What is a study unit?
NAWQA study units are major river basins and aquifers across the Nation. A study unit boundary frequently crosses state boundaries and usually encompasses more than 10,000 square kilometers (about 3,900 square miles).

Study units are the building blocks of NAWQA and form the basis for national synthesis assessments.

How were study units selected?
Geographic areas were selected to represent a variety of important hydrologic and ecological resources; critical sources of contaminants, including agricultural, urban, and natural sources; and a high percentage of population served by municipal water supply and irrigated agriculture.

What is the scope of study-unit investigations?
Studies are long-term and cyclical. One-third of all study units are intensively investigated at any given time for 3-4 years, followed by low-intensity monitoring. Trends are assessed about every 10 years.

1991 - 2001 (51 study units)
The first cycle of intensive investigations began with 20 study units in fiscal year 1991; investigations of additional study units began in 1994, 1997, and 1999.

2001 - 2012 (42 study units)
The second cycle of intensive investigations began in 2001 in 14 study units that were intensively assessed during the first cycle from 1991-95. Investigations will begin in 14 more study units in 2004 and in another 14 in 2007.