The topographically defined Black Hills and adjacent areas (Black Hills area) of Wyoming (fig. 1) are underlain by two regionally important aquifers-the Minnelusa and the Madison. The Minnelusa aquifer is used extensively in the Black Hills area as a source of domestic and livestock water. The Madison aquifer is an important source of municipal, industrial, agricultural, and domestic water in both the Black Hills area and other parts of Wyoming. Increased demand for water from the Minnelusa and Madison aquifers in the Black Hills area of Wyoming and South Dakota have created a need for better understanding of the hydrology of these two important aquifers.

This report presents information on the potentiometric surfaces and altitudes of the tops of the Minnelusa and Madison aquifers in the Black Hills area of northeastern Wyoming using new data collected since completion of earlier studies. In addition, ground-water levels in selected wells are examined, and the relative age of a limited number of ground-water samples are discussed. The information will be of use to government officials, land planners and other individuals who must manage the limited water resources of this growing and developing area.

The scope of the project was limited to wells and springs in the Black Hills area of Wyoming. Water levels were measured during 1998 and 1999 in wells completed in the Minnelusa and Madison aquifers to construct potentiometric-surface maps. The potentiometric surfaces and altitudes of the tops of the Minnelusa and Madison aquifers were contoured at the same scale (1:100,000) as studies of the aquifers recently conducted by the U.S. Geological Survey (USGS) in the Black Hills area within South Dakota (Carter and Redden, 1999a, 1999b; Strobel and others, 2000a, 2000b). The contours in these studies of the Minnelusa and Madison aquifers in South Dakota were "edge matched" as closely as possible along the Wyoming-South Dakota State line using the same contour interval. Use of the same contour interval and edge matching will allow for the use of the maps across State boundaries and improve understanding of the aquifers in a larger area within the Black Hills uplift. In places, some contours could not be matched across the Wyoming-South Dakota State line because this investigation was completed after the South Dakota investigations, and additional control points used in this investigation were located in Wyoming near the State line. The reader should note that Minnelusa and Madison outcrop areas and some structural features may not match in some areas along the Wyoming-South Dakota State line when maps in this study area are aligned or "edge matched" with maps produced for the South Dakota studies because different sources of geological mapping were used for the studies. Ground-water samples were collected from a subset of measured wells and analyzed for tritium to qualitatively estimate the time of ground-water recharge.

This project was conducted by the USGS in cooperation with the Wyoming State Engineer's Office (WSEO). The study area was almost entirely within Crook and Weston Counties in Wyoming and was bordered on the east by the Wyoming-South Dakota State line.