Flow System Analysis Of The Madison And Minnelusa Aquifers In The Rapid City Area South Dakota PDF Download

More than 50 percent of the public drinking water systems and more than 90 percent of the population in South Dakota rely solely on groundwater. This dependence on groundwater raises important questions regarding the Madison and Minnelusa aquifers in and near the Black Hills of South Dakota, including groundwater availability, the effects of water use or drought, mixing of regional flow and local recharge, and the effects of capture zones of springs and wells on the groundwater-flow system. These questions are best addressed with a three-dimensional numerical groundwater-flow model that includes the entire Black Hills area. In preparation for such a model, a three-dimensional hydrogeologic framework was constructed for the Black Hills and surrounding area. The study area includes approximately 60,000 square miles, extending approximately 150 miles from the center of the Black Hills in all directions. Structural-contour maps, potentiometric maps, and summaries of aquifer properties presented in this report will enhance groundwater modeling of the Madison and Minnelusa aquifers on a regional scale and allow for more realistic modeling of boundary conditions on a local, site-specific scale. Structural-contour maps and well logs quantifying the top and bottom altitudes of the Madison and Minnelusa aquifers were aggregated from numerous previous investigations to construct continuous surfaces defining the hydrogeologic framework. The primary challenge in this aggregation was that structural-contour maps from different sources frequently were inconsistent for overlapping areas, usually as a result of varying resolution in spatial data. For these inconsistencies, a systematic workflow was developed to determine which source was most accurate or reliable and would be used in the final aggregation. Potentiometric maps delineating the hydraulic head of the Madison and Minnelusa aquifers are a result of aggregating numerous previous investigations using a method similar to the construction of the structural-contour maps, with modifications based on additional groundwater-level measurements. The data were combined to construct continuous surfaces defining the regional potentiometric surface for the Madison and Minnelusa aquifers. The Minnelusa aquifer potentiometric map is largely similar to recent publications. The Madison aquifer potentiometric map enhances understanding of a trough, or valley-shaped feature, in the potentiometric surface extending from Rapid City through Philip and eastward. This trough was previously identified by Downey in U.S. Geological Survey Professional Paper 1402-E but not shown in many other recent publications. Aquifer properties, including hydraulic conductivity, transmissivity, and storage coefficient, also were summarized from 40 wells for which estimates were available from various types of aquifer tests. Hydraulic ranged from [2x10-3] ft/day to 113.62 ft/day for the Madison aquifer and from 0.36 ft/day to 24.43 ft/day for the Minnelusa aquifer. Storage coefficient values derived from pumping tests ranged from [1x1O-7] to [2x10-3] for the Madison aquifer and from [7x10-5] to [2x10-3] for the Minnelusa aquifer.