Sources of nitrate in drinking water
Nitrate is a form of nitrogen that occurs naturally in the environment and is introduced from human and animal sources. Nitrogen is the nutrient most often applied for lawn and garden care and crop production to increase productivity. Feedlots, animal yards, septic systems, and other waste treatment systems are additional sources of nitrogen that are carried in waste. Even properly designed and efficiently operating traditional septic tank/drainfield systems discharge nitrate to the environment. A sufficient distance between a septic system and well will reduce the risk of nitrate contamination of drinking water at concentrations above that allowed in a public water supply. Nitrogen occurs naturally in the soil in organic forms from decaying plant and animal residues. Nitrate is highly soluble and readily moves with water through layers of soil. Nitrate in water is undetectable without testing because it is colorless, odorless, and tasteless.
Potential health effects of nitrate in drinking water
The U.S. Environmental Protection Agency (EPA) Maximum Contaminant Level (MCL) for nitrate-nitrogen in a public water supply is 10 milligrams per liter (mg/L, also equivalent to parts per million or ppm) and is based on acute health effects, specifically the 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 drinking water with nitrate occurs when bacteria in the digestive system transform nitrate to nitrite. The nitrite reacts with iron in the hemoglobin of red blood cells to form methemoglobin, which lacks the oxygen-carrying ability of hemoglobin. This creates the condition known as methemoglobinemia (sometimes referred to as “blue baby syndrome”), in which blood lacks the ability to carry sufficient oxygen to the individual body cells.
Infants under one year of age have the highest risk of developing methemoglobinemia. Contributing risk factors include digestive and enzyme systems that are not fully developed. Older persons who have a gastrointestinal system disorder resulting in increased bacteria growth may be at greater risk than the general population. In addition, individuals who have a genetically impaired enzyme system for metabolizing methemoglobin may be at greater risk. The general population has a low risk of developing methemoglobinemia, even when ingesting relatively high levels of nitrate/nitrite.
Historical information on infants with methemoglobinemia suggests that a number of infants with the condition also showed signs of diarrhea, inflammation and infection of the gastrointestinal track, or protein intolerance. The significance of these factors in regard to methemoglobinemia risk, if any, is not known.
The EPA has established the regulatory threshold for acute health effects based on best available science. The intake from food, drugs and other sources also is important and must be considered.
Although the EPA standard was set at 10 mg/L based on acute health effects, questions have been raised regarding possible chronic health effects from consuming water with nitrate at higher concentrations. Chronic health effects are those that can occur when a contaminant has been ingested over long periods of time. Research is limited regarding the possibility of chronic health effects due to long-term ingestion of drinking water with nitrate above the MCL. While it is recognized that research is limited, largely due to the complexity and cost of this type of research, some studies have shown a correlation between long-term ingestion of elevated nitrate and increased incidence of certain cancers, and increased birth defects. Uncertainty exists in nitrate risk assessment, and the connections between the level of nitrate in drinking water, volume ingested, duration of exposure, and possible chronic risks are not fully understood.
Testing for nitrate in drinking water
The quality of water supplied by Public Water Systems is regulated by the U.S. Environmental Protection Agency (EPA). The EPA set an enforceable standard of 10 parts per million for nitrate-nitrogen (ppm = mg/L). Information is available through annual water quality reports, which suppliers must provide, or by contacting your water supplier.
Testing a private water supply is most often optional, but may be justified. An initial test of a new water supply is recommended to determine the baseline nitrate concentration in the water source. Private drinking water should be tested annually to monitor changes in nitrate concentration. In addition, a water test is recommended for households with infants, pregnant women, nursing mothers, or elderly people. These groups are believed to be the most susceptible to nitrate health effects.
Tests to determine the presence of nitrate in drinking water should be done by a state certified laboratory approved for nitrate testing. To have water tested, select a laboratory and obtain a drinking water nitrate test kit from the laboratory. The sampling instructions provide information on how to collect the sample. Follow these instructions carefully to avoid contamination and to obtain a representative sample. While a variety of test kits and dip strips are available for nitrate testing outside of a laboratory environment, laboratory analysis is needed for an accurate and reliable nitrate measurement.
Users of private drinking water may consider the EPA guideline of 10 ppm nitrate-nitrogen in considering the risk associated with their water supply. If nitrate-nitrogen concentrations are found to be above 10 ppm, private drinking water users might voluntarily consider EPA guidelines, and try to reduce the nitrate-nitrogen concentration in the water, taking into account health risks, cost, and benefits.
It is important to note that some labs may report total nitrate (NO3–). Be sure to check your test report for which quantity, nitrate-nitrogen (NO3–-N) or total nitrate (NO3–), is reported. Use the following to compare the two reporting systems: 10 mg/L nitrate-nitrogen (NO3–-N) = 44.3 mg/L nitrate (NO3–). One of the test methods converts nitrate to nitrite and measures the total amount of nitrate and nitrite; in this case, results may be reported as nitrate + nitrite. In most well water, most of the nitrate + nitrite will have been nitrate originally.
Options for managing nitrate in drinking water
If a test indicates that the nitrate-nitrogen concentration of public water exceeds the standard, the public must be notified and steps must be taken to bring the water into compliance. If nitrate-nitrogen exceeds a desirable concentration in a private water supply, users might voluntarily consider an alternative drinking water source or water treatment. 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. Another alternate source of water is bottled water that can be purchased in stores or direct from bottling companies. This alternative especially might be considered if the primary concern is water for infant food and drinking.
Drinking water can be treated for nitrate-nitrogen by three treatment methods: distillation, reverse osmosis, and ion exchange. Home treatment equipment using these processes is available from several manufacturers. Carbon filters and standard water softeners do not remove nitrate-nitrogen. Merely boiling water will increase rather than decrease the nitrate-nitrogen concentration.