Nitrates in Drinking Water
Nitrates are colorless, odorless compounds that occur naturally in soil but reach dangerous levels in water primarily from agricultural fertilizer runoff and septic system leakage. They pose an acute, potentially fatal risk to infants under 6 months and are increasingly linked to cancer risk even at levels below the EPA's 10 mg/L limit. An estimated 62 million Americans drink water with nitrate levels above 3 mg/L.
Quick Reference · Nitrates
Quick Answer
Nitrate (NO₃⁻) is a nitrogen-containing compound that forms naturally through the decomposition of organic matter. At elevated concentrations — almost always caused by human activity — nitrate is converted in the digestive system to nitrite, which then reacts with hemoglobin to form methemoglobin, a form of hemoglobin that cannot carry oxygen. In the body, nitrite also reacts with amines in food to form N-nitroso compounds (nitrosamines) — known carcinogens classified by the IARC as Group 2A (probable human carcinogens). The United States applies over 23 million tons of nitrogen fertilizer annually, making agricultural runoff the dominant source of nitrate contamination in U.S. groundwater.
Why Is Nitrates in Drinking Water a Concern?
Nitrate is the single most widespread agricultural contaminant in U.S. groundwater. The Environmental Working Group found that approximately 62 million Americans are served drinking water with nitrate above 3 mg/L — a level associated with increased colorectal cancer risk in a 2021 study published in the International Journal of Epidemiology. The current EPA limit of 10 mg/L was set in 1991 specifically to prevent infant methemoglobinemia, not to address cancer risk. Emerging research suggests cancer risk may begin well below the legal limit. The USGS estimates 4% of private wells exceed the EPA limit nationally, with rates exceeding 20% in high-intensity agricultural counties.
Infants under 6 months face the most acute risk — their digestive systems have higher bacterial activity that converts more nitrate to nitrite, and fetal hemoglobin is more susceptible to methemoglobin formation. Pregnant women are advised to limit exposure because nitrate may affect fetal oxygenation and is associated with adverse birth outcomes at higher levels. People with hereditary methemoglobin reductase deficiency cannot reduce methemoglobin effectively. Rural residents relying on private wells in the Midwest Corn Belt (Iowa, Nebraska, Kansas, Indiana, Illinois), California's Central Valley (Tulare, Fresno, Merced counties), and the Mid-Atlantic are at the highest risk of exceeding the EPA limit.
Health Effects of Nitrates in Drinking Water
Methemoglobinemia ('blue baby syndrome') in infants under 6 months — potentially fatal without emergency treatment
Reduced blood oxygen-carrying capacity — causes bluish skin discoloration, rapid heart rate, and in severe cases death
Colorectal cancer: a 2021 epidemiological study found 17% higher risk at 5–9.9 mg/L nitrate exposure
Bladder and kidney cancer association with long-term exposure above 5 mg/L (IARC Group 2A)
Adverse birth outcomes: preterm birth and neural tube defects associated with elevated exposure during pregnancy
Thyroid disruption: nitrate competes with iodide uptake, potentially impairing thyroid function with chronic exposure
How Does Nitrates Get Into Drinking Water?
Agricultural fertilizer runoff — the dominant source, responsible for over 70% of nitrate in U.S. groundwater
Livestock operations and concentrated animal feeding operations (CAFOs)
Improperly sited or failing septic systems — a primary source in suburban well areas
Sewage treatment plant effluent discharged to surface water
Natural soil nitrogen mineralization in certain geologies
Urban lawn fertilizer and golf course runoff
Regulatory Limit
EPA Maximum Contaminant Level (MCL)
10 mg/L
The EPA Maximum Contaminant Level (MCL) for nitrate is 10 milligrams per liter (mg/L) measured as nitrogen — equivalent to 10 parts per million (ppm). A separate limit of 1 mg/L applies to nitrite. The MCL was set in 1991 primarily to protect infants against methemoglobinemia, not based on cancer risk data. The EPA has not revised the nitrate MCL since 1991 despite growing evidence that cancer risk may begin well below 10 mg/L. The Environmental Working Group and several independent researchers advocate for a lower limit of 3–5 mg/L. California has set a public health goal of 45 mg/L for nitrate as NO₃ (equivalent to 10 mg/L as nitrogen) but has not lowered its regulatory limit.
How to Test for Nitrates in Your Water
Nitrate is readily detectable through standard certified laboratory water tests ($15–$40 for a basic nitrate panel). Home test strips can flag elevated nitrate but are not accurate enough to determine whether levels are near the 10 mg/L limit — use a lab for meaningful results. Public water systems test regularly and report results in annual Consumer Confidence Reports (CCRs). Private well owners are not required to test and receive no automatic notification — self-initiated annual testing is the only way to monitor a well. If you're on a public system, check your utility's most recent CCR for detected nitrate levels.
How to Remove Nitrates from Drinking Water
These treatment methods have demonstrated effectiveness for Nitrates removal.
Utilities With Nitrates Violations(8+ in database)
These utilities have reported Nitrates-related violations in EPA SDWIS. Ordered by population served.
Chicago
IL · 2.7M served
San Antonio Water System
TX · 2.1M served
Philadelphia Water Department
PA · 1.6M served
Charlotte Water
NC · 1.2M served
Atlanta
GA · 1.1M served
Gwinnett Co. Dept. of Water Resources
GA · 975K served
Louisville Water Company
KY · 765K served
Dekalb County
GA · 743K served
Frequently Asked Questions
From Nitrates to Action
Find a certified lab to test for Nitrates
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Reverse osmosis — removes Nitrates
NSF certification details, cost range, and maintenance notes
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Nitrates in California
State-specific violations, utilities, and testing guidance
Nitrates in Texas
State-specific violations, utilities, and testing guidance
Nitrates in Florida
State-specific violations, utilities, and testing guidance
Nitrates in Arizona
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Data sources and methodology
How EPA compliance and UCMR 5 monitoring data is sourced and validated
Questions About Nitrates in Drinking Water
How do I test my water for Nitrates?
Find state-certified labs — confirm EPA Method 533/537.1 for PFAS
Which utilities have Nitrates violations?
Search by ZIP code or utility name for official EPA compliance records
What removes Nitrates from tap water?
Reverse Osmosis Filtration — NSF certification details, cost, and maintenance
How is Nitrates data sourced on this site?
EPA SDWIS, UCMR 5, and CCR data sources — update cadence and accuracy notes
Related Pages
Data Sources & Provenance
All data on this page is sourced from official U.S. government or public datasets.
Quick Reference
Category
Agricultural Chemicals
Risk Level
moderate
EPA Limit
10 mg/L
Most at Risk
Infants under 6 months face the most acute risk — their digestive systems have higher bacterial activity that converts more nitrate to nitrite, and fetal hemoglobin is more susceptible to methemoglobin formation.
Well Water Relevant
Yes
Nitrates by State
Treatment Options