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Nitrate is a form of nitrogen found extensively in the environment. It is a major component of various proteins (i.e., organic nitrogen) in plant and animal tissues. It occurs naturally in the soil as the end product of the nitrogen cycle as these plant and animal residues decay and release nitrate.
Nitrate is highly soluble and readily moves with water from the surface into the subsoil and eventually into the underlying aquifer materials.
Nitrate is colorless, odorless, and tasteless, so is undetectable in water without testing. Drinking water nitrates have potential to cause adverse health effects for susceptible individuals.
Nitrogen is an essential plant nutrient. It is the most commonly applied nutrient for lawn and garden care and for crop production to increase productivity. Nitrate is one of several common chemical forms found at various points in the nitrogen cycle.
Nitrate can be introduced into drinking water from human and animal sources including septic systems, fertilizer. animal confinements, and other waste treatment systems. Nitrate is soluble and is transported readily through the environment by water movement.
Nitrate can enter ground water via recharge, as surface water percolates through the overlying soil. Contaminated surface water may also mingle with clean water through direct entry pathways, like improperly sealed water wells.
Even properly designed and efficiently operating traditional septic-tank/drain-field systems can discharge nitrate to the environment. Nitrate contamination risk can be reduced with adequate separation distances between septic systems and water wells.
The US-EPA has set a “maximum contaminant level (MCL)” for nitrate-nitrogen in a public water supply at 10 milligrams per liter1 (10 mg/L NO3-N). The MCL is based on acute health effects, specifically the increased risk of “methemoglobinemia”. Acute health effects are those that result from ingestion of a contaminant over a short period of time.
The acute health hazard associated with nitrate occurs when bacteria in the digestive system transform nitrate
(NO3) to nitrite (NO2), another form of nitrogen. Nitrite reacts with iron in the hemoglobin of red blood cells, forming “methemoglobin” - which lacks the oxygen-carrying ability of hemoglobin.
The general population has a low risk of developing methemoglobinemia, even when ingesting relatively high levels of nitrate/nitrite.
Consuming excess dietary nitrates can result in methemoglobinemia (sometimes called “blue baby syndrome”), in which blood lacks the ability to carry sufficient oxygen to the individual body cells.
Some nitrate is necessary for cardiovascular health, but excess nitrate can trigger acute health effects. Formula-fed infants under one year of age have the highest risk of developing methemoglobinemia. Older persons with a gastrointestinal system disorder or renal (kidney) problems may be at greater risk than the general population.
Total dietary nitrate intake from food, drugs and other sources must be considered along with nitrate intake from drinking water. The potential risk of chronic health problems resulting from long-term drinking water nitrate intake is uncertain.
¶ Table 1. Relative Rankings of Nitrate Concentrations in Drinking Water |
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| Nitrate-nitrogen as NO3-N mg/L | Nitrate as NO3 mg/L | Relative ranking |
| Less than 3.0 | < 13 | Low |
| 3.0 - 7.0 | 13 - 30 | Moderate |
| 7.0 - 10.0 | 30 - 45 | High |
| 10 | 45 | MCL |
| 10 - 20 | 45 - 90 | Very High |
| 20 - 40 | 90 - 175 | Extremely high |
| Greater than 40 | > 175 | Excessively high |
A public water supply must report nitrate levels annually to its users and when levels exceed the MCL.
Testing a private water supply is optional, but is important. New water supplies should be tested to determine the baseline nitrate concentration in the water source.
Private drinking water should be tested annually to monitor changes in nitrate concentration. This is especially important for households with infants, pregnant women, nursing mothers, or elderly people. These groups are believed to be the most susceptible to nitrate health effects.
Nitrate in drinking water should be tested by a state certified laboratory. There are a variety of test kits and dip strips are available for nitrate testing outside of a laboratory environment. Laboratory analysis is essential for an accurate and reliable nitrate measurement.
A small amount of nitrite (NO2)may be associated with nitrate (NO3) in a water source depending on conditions.
nitrite. The health effect of nitrite is considered to have ten times the impact as does nitrate. The MCL for nitrite-nitrogen is only 1.0 milligram per liter (1.0 mg/L NO2-N).
Nitrite can be analyzed separately from nitrate. To estimate the combined effect of nitrite and nitrate, multiply the nitrite level by 10 and add it to the nitrate level. If the sum is 10 mg/L or above, the sample does not meet the drinking water standard.
ServiTech Laboratories report concentrations as “nitrate-nitrogen (NO3-N)”. Some labs report concentrations as nitrate (NO3). Table 1 lists the relative rankings of nitrate concentrations in drinking water.
It is important to check the reporting units to properly interpret a lab result. The two expressions compare as follows:
10 mg/L NO3-N (nitrate-nitrogen) = 44.3 mg/L NO3 (nitrate)
The MCL for nitrate is often rounded to “45 mg/L NO3” for convenience.
From a practical standpoint, the units are interchangeable as long as there is consistent reporting. It is really no different than reporting temperatures as Fahrenheit (°F) or centigrade (°C) as long as the interpretation of the numbers is consistent.
One laboratory method converts nitrate to nitrite, then measures the total amount of both. In this case, final results are reported as “nitrate-plus-nitrite”.
If nitrate exceeds a desirable concentration, management options are to consider an alternative drinking water source, water treatment, or both. Nitrate does not affect other domestic uses including bathing, washing, or watering.
Users of private drinking water may wish to consider the EPA guideline of 10 mg/L NO3-N when considering potential health risk, even though the general population has a low risk of developing chronic health problems. Dietary intake of naturally-occurring nitrate from vegetables and milk likely exceeds that of drinking water. Some adults include nitrate-containing foods or supplements to improve cardiovascular health.
It may be possible to obtain a satisfactory alternate water supply by drilling a new well in a different location or a deeper well in a different aquifer. Local conditions and regulations will affect this decision.
Bottled water, purchased in retail stores or direct from bottling companies, is an alternative source. This is a good short-term alternative especially if the primary concern is water for infant food and drinking.
Drinking water can be treated for nitrate-nitrogen by three treatment methods:
Equipment using these processes is available from several manufacturers for installation in homes and businesses.
Carbon filters and standard water softeners do not remove nitrate. Carbon filters remove certain positively charged organic molecules that affect taste and odor. Softeners exchange positively charged sodium molecules with other positively charged molecules, like calcium or magnesium that contribute to water hardness. Nitrate molecules are negatively charged, so are not affected by the exchange media in carbon filters or softeners.
Boiling water will actually increase rather than decrease nitrate levels. The nitrate molecule is not affected by heating so remains in solution during boiling. The nitrate also remains in the container as pure water is removed by escaping steam, resulting in a more concentrated solution.
The U.S. Public Health Service considers drinking water with nitrate nitrogen levels at or below 10 mg/L (or nitrate levels at or below 45 ppm NO3) to be acceptable for unlimited consumption. Recommend testing this water supply routinely to confirm low nitrate concentrations. Nitrate occurs in many natural waters at a concentration of 1 to 3 mg/L. Levels above this may suggest nitrate impact from man-made sources.
The U.S. Public Health Service considers drinking water with nitrate-nitrogen levels at or below 10 mg/L NO3-N (or nitrate levels at or below 45 ppm NO3) to be acceptable for unlimited consumption. Recommend that this water supply be tested routinely to confirm low nitrate concentrations.
The U.S. Public Health Service considers drinking water with nitrate-nitrogen levels at or below 10 mg/L NO3-N (or nitrate levels at or below 45 ppm NO3) to be acceptable for unlimited consumption. Recommend that this water supply be tested routinely to confirm low nitrate concentrations.
The U.S. Public Health Service considers nitrate-nitrogen levels above 10 mg/L NO3-N (or nitrate levels at or below 45 ppm NO3) to be unsafe for unlimited consumption by infants less than one year of age and pregnant women. An alternate water supply low in nitrate or proper water treatment to reduce nitrate (such as distillation or reverse osmosis) is suggested for individuals at risk. This water is considered safe to for other uses, like washing, bathing, etc. This water source should be tested annually to monitor nitrate levels and assure they are not increasing. If nitrate levels remain elevated, it may be prudent to have the water system should be examined by a trained professional to identify the potential for contamination from outside sources. The entire water system should be inspected to assure it meets current standards for well construction, cross connection, etc. which may allow contaminants to be introduced into the water supply.
The maximum contaminant level for nitrate-nitrogen is 10 mg/L. Above 10 mg/L, nitrate nitrogen is considered unsafe for consumption by infants and pregnant women. Levels exceeding 20 mg/L NO3-N, this water is considered unsafe to use for unlimited human consumption. This condition (two to four times the U.S. Public Health Service standard) indicates nitrate contaminants which should be corrected. The entire water system should be examined by a trained professional to determine the origin of the nitrate contamination, including: potential contaminant sources, improperly constructed or maintained water wells, cross-connection, etc. An alternate water supply or treatment (like distillation or reverse osmosis) is recommended to reduce nitrate for drinking or cooking. This water is considered safe to for other uses, like washing, bathing, etc. Test water at least twice per year to monitor nitrate levels.
If the nitrate nitrogen level is over four times the U.S. Public Health Service standard, the water source is considered hazardous for unlimited human consumption, both infants and adults. Do not use this water supply for drinking or cooking unless a safe level has been achieved through treatment, such as distillation or reverse osmosis. An alternative water supply for drinking and cooking may be required. The water may or may not be acceptable for other household uses because the high nitrate level indicates severe contamination. The entire water system should be examined by a trained professional to determine the origin of the nitrate contamination, locate potential contaminant sources, and evaluate the water system. The water system should meet current standards for well construction, cross-connection, equipment maintenance, etc. Test water at least twice per year to monitor nitrate levels.
USDA Extension. 2019. Drinking Water Contaminant – Nitrate. accessed 06July2022. https://drinking-water.extension.org/drinking-water-contaminant-nitrate/
Bradshaw & Powell. 2000. Bull. MF-912. Understanding Your Water Test Report. Kansas State Univ. Coop. Ext. Svc., Manhattan KS 12 pg.