Nitrate in Drinking Water

DNR publication number WS-001.

What is Nitrate?

Nitrate (NO3^-) is a water-soluble molecule made up of nitrogen and oxygen. It is formed when nitrogen from ammonia or other sources combines with oxygenated water. Nitrate is a natural constituent of plants and is found in vegetables at varying levels depending on the amount of fertilizer applied and on other growing conditions. According to the World Health Organization, most adults ingest 20-70 milligrams of nitrate-nitrogen per day with most of this coming from foods like lettuce, celery, beets, and spinach. When foods containing nitrate are eaten as part of a balanced diet the nitrate exposure is not thought to be harmful.

Nitrate Levels in Drinking Water

Water naturally contains less than 1 milligram of nitrate-nitrogen per liter and is not a major source of exposure. Higher levels indicate that the water has been contaminated. Common sources of nitrate contamination include fertilizers, animal wastes, septic tanks, municipal sewage treatment systems, and decaying plant debris. The ability of nitrate to enter well water depends on the type of soil and bedrock present, and on the depth and construction of the well.

State and federal laws set the maximum allowable level of nitrate-nitrogen in public drinking water at 10 milligrams per liter (10 parts per million). These laws apply to all city and village water supplies and are used as an advisory for private wells.

Health Effects

High nitrate levels in drinking water pose a risk to infants. Infants who are fed water or formula made with water that is high in nitrate can develop a condition that doctors call methemoglobinemia. The condition is also called "blue baby syndrome" because the skin appears blue-gray or lavender in color. This color change is caused by a lack of oxygen in the blood.

All infants under six months of age are at risk of nitrate poisoning, but some babies may be more sensitive than others. Infants suffering from "blue baby syndrome" need immediate medical care because the condition can lead to coma and death if it is not treated promptly.

When nursing mothers ingest water that contains nitrate, the amount of nitrate in breast milk may increase. Although no confirmed cases of "blue-baby syndrome" have been associated with nitrate in breast milk, it may be advisable for nursing women to avoid drinking water that contains more than 50 milligrams per liter nitrate-nitrogen.

Some scientific studies have found evidence suggesting that women who drink nitrate-contaminated water during pregnancy are more likely to have babies with birth defects. Nitrate ingested by the mother may also lower the amount of oxygen available to the fetus.

People who have heart or lung disease, certain inherited enzyme defects, or cancer may be more sensitive to the toxic effects of nitrate than others. In addition, some experts believe that long-term ingestion of water high in nitrate may increase the risk of certain types of cancer.

Recommendations

When laboratory tests determine that water contains more than 10 milligrams per liter nitrate-nitrogen, the following actions are recommended:

1. Do not give the water to infants less than 6 months of age or use the water to prepare infant formula. Avoid drinking the water on a daily basis during pregnancy. Do not attempt to remove the nitrate by boiling the water. This will only concentrate the nitrate making levels even higher. Seek medical help immediately if the skin of an infant appears bluish or gray in color. Sometimes the color change is first noticed around the mouth, or on the hands and feet. Identify the nitrate source and take action to reduce contamination. Remedial actions may include reducing fertilizer use, improving manure handling methods, pumping septic tanks, or upgrading wells.

2. Limit your daily intake if you have chronic health problems that increase your sensitivity to nitrate, or if you are concerned about scientific uncertainty regarding the health effects of long-term exposure to nitrate-contaminated water.