Digital data set describing ground-water regions in the conterminous US

This data set describes ground-water regions in the United
States defined by the U.S. Geological Survey. These
ground-water regions are useful for dividing the United States
into areas of roughly similar hydrologic characterstics and
water-use patterns. These regions are very generalized and
were developed from a illustration published at a scale of
approximately 1:20 million.