Nitrate in Drinking Water: Risks, Sources, and Solutions

What Is Nitrate and Why Does It Matter

Nitrate (NO3) is one of the most common groundwater contaminants in the United States. It enters water supplies primarily through agricultural runoff, and at elevated levels it poses a serious health risk, especially to infants. The EPA Maximum Contaminant Level (MCL) for nitrate is 10 mg/L (measured as nitrogen). This is a legally enforceable limit for public water systems.

Check your city\'s water to see the nitrate level in your water supply.

Why Infants Are at Greatest Risk

Nitrate itself is not directly toxic. The danger comes when bacteria in the digestive system convert nitrate to nitrite (NO2). Nitrite binds to hemoglobin in blood, reducing its ability to carry oxygen. This condition is called methemoglobinemia, commonly known as blue baby syndrome.

Infants under 6 months are most vulnerable because:

• Their stomachs have lower acidity, allowing more nitrate-converting bacteria to thrive
• Their hemoglobin (fetal hemoglobin) is more easily oxidized by nitrite
• They consume large amounts of water relative to body weight, especially when formula is mixed with tap water

Symptoms include bluish skin color (especially around the lips, hands, and feet), difficulty breathing, lethargy, and in severe cases, loss of consciousness. Methemoglobinemia is a medical emergency.

Other at-risk groups

Pregnant women should also limit nitrate exposure. Some studies link high nitrate intake during pregnancy to increased risk of certain birth defects and low birth weight. Adults with certain enzyme deficiencies may also be more susceptible to methemoglobin formation.

Where Nitrate Comes From

Nitrate enters water through several pathways, but agricultural activity is the dominant source in most of the US.

Agricultural sources

• Fertilizer runoff: Nitrogen-based fertilizers applied to crops are the largest contributor. Rain and irrigation wash excess nitrogen into streams, rivers, and groundwater
• Animal waste: Concentrated animal feeding operations (CAFOs) produce massive volumes of manure. Nitrogen from manure seeps into groundwater and runs off into surface water
• Irrigation return flows: Water that drains from irrigated fields carries dissolved nitrate back into waterways

Other sources

• Septic systems: Failing or improperly maintained septic systems leach nitrogen into groundwater. This is a common issue for private well owners
• Wastewater discharge: Municipal wastewater treatment plants release nitrogen into rivers and streams
• Natural deposits: Some geological formations naturally contain nitrogen-bearing minerals, though this is a minor source compared to agriculture

US Regions with the Highest Nitrate Levels

Nitrate contamination correlates strongly with intensive agriculture. The areas of highest concern:

Region	States Most Affected	Primary Cause
Central Valley, California	CA	Intensive crop irrigation, dairy operations
Corn Belt	IA, IL, IN, OH, MN	Corn and soybean fertilization
Great Plains	NE, KS	Irrigated agriculture over the Ogallala Aquifer
Delmarva Peninsula	DE, MD, VA	Poultry operations
Central Wisconsin	WI	Dairy farming, sandy soils allow fast infiltration

The USGS estimates that about 20% of private wells in agricultural areas exceed the EPA limit of 10 mg/L. Public water systems are required to treat and monitor, but private wells are unregulated.

How Nitrate Differs from Nitrite

The EPA regulates both nitrate (NO3) and nitrite (NO2) separately:

Contaminant	EPA MCL	Measured As
Nitrate	10 mg/L	Nitrogen (N)
Nitrite	1 mg/L	Nitrogen (N)
Combined (nitrate + nitrite)	10 mg/L	Nitrogen (N)

When reading water reports, make sure you know whether the result is reported as nitrogen (N) or as nitrate (NO3). The conversion: 10 mg/L as N = 44.3 mg/L as NO3. The EPA MCL of 10 mg/L is expressed as nitrogen. Some labs report the higher number (as NO3), which can cause confusion.

Testing Your Water

Public water systems test for nitrate at least annually and report results in the annual Consumer Confidence Report. If you are on a private well, you are responsible for your own testing.

• Lab testing: Send a sample to a state-certified lab. Costs $15-30 for nitrate analysis
• Home test strips: Nitrate strips provide a rough estimate but are not precise enough for regulatory comparison. Use them for screening only
• When to test: Test wells in spring and after heavy rain when runoff is highest. Test annually at minimum if you are in an agricultural area

For more on testing methodology, see our water testing guide.

Removal Methods

Nitrate is one of the more difficult contaminants to remove from drinking water. Standard carbon filters do NOT remove nitrate. Boiling makes it worse by concentrating the nitrate as water evaporates.

Effective technologies

Technology	Nitrate Removal	Cost Range	Notes
Reverse osmosis (under-sink)	85-95%	$150-400	Most practical for home use; treats drinking/cooking water
Ion exchange (nitrate-selective resin)	90-98%	$500-2,000	Whole-house or point-of-entry; uses specialty resin, not standard softener resin
Distillation	95%+	$100-500	Effective but slow; produces 1-5 gallons per day

An under-sink RO system is the most common residential solution. It handles nitrate along with many other contaminants in a single system.

What to Do If You Have an Infant and High Nitrate

If your water exceeds 10 mg/L nitrate (as nitrogen) and you have an infant under 6 months:

• Use bottled water for formula preparation immediately. Do not wait for filtration installation
• Do not boil the tap water; boiling concentrates nitrate
• Install an RO system or nitrate-selective ion exchange filter as soon as possible
• Continue using bottled or filtered water until you can verify the filter is reducing nitrate below 10 mg/L
• Breastfeeding is protective; nitrate does not transfer significantly through breast milk

Long-Term Health Concerns for Adults

While methemoglobinemia in adults is rare, long-term nitrate exposure at levels above the MCL is associated with increased risk of colorectal cancer according to some epidemiological studies. The WHO has classified ingested nitrate (under conditions that lead to nitrosation) as probably carcinogenic to humans (Group 2A). Reducing exposure is prudent for everyone, not just infants.

Check your city\'s water for nitrate levels and other contaminants. If you are on a private well in an agricultural area, make testing a priority. For related contaminants that affect well water, see our guide on arsenic in well water.