What is Nitrate and Nitrite Element Definition
Nitrate is one of the major anions in natural waters, but concentrations can be greatly elevated due to leaching of nitrogen from farm fertilizer or from feed lots or from septic tanks. The mean concentration of nitrate nitrogen (NO3-N, nitrate measured as nitrogen in testing) in a typical surface water supply would be around 0.2 to 2 mg/L; however, individual wells can have significantly higher concentrations. Adult daily dietary nitrate intake is approximately 20 mg, mostly from vegetables, like lettuce, celery, beets, and spinach (National Academy of Sciences Committee on Nitrite and Alternative Curing Agents in Food, 1981).
Nitrite does not typically occur in natural waters at significant levels, except under reducing conditions. It can also occur if water with sufficient ammonia is treated with permanganate. Sodium nitrite is widely used for cured meats, pickling, and beer. Rarely, buildings have been contaminated by faulty cross connections or procedures during boiler cleaning with nitrous acid. Nitrite, or nitrate converted to nitrite in the body, causes two chemical reactions that can cause adverse health effects: induction of methemoglobinemia, especially in infants under one year of age, and the potential formation of carcinogenic nitrosamides and nitrosamines. Methemoglobin, normally present at 1 to 3 percent in the blood, is the oxidized form of hemoglobin and cannot act as an oxygen carrier in the blood. Certain substances, such as nitrite ion, act as oxidizers. Nitrite is formed by reaction of nitrate with saliva, but in infants under one year of age the relatively alkaline conditions in the stomach allow bacteria there to form nitrite. Up to 100 percent of nitrate is reduced to nitrite in infants, compared with 10 percent in adults and children over one year of age.
Furthermore, infants do not have the same capability as adults to reconvert methemoglobin back to hemoglobin. When the concentration of methemoglobin reaches 5 to 10 percent, the symptoms can include lethargy, shortness of breath, and a bluish skin color. Anoxia and death can occur at high concentrations of nitrites or nitrates. Carcinogenic nitrosamines and nitrosamides are formed when nitrate or nitrite are administered with nitrosatable amines, such as the amino acids in proteins. However, epidemiological studies, primarily on gastric cancer, have not yielded consistent results . The carcinogenicity of nitrate and nitrite is currently under review. The data on the role of nitrates in developmental effects, such as birth defects, are regarded as being inconclusive. Several epidemiologic case-control studies found an increased risk of developmental brain defects, but more studies are needed. If there is an association, maternal methemoglobinemia might be the critical risk factor. The MCLGs and MCLs are 10 mg/L for nitrate measured as nitrogen (or 45 mg/L nitrate) and 1 mg/L for nitrite measured as nitrogen. In addition, the MCL for total nitrate-N and nitrite-N is 10 mg/L.
Learn more about other elements and their effects in water supplies by exploring our water treatment blog.