas that contain extensive outcrops of permeable clinker will decrease recharge to the local groundwater flow systems. Mining of lease areas in or near locations of groundwater discharge may decrease discharge to nearby streams, springs, and flowing wells. Mining also will destroy the wells in the coal-lease areas and may affect water levels in nearby wells outside the lease areas. Mining also will affect the quality of ground water available in the lease areas. Concentrations of dissolved solids in water from spoil aquifers generally are two to three times greater than in water from undisturbed coal aquifers. Large concentrations of dissolved solids (greater than 5,000 milligrams per liter) or dissolved sulfate (greater than 3,000 milligrams per liter) or both in spoil water occasionally may render the water unsuitable for livestock watering. Water suitable for domestic use generally is available only in the southern part of the basin, and mining of lease areas in this part of the basin likely will leave spoil water unsuitable for domestic use. Excessive nitrate (greater than 100 milligrams per liter) and selenium (greater than 0.10 milligram per liter) concentrations, such as those measured in spoil water at one existing mine, could render spoil water unsuitable for livestock or domestic use.