National Water-Quality Assessment (NAWQA) Program
How the figure was made:
This table is derived from Puckett (2004).
Puckett, L.J., 2004, Hydrogeologic controls on the transport and fate of nitrate in ground water beneath riparian buffer zones: Results from thirteen studies across the United States: Water Science and Technology, v. 49, no. 3, p. 47–53, accessed June 17, 2010, at http://water.usgs.gov/nawqa/nutrients/pubs/wst_v49_no3/wst_v49_no3.pdf.