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 a serious risk to infants under 6 months, who can develop methemoglobinemia (blue baby syndrome). The EPA MCL is 10 mg/L as nitrogen.
Quick Answer
Nitrate (NO₃⁻) is a nitrogen-containing compound that forms naturally through the decomposition of organic matter. At elevated concentrations — almost always from human activity — nitrate interferes with the blood's ability to carry oxygen. The United States produces over 23 million tons of nitrogen fertilizer annually, making agricultural runoff the dominant source of nitrate contamination.
Why Do People Care?
Nitrate is the most widespread agricultural contaminant in U.S. groundwater. The USGS estimates that 4% of private wells exceed the EPA limit, with much higher rates in agricultural regions of the Midwest, California's Central Valley, and the Southeast.
Infants under 6 months are at critical risk because their digestive systems convert nitrate to nitrite more readily. Pregnant women, the elderly, and people with certain enzyme deficiencies are also more vulnerable. Adults with normal health can tolerate the EPA limit without acute effects, though long-term exposure research is ongoing.
Known Health Effects
Methemoglobinemia ('blue baby syndrome') in infants — potentially fatal
Reduced oxygen-carrying capacity of blood
Potential increased cancer risk with long-term exposure (IARC Group 2A)
Adverse reproductive outcomes at high levels during pregnancy
Thyroid disruption with chronic high-level exposure
Common Sources
Agricultural fertilizer runoff — the dominant source in rural areas
Livestock operations and feedlot runoff
Leaking or improperly sited septic systems
Sewage treatment plant effluent
Natural soil nitrogen mineralization
Urban lawn fertilizer and storm 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. Some health researchers argue the standard should be lower given emerging evidence of cancer risk at sub-MCL levels.
How to Test for It
Nitrate is readily detectable through standard water quality tests available from certified labs. Many utilities test regularly and report results in Consumer Confidence Reports. Home test strips can indicate elevated nitrate but are less accurate than lab analysis. Public water systems are required to notify customers if levels exceed the MCL.
Effective Treatment Options
These treatment methods have demonstrated effectiveness for Nitrates.
Frequently Asked Questions
Related Pages
Data Sources & Provenance
All data on this page is sourced from official U.S. government or public datasets.