National Water-Quality Assessment (NAWQA) Study-Unit Investigations in the conterminous United States 2001-2012
This is a coverage of the boundaries and codes used for the
U.S. Geological Survey's National Water-Quality Assessment 
(NAWQA) Program Study-Unit investigations in the conterminous United
States, excluding the High Plains Regional Ground-Water Study.
The data set represents the areas to be studied during the second …