SUMMARY
Runoff from our farms and cities is a major source of phosphorus (P) and nitrogen (N) entering rivers, lakes, and coastal
waters. Acid rain and airborne pollutants generated by human activities also supply N to surface waters. These nutrient
sources are called nonpoint because they involve widely dispersed activities. Nonpoint inputs are difficult to measure and
regulate because of their dispersed origins and because they vary with the seasons and the weather.