Nutrients |
U.S. Geological Circular 1225--The Quality of Our Nation's Waters--Nutrients and Pesticides
Human activities--including agricultural and urban uses of fertilizer, agricultural use of manure, and combustion of fossil fuels--have caused widespread increases of nitrate in shallow ground water and total nitrogen and total phosphorus in streams across the Nation.
Nitrate did not pose a national health risk for residents whose drinking water came from streams or from major aquifers buried relatively deep beneath the land surface. Some con'cerns were evident in 4 of the 33 major aquifers sampled, where nitrate concentrations in more than 15 percent of each aquifer exceeded the USEPA drinking-water standard. The most prevalent nitrate contamination of ground water, however, was found in relatively shallow ground water in rural areas where the water commonly is used for domestic supply.
In more than one-half of sampled streams, concentrations of total nitrogen and total phosphorus were above national background concentrations. Elevated phosphorus levels, in particular, can lead to excessive plant growth (eutrophication) in freshwater environments; in more than one-half of sampled streams and in three-fourths of agricultural and urban streams, average annual concentrations of total phosphorus exceeded the USEPA desired goal for prevention of nuisance plant growth. The highest total nitrogen and total phosphorus concentrations were found in small streams draining watersheds with large proportions of agricultural or urban land. Long-term monitoring of streams indicates that programs to control point-source discharges of phosphorus and ammonia have been effective, despite population increases in most metropolitan areas. Phosphorus concentrations have decreased as a result of reductions in the use of phosphate detergents and in the amount of phosphorus discharged from upgraded wastewater treatment plants. Improved wastewater treatment, which converts ammonia to nitrate, generally has resulted in a decrease in ammonia concentrations and an increase in nitrate concentrations in streams. Thus, concentrations of total nitrogen downstream from metropolitan areas have changed little during the past 20 years, although toxicity to fish has decreased with decreasing ammonia levels.
Results from NAWQA studies have shown regional and seasonal differences in nutrient concentrations that can be explained largely by the amounts and timing of fertilizer and manure applications and by the variety of soils, geology, climate, and land- and water-management practices across the Nation. Recognition of these differences is important for efficient protection of ground water needed for drinking and for curbing eutrophication of surface water.
