Aquifers in Quaternary glacial deposits and the underlying Silurian and Devonian carbonate bedrock in parts of Indiana, Ohio, Illinois, and Michigan compose the regional aquifer system under investigation as part of the Midwestern Basins and Arches Regional Aquifer System Analysis (Midwestern Basins and Arches—RASA) project of the U.S. Geological Survey (USGS). The Midwestern Basins and Arches—RASA is part of a USGS program to assess the regional hydrology, geology, and water quality of the Nation's most important aquifers (Sun, 1986). An objective specific to the Midwestern Basins and Arches—RASA project is to conceptualize and describe regional ground-water flow in the glacial-deposit and carbonate-bedrock aquifer system, including regional recharge and discharge areas and regional relations between surface and ground water (Bugliosi, 1990).
Water-level and ground-water discharge data were collected and (or) analyzed to help meet the above objective. Specifically, data from the USGS Ground-Water Site Inventory (GWSI) data base were used to determine relations between land-surface altitude and water levels in glacial-deposit aquifers. Water levels in the carbonate-bedrock aquifer were synoptically measured during July 1990, and the data were used to construct a potentiometric surface map of the aquifer. Regional hydraulic gradients and general directions of regional flow in the carbonate-bedrock aquifer can be inferred from this map. Steady-state groundwater discharge to streams that drain the area underlain by the glacial-deposit and carbonate bedrock aquifer system was estimated from base-flow daily values computed from streamflow records.
Water-level and ground-water-discharge data collectively form the sample information necessary to develop calibration targets for calibration of a ground-water-flow model (Anderson and Woessner, 1992). Such a ground-water-flow model of the glacial-deposit and carbonate-bedrock aquifer system was constructed to help conceptualize and describe regional ground-water flow in the aquifer system. The model was calibrated to the water-level and ground-water-discharge data presented in this atlas.