Ground-water resources in the Hanna and Carbon basins were assessed in a study from 1974 through 1980 because of the development of coal mining in the area. Data collected from 105 wells during that study are presented in this report. The data include well-completion records, lithologic logs, and water levels. The data are from stock wells, coaltest holes completed as observation wells by the U.S. Geological Survey, and mining company observation wells monitored by the U.S. Geological Survey. The data are mostly from mined coal-bearing formations: The Tertiary Hanna Formation and the Tertiary and Cretaceous Ferris Formation. Well-completion data and lithologic logs were collected during drilling of the wells or from U.S. Geological Survey files, company records, Wyoming State Engineer well-permit files, and published reports. PREVIOUS INVESTIGATIONS DESCRIPTION OF THE AREA The Hanna and Carbon basins, in Carbon County, south-central Wyoming, have a combined area of 1,500 square miles. Resistant rocks of the Late Cretaceous Mesaverde Group, which forms the Saddleback Hills, separate the Hanna basin on the vest from the Carbon basin on the east (fig. 2). The basins are bounded on the north by the Seminoe, Shirley, and Freezeout Mountains; on the vest by the Haystack Mountains; and on the south by the Medicine Bov Mountains* The average altitude is about 7,000 feet; the topography principally consists of undulating plains broken by high ridges. Typically, the topography closely reflects the structure of the underlying rocks. Stratigraphic and structural features of the area are shown in figure 3. Outcropping formations range in age from Late Cretaceous to Holocene. The Late Cretaceous Medicine Bov Formation, late Cretaceous and early Tertiary Ferris Formation, and early Tertiary Hanna Formation contain coal layers. Coal in the Hanna and Ferris Formations is mined by surface and underground methods; coal in the Medicine Bov Formation is not mined presently (1980). DESCRIPTION OF THE DATA Ground-vater data from 105 veils in the area (fig. A) are presented in tables 1,2, and 3, at the end of this report. This information includes veil-completion records, lithologic logs, and vater levels. Well-completion records and lithologic logs vere collected on-site during drilling of the veils or from U.S. Geological Survey files, company records, Wyoming State Engineer veil-permit files, and published reports. Well-completion data, vater levels, and additional information are available in the U.S. Geological Survey's computer data base, the Ground-Water Site-Inventory (GWSI) file of the National Water Data Storage and Retrieval System (WATS TORE). The lithologic logs are not in the computer data base.The lithologic logs quoted from published sources have not been edited to conform to U.S. Geological Survey style. Coal bed names are those used by the source and are not necessarily approved by the U.S. Geological Survey.

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.

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The Powder River structural basin in northeastern Wyoming is an area where the Wyoming Department of Environmental Quality is required to assess cumulative effects of mining because of existing and pending applications for surface-coal-mining operations. This investigation was conducted to determine the effects of surface-coal mining on the surface-and groundwater systems in a 5,400-square-mile area that includes 20 major coal mines. Three approaches were used in the investigation: A surface-water model, a landscape-stability analysis, and a groundwater flow model. A surface-water model was developed for the Belle Fourche River basin. The Hydrological Simulation Program-Fortran model was used to simulate changes in streamflow and changes in dissolved-solids and sulfate concentrations. For the calibration period, May and June 1978, simulated streamflow volume at the downstream station, Belle Fourche River below Moorcraft, was about 6 percent less than measured streamflow volume. During verification, simulated peak flows approximated the measured peak flows, but the simulated volume was much larger than the measured volume. Simulated and measured concentrations of dissolved solids differed by 18 percent and of sulfate by 35 percent during calibration and verification. The effects of mining on streamflow and dissolved-solids and sulfate concentrations were simulated by the model, using less than and greater than average rainfall for premining, during-mining, and postmining conditions. Values of hydrologic characteristics resulting from adjustment during calibration were used in the simulation of the premining condition. The following values of hydrologic characteristics were changed to represent the disturbed and reclaimed areas: Average length and slope of overland-flow path, infiltration capacity, and dissolved-solids concentration in interflow and in groundwater contribution. The during-mining condition assumed the maximum disturbed area for all mines at the same time. The postmining condition assumed the combined permit areas were reclaimed, and the sediment and flood-storage ponds remained in place for 10 years after completion of reclamation. Simulated streamflows resulting from less than average rainfall were small, changes in flow from premining to during-mining and postmining conditions were less than 2.5 percent, and changes in mean dissolved-solids and sulfate concentrations ranged from 1 to 7 percent. Between premining and postmining conditions, changes in median streamflows simulated using greater than average rainfall ranged from 4 to 22 percent at four sites downstream from mining, and simulated dissolved-solids and sulfate concentrations for streamflows exceeding 1.0 cubic foot per second were decreased by as much as 49 percent. Physical characteristics were measured for 102 drainage basins, and regression relations were developed for characteristics important to landscape stability to aid in designing the reconstruction of drainage networks. The results of hypsometric analyses for evaluating the sta...

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