Nitrate in Water: Health Effects, Limits & Treatment
What Is Nitrate and Where Does It Come From?
Nitrate (NO3-) is a highly soluble inorganic compound composed of nitrogen and oxygen. It is one of the most widespread groundwater contaminants in the world, primarily associated with agricultural activity. Nitrate moves readily through soil into groundwater because it carries a negative charge and is not retained by soil particles.
The dominant source of nitrate contamination is agricultural runoff, including synthetic nitrogen fertilizers, animal manure from concentrated animal feeding operations (CAFOs), and irrigation return flows. In the United States, the USGS estimates that agricultural sources account for the majority of nitrate in groundwater, particularly in regions with intensive row-crop farming.
Other significant sources include septic system leachate, municipal wastewater discharge, and natural decomposition of organic nitrogen in soil. Urban stormwater runoff carrying lawn fertilizers also contributes to nitrate loading in surface water bodies.
Nitrate contamination is most severe in shallow wells in agricultural areas, particularly in regions with sandy or permeable soils that allow rapid infiltration. Once in groundwater, nitrate can persist for decades because subsurface conditions often lack the organic carbon and anaerobic environment needed for natural denitrification.
Health Effects of Nitrate in Water
The most well-documented health effect of nitrate in drinking water is methemoglobinemia, commonly known as blue baby syndrome. This condition occurs when nitrate is converted to nitrite in the gastrointestinal tract, and the nitrite then oxidizes hemoglobin to methemoglobin, which cannot transport oxygen effectively.
Infants under six months of age are at the greatest risk because their stomachs have a higher pH that promotes the bacterial conversion of nitrate to nitrite, and fetal hemoglobin is more susceptible to oxidation. Symptoms include bluish discoloration of the skin (cyanosis), shortness of breath, and in severe cases, death.
Emerging research has suggested potential associations between chronic nitrate exposure and other health effects, including certain cancers (colorectal, bladder, ovarian, and thyroid), thyroid disease, and adverse reproductive outcomes. The endogenous formation of N-nitroso compounds from ingested nitrate is the hypothesized mechanism for the cancer association.
Pregnant women may also be at elevated risk, as some studies have reported associations between nitrate exposure and neural tube defects, preterm birth, and intrauterine growth restriction, though these findings are not yet conclusive.
Regulatory Limits for Nitrate in Drinking Water
| Regulatory Body | Standard | Limit (mg/L as NO3-N) |
|---|---|---|
| World Health Organization (WHO) | Guideline Value | 11.3 (50 mg/L as NO3-) |
| U.S. EPA | Maximum Contaminant Level (MCL) | 10 |
| U.S. EPA | MCL for Nitrite | 1 |
| European Union | Drinking Water Directive | 11.3 (50 mg/L as NO3-) |
The EPA MCL of 10 mg/L (as nitrogen) was established to protect against methemoglobinemia in infants. Note that nitrate results can be reported as nitrate-nitrogen (NO3-N) or as nitrate (NO3-); the conversion factor is 4.43 (multiply NO3-N by 4.43 to get NO3-).
How to Test for Nitrate in Water
Nitrate is colorless, odorless, and tasteless in water, so testing is the only way to determine whether it is present. Laboratory analysis using EPA Method 300.0 (ion chromatography) or EPA Method 353.2 (automated colorimetric reduction) provides accurate quantification.
Samples for nitrate analysis should be collected in clean polyethylene containers and kept cold (below 6 degrees Celsius) during transport. Analysis should be completed within 48 hours of collection, or the sample should be preserved with sulfuric acid to a pH below 2.
For private wells in agricultural areas, the EPA recommends testing for nitrate at least annually, and more frequently if previous tests have shown concentrations approaching the MCL. Testing should be conducted in spring or after periods of heavy rain or irrigation when nitrate leaching is most active.
Treatment Methods for Nitrate Removal
Reverse Osmosis
Reverse osmosis systems effectively remove nitrate from water, typically achieving 85% to 95% rejection. RO is widely used for both point-of-use and commercial applications where nitrate reduction is required. System design should account for the concentrate stream, which contains elevated nitrate levels and requires proper disposal.
Ion Exchange
Nitrate-selective strong base anion exchange resins are specifically designed for nitrate removal and are the most common technology used in municipal nitrate treatment plants. Standard anion exchange resins preferentially remove sulfate over nitrate, which can lead to nitrate dumping (sudden release of accumulated nitrate) when the resin becomes exhausted. Nitrate-selective resins avoid this problem by preferentially removing nitrate.
Biological Denitrification
Biological treatment uses denitrifying bacteria to convert nitrate to nitrogen gas under anoxic conditions with an organic carbon source. This technology is used in municipal and industrial applications and produces no brine waste stream, making it environmentally favorable for large-scale treatment.
Electrodialysis
Electrodialysis (ED) and electrodialysis reversal (EDR) use ion-selective membranes and an electrical potential to separate nitrate and other ions from water. This technology is used in municipal applications and offers advantages in water recovery compared to RO.
Frequently Asked Questions
What is blue baby syndrome and how is it related to nitrate?
Blue baby syndrome (methemoglobinemia) occurs when nitrate in drinking water is converted to nitrite in an infant's digestive system. Nitrite interferes with the blood's ability to carry oxygen, causing the skin to turn blue. Infants under six months are most vulnerable because their stomachs have lower acidity, which promotes the bacterial conversion of nitrate to nitrite. This is why the EPA MCL for nitrate is set at 10 mg/L as nitrogen.
Can you boil water to remove nitrate?
No. Boiling water does not remove nitrate. In fact, boiling concentrates nitrate by reducing water volume through evaporation while the dissolved nitrate remains. Effective nitrate removal requires reverse osmosis, ion exchange, or biological denitrification.
What causes high nitrate levels in well water?
High nitrate in well water is most commonly caused by agricultural activities, including fertilizer application and animal waste from concentrated feeding operations. Other sources include septic system leachate, wastewater treatment discharge, and natural decomposition of organic matter. Shallow wells in agricultural areas are at the highest risk.
Need to Remove Nitrate from Your Water?
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