National Water-Quality Assessment (NAWQA) Project
A major focus of the NAWQA Program in its second decade (2002-2013) is on regional- and national-scale assessments of groundwater-quality status and trends in principal aquifers. The U.S. Geological Survey Office of Groundwater has identified 62 principal aquifers in the U.S. (U.S. Geological Survey, 2003). About 1/3 of the Nation's principal aquifers are the focus of water-quality assessments at the regional scale by NAWQA.
The NAWQA framework for principal aquifer assessments considers the physical setting of the aquifer, in addition to its susceptibility and vulnerability to contamination.
More information (USGS Fact Sheet 2005-3013, PDF, Adobe reader is freely available to read a PDF)
Variations in groundwater quality among aquifers
What is a Principal Aquifer?
Definition: A principal aquifer is defined as a regionally extensive aquifer or aquifer system that has the potential to be used as a source of potable water. An aquifer is a geologic formation, a group of formations, or a part of a formation that contains sufficient saturated permeable material to yield significant quantities of water to wells and springs. Aquifers are often combined into aquifer systems. (U.S. Geological Survey, 2003).