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06/24/03

Nitrates in Drinking Water



Although nitrate occurs naturally in drinking water, elevated levels in groundwater usually result from human activities such as overuse of chemical fertilizers and improper disposal of human and animal wastes. These fertilizers and wastes are sources of nitrogen, containing compounds which are converted to nitrates in the soil. Nitrates are extremely soluble in water and can move easily through soil into the drinking water supply.

High levels can build up over time as nitrate accumulates in the water, but even at elevated levels, they are not likely to be a health hazard for most adults. However, the ingestion of excessive amounts of nitrate can cause adverse health effects in very young infants and susceptible adults. Consequently, the federal government has established a maximum acceptable level, known as the Maximum Contaminant Level (MCL), for nitrate in public drinking water supplies. This level is 10 milligrams per liter (mg/l)--often expressed as 10 parts per million (ppm)---measured on the basis of the nitrogen content of nitrate.

Nitrate in the Environment

The most common sources of nitrate are municipal and industrial wastewaters, refuse dumps, animal feed lots, and septic systems. Once nitrate is formed, its movement in soil and potential for contamination of ground, water depend on several factors including the soil characteristics, location and characteristics of the underground water formations (aquifers), and climatic conditions. Potential for nitrate contamination of drinking water also depends on the depth and construction of wells.

Identifying the source of nitrates for an individual well is often very difficult. Because nitrates move with the flow of groundwater, the source may be located a considerable distance from the well. In many cases, the time needed for nitrate to pass through the soil into groundwater is difficult to predict due to many variables including application rate, the soil type, and the depth to the water table.

Human Exposure

Human exposure to nitrates occurs primarily through the diet because nitrate is a natural substance found in both water and plants. In the United States, the average dietary intake of nitrate is about 75 to 100 mg per day.About 80 to 90 percent of this amount comes from vegetables. Some common vegetables with high nitrate content are beets, celery, lettuce, and spinach. People following a vegetarian diet may have nitrate intakes of up to 250 mg per day.

Drinking water generally accounts for 5 to 10 percent of nitrates consumed. However, where drinking water is contaminated to a level of 50 mg/l (5 times the MCL), it may supply as much as half of the total daily intake Ingested nitrate is the major source of nitrite in the body, high levels of nitrate in drinking water are generally responsible for high levels of nitrite in the body. Nitrate intake depends on a variety of factors, including diet, and amount and quality of water consumed.

Health Effects

The most significant health effect associated with nitrate ingestion is methemoglobinemia in infants under six months of age. This condition results from the presence of high nitrite levels in the blood. Untreated, severe methemoglobinemia can result in brain damage and even death. Infants in the first six months of life are particularly susceptible to nitrite induced methemoglobinemia. Finally, infants have a higher intake of water for their weight than adults, so consequently, they ingest a relatively higher amount of nitrate. In addition to small infants, some adults may be susceptible to the development of nitrite, induced methemoglobinemia.These include pregnant women with a particular enzyme deficiency, adults with reduced stomach acidity, and those with a deficiency in the enzyme needed to change methemoglobin back to normal hemoglobin, a condition which can be hereditary. Fortunately, methemoglobinemia is easily recognized by the medical and public health communities and can be readily diagnosed and treated.

Another concern about nitrate ingestion is the possibility that nitrites in the stomach and intestines may contribute to the development of some cancers. Nitrate in groundwater is of concern not only because of its toxic potential, but also because it may indicate contamination of the groundwater. If the source of contamination is animal waste or effluent from septic tanks, bacteria, viruses, and protozoa may also be present. Contamination of groundwater by fertilizers may also indicate the presence of other agricultural chemicals such as pesticides. The source of the nitrate may be a clue as to which other contaminants may be present.

Remedies

Drinking water containing more nitrates than the maximum Contaminant Level of 10 mg/l should not be consumed by infants or other susceptible individuals. Water that is bottled or taken from another safe source should be used. Simple in-line filters do not remove nitrates; but deionization, reverse osmosis, or distillation can be effective in the removal of nitrate. However, these treatments are expensive and require careful maintenance. In some cases drilling a deeper well extending into a noncontaminated water source may be the best, and in the long run, the least expensive remedy.
References

This information comes from Michigan State University Extension bulletin WQ19, Nitrate-A Drinking Water Concern.

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MSU is an affirmative-action, equal-opportunity employer. Michigan State University Extension programs and materials are open to all without regard to race, color, national origin, gender, gender identity, religion, age, height, weight, disability, political beliefs, sexual orientation, marital status, family status or veteran status. Issued in furtherance of MSU Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Thomas G. Coon, Director, MSU Extension, East Lansing,MI 48824. This information is for educational purposes only. Reference to commercial products or trade names does not imply endorsement by MSU Extension or bias against those not mentioned.

This information.was reviewed as of June 2008.  For more information about the contents please contact costner@msu.edu for webpage problems strausc@msu.edu .