National Water-Quality Assessment (NAWQA) Study-Unit Investigations in the conterminous United States 1991-2001
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 studied during the first decade
of …