National Water-Quality Assessment (NAWQA) Project
Modeled predictions for concentrations of nitrate in shallow, recently recharged U.S. ground water show some of the highest concentrations (greater than 10 milligrams per liter) in the High Plains and moderate concentrations (less than 10 milligrams per liter but greater than 5) in the northern Midwest. These areas generally are associated with relatively high nitrogen inputs from fertilizers and manure; low-to-moderate clay content in the soils and aquifer materials, which can affect movement of nitrate in the ground water; sufficient water input; and low potential for transformation of nitrate to other forms (denitrification).
The full article is accessible at: http://water.usgs.gov/nawqa/nutrients/pubs/est_v40_no24/.
Additional information on nutrients in streams and ground water can be found at the Nutrients National Synthesis Project website