National Water-Quality Assessment (NAWQA) Program
Alexander, R.B. and Smith, R.A., 1990, County-Level Estimates of Nitrogen and Phosphorus Fertilizer Use in the United States, 1945 to 1985: USGS Open-File Report 90-130.
Alexander, R.B., Slack, J.R., Ludtke, A.S., Fitzgerald, K.K., Schertz, T.L., 1996, Data from Selected U.S. Geological Survey National Stream Water-Quality Monitoring Networks (WQN): USGS Digital Data Series DDS-37.
Hitt, K.J., 2007, GWAVA-S -- Vulnerability of shallow ground water and drinking-water wells to nitrate in the United States: Model of predicted nitrate concentration in shallow, recently recharged ground water (ARC/INFO Grid data set)
Hitt, K.J. 2007, GWAVA-DW -- Vulnerability of shallow ground water and drinking-water wells to nitrate in the United States: Model of predicted nitrate concentration in U.S. ground water used for drinking (simulation depth 50 meters) (ARC/INFO Grid data set)
Nolan, B.T., Hitt, K.J., and Ruddy, B.C., Probability of nitrate contamination of recently recharged ground waters of the conterminous United States (ARC/INFO Grid data set)
Puckett, L.J., Alexander, R.A., and Hitt, K.J., 1998, County-based estimates of nitrogen and phosphorus content of animal manure in the United States for 1982, 1987, and 1992.