This geospatial dataset represents a prototype of a finer-scale representation of the principal aquifers of the United States, using four original principal aquifers in the Upper Midwest, United States, which were re-analyzed and condensed into three updated principal aquifers. The original principal aquifer definitions and extents were published in the Ground Water Atlas of the United States at a scale of 1:5,000,000. The updated version in this dataset are based on a reinterpretation of the original definitions using geologic data from the U.S. Geological Survey (USGS) State Geologic Map Compilation (v 1.1, 2017), which contains published state data at scales from 1:50,000 to 1:1,000,000. This shapefile includes fields describing the original principal aquifer definitions and data links as well as ages, major and minor lithologies, and source data for the updated aquifer unit polygons. A detailed description of the process followed to evaluate the old definitions of the principal aquifers in the Ground Water Atlas of the United States in the context of the State Geologic Map compilation datasets can be found in the related primary publication.
