Every year, farm operators apply more than 12 million tons of nitrogen fertilizer and 8 million tons of phos- phorus fertilizer to agricultural land in the . Unless carefully managed, much of it is carried off the fields by runoff or percolates into drainage systems, eventually ending up in streams, rivers, lakes and underground aquifers. Animal manure from livestock is also an important contributor to nutrient pollution, particularly phos- phorus. Two recent reports have estimated that nationally, livestock production generates between 350 million and 1 billion tons of manure each year. According to the . Geological Survey, manure contributes 7-to-48 percent of.
