This polygon shapefile contains the shallowest principal aquifers of the conterminous United States, Hawaii, Puerto Rico, and the U.S. Virgin Islands, portrayed as polygons. The map layer was developed as part of the effort to produce the maps published at 1:2,500,000 in the printed series "Ground Water Atlas of the United States". The published maps contain base and cultural features not included in these data. This is a replacement for the July 1998 map layer called Principal Aquifers of the 48 Conterminous United States. This map layer was created and modified over a period of at least five years by several staff members of the U.S. Geological Survey Water Resources Discipline, Cartographic and Publications Program in Madison, Wisconsin. Completion of this map layer and associated metadata was funded, in part, under a cooperative joint funding agreement between the U.S. Geological Survey and the U.S. Environmental Protection Agency. This layer is part of the 1997-2014 edition National Atlas of the United States.These data are intended for use in publications, at a scale of 1:2,500,000 or smaller. Due to the small scale, the primary intended use is for regional and national data display and analysis, rather than specific local data analysis.