Proportions of nonpoint and point sources of nitrogen vary in watersheds across the continental United States.⁽¹²⁾ Commercial fertilizer and manure typically constitute the major sources of nitrogen to the first 20 NAWQA Study Units. Atmospheric nitrogen is significant in most Study Units except in the far West and the Northern Great Plains. Point sources are an important source of nutrients to watersheds near large urban areas, such as Denver in the South Platte River Basin and Hartford in the Connecticut, Housatonic, and Thames River Basins.

The use of nitrogen and phosphorus fertilizer increased twentyfold and fourfold, respectively, between 1945 and the early 1980s, and then leveled off through 1993.

Changes in agricultural pesticide use over the decades.(13-17)