Nitrate (NO3) is a common inorganic form of nitrogen. Chemically, it is an anion with a single negative charge, consisting of one atom of nitrogen and three atoms of oxygen. Because it is an anion, it is soluble in water. Plants normally use nitrate as their source of the nitrogen needed by all living things, and so nitrate is considered a nutrient for plants. Excessive concentrations of nitrate in lakes and streams greater than about 5 milligrams per liter (measured as nitrogen), depending on the water body, can cause excessive growth of algae and other plants, leading to accelerated eutrophication or "aging" of lakes, and occasional loss of dissolved oxygen. Animals and humans cannot use inorganic forms of nitrogen, so nitrate is not a nutrient for us. If nitrate-nitrogen exceeds 10 milligrams per liter in drinking water, it can cause a condition called methemoglobinemia or "blue baby syndrome" in infants. Some recent studies have indicated a possible connection between elevated nitrate concentrations and cancer.
Nitrate can get into water directly as the result of runoff of fertilizers containing nitrate. Some nitrate enters water from the atmosphere, which carries nitrogen-containing compounds derived from automobiles and other sources. Nitrate can also be formed in water bodies through the oxidation of other, more reduced forms of nitrogen, including nitrite, ammonia, and organic nitrogen compounds such as amino acids. Ammonia and organic nitrogen can enter water through sewage effluent and runoff from land where manure has been applied or stored.
Water-quality regulatory agencies seek to avoid high concentrations of nitrate in water to minimize both of the problems noted above. Nitrate standards take two forms: drinking-water standards, designed to prevent adverse human-health effects, and ambient-water standards, designed to prevent excessive eutrophication in lakes and streams. Drinking-water standards for nitrate have been around since at least 1974, when the Safe Drinking Water Act was passed, and probably well before. States may set their own drinking-water standard for nitrate, but most or all use the EPA standard of 10 milligrams per liter (measured as nitrogen). Ambient-water standards have also been around for years, but each State has decided on what standards to use, if any. The EPA is just now setting guidelines for determination of ambient nitrate standards for different water bodies in different regions. General information of EPA's programs for water-quality standards and criteria is available at:
A good contact at EPA to answer questions about standards is the Office of Ground Water and Drinking Water.
Keeping drinking water free of excessive concentrations of nitrate involves a multiple-barrier approach. The most effective strategy is prevention–keeping chemicals that contain or can generate nitrate out of the water. This means managing agricultural operations to minimize application of fertilizer and to minimize runoff of fertilizer that is applied. Some farmers are now using computerized maps of their fields, calibrated to the specific soil and water conditions in various parts of their fields, to restrict the application of fertilizer to only what is needed for each part of the field. In some countries, for example Switzerland, drinking-water providers enter into contracts with farmers in their source areas in which farmers receive subsidies to eliminate fertilizers and use organic farming methods. Prevention also means proper handling of manure and animal waste lagoons, to minimize the discharge of animal waste or waste runoff to streams. Nitrate contributions from other sources can also be curtailed, for example by adding tertiary treatment, or by nutrient removal, to sewage treatment plants, and by controlling emissions from automobiles.
In addition to prevention, drinking-water providers may use advanced treatment techniques to remove nitrate from water. For example, Des Moines Water Works uses advanced ion-exchange technology to remove excess nitrate and remain below the 10 mg/L standard. In a typical year, this is needed mostly during the spring, following spring runoff after the application of fertilizer.
A good article about the occurrence of methemoglobinemia can be found at: