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Vertical joints and bedding planes in massive sandstone beds above and below the dark shale confining unit act as channels for the movement of the water that is visible as dark stains on this quarry face in New Jersey.
Photo by H. Trapp Jr.,
U.S. Geological Survey
The United States Geological Survey (USGS) monitors the quantity and quality of surface and ground waters throughout the nation. In the 1980s and 90s, the USGS investigated 24 of the most important aquifers and aquifer systems of the Nation with these objectives:

• to define the geologic and hydrologic frameworks of each aquifer system;
• to assess the geochemistry of the water in the system;
• to characterize the ground water flow system; and
• to describe the effects of development on the flow system.

These studies compiled much of the data needed to make a national assessment of ground water resources. The national assessment is provided in the Ground Water Atlas of the United States, a summary of the most important information available for each principal aquifer or rock unit that will yield usable quantities of water to wells.
Outwash deposits, which consist of well sorted and stratified sand and gravel deposited primarily by streams during the melting and retreat of the glacial ice, form productive valley-fill glacial aquifers.
Photo by P.G. Olcott, U.S. Geological Survey

The National Atlas online interactive Map Maker includes two nationwide maps of principal aquifers, the Principal Aquifers of the United States, and the Aquifers of Alluvial and Glacial Origin north of the line of continental glaciation. Each individual aquifer is linked to the Aquifer Basics Web site, which provides information about all 63 principal aquifers and the widespread, but often fragmented, sand and gravel aquifers of alluvial and glacial origin.

Aquifer Basics includes information about the six types of permeable geologic material, links to source documents that describe the location, extent, and geologic and hydrologic characteristics of all the important aquifers in the United States, and includes underground aquifer extent when applicable.