Nitrates are nitrogen-oxygen compounds that dissolve easily in water and show up in groundwater, wells, and sometimes public water supplies. They occur naturally at low levels, but human activity, especially agriculture, has pushed concentrations high enough to pose health risks in many regions. The U.S. EPA sets the legal limit for nitrate in drinking water at 10 mg/L (measured as nitrogen), and water above that threshold is unsafe to drink without treatment.
Where Nitrates in Water Come From
Nitrate is a stable form of nitrogen and exists naturally in soil and water at low concentrations. It’s also an essential plant nutrient, which is exactly why it becomes a pollution problem. The main sources of nitrate contamination in water are agricultural fertilizers, animal waste from feedlots and barnyards, septic system leachate, and domestic sewage. When rain or irrigation water moves through soil loaded with these materials, it carries dissolved nitrate down into groundwater.
Private wells are particularly vulnerable because they tap directly into groundwater and aren’t regulated the way public water systems are. Shallow wells near farmland or older septic systems face the highest risk. Nitrate is colorless, odorless, and tasteless, so contamination is invisible without testing.
Nitrate vs. Nitrite
Nitrate (NO₃) and nitrite (NO₂) are closely related but behave differently. Nitrate is the more stable form and is what you’ll typically find in water. Nitrite is unstable and converts to nitrate quickly when oxygen is present. However, bacteria can reverse this process, converting nitrate back into nitrite under the right conditions, such as inside stagnant pipes or in the low-oxygen environment of an infant’s stomach.
This conversion matters because nitrite is the form that actually causes health problems. Most water treatment and testing focuses on nitrate, since it’s the dominant form in water and readily converts to the more harmful nitrite inside the body. The EPA limit for nitrite is much lower than for nitrate: just 1 mg/L compared to 10 mg/L.
Health Risks for Infants
The most well-known danger of nitrate in drinking water is methemoglobinemia, commonly called blue baby syndrome. When an infant ingests nitrate-contaminated water (typically through formula mixed with well water), bacteria in the baby’s stomach convert nitrate into nitrite. That nitrite then reacts with hemoglobin, the protein in red blood cells that carries oxygen, transforming it into a form called methemoglobin that can no longer deliver oxygen to tissues. The baby’s skin can turn bluish, and severe cases can be fatal without treatment.
Infants under six months are the most vulnerable because their stomachs have lower acid levels, creating a better environment for the bacteria that perform this conversion. Their hemoglobin is also more susceptible to the chemical change. CDC data from the early 1990s documented over 500 nitrate exposures among children under six in a two-year period, with contaminated well water being the most common environmental cause.
Long-Term Risks for Adults
Adults can handle moderate nitrate exposure better than infants, but long-term consumption is not without concern. Emerging evidence links chronic nitrate exposure to thyroid problems, adverse pregnancy outcomes, and an increased risk of certain cancers, particularly colorectal cancer. The Minnesota Department of Health notes that some of these associations appear at concentrations below the current 10 mg/L regulatory standard, though the findings are still being evaluated.
Pregnant women face particular risk. The same nitrate-to-nitrite conversion that harms infants can affect fetal development, and some studies suggest links to complications like preterm birth and low birth weight.
What Nitrates Do to Lakes and Rivers
Nitrate pollution isn’t just a drinking water issue. When excess nitrogen from fertilizers and waste washes into lakes, rivers, and coastal waters, it fuels explosive growth of algae and aquatic plants. This process, called eutrophication, depletes dissolved oxygen in the water as the algae die and decompose, creating dead zones where fish and other aquatic life suffocate. Some algal blooms also produce toxins harmful to wildlife, pets, and people.
The scale of the problem is significant. Average nitrogen concentrations in lakes worldwide now exceed eutrophication thresholds by nearly double. Nitrate is one of the primary nutrients targeted in global efforts to control water eutrophication, alongside phosphorus.
Legal Limits and Guidelines
The EPA’s Maximum Contaminant Level for nitrate is 10 mg/L measured as nitrogen. Public water systems are required to test for nitrate and treat or notify customers if levels exceed this limit. The World Health Organization sets a similar guideline at 50 mg/L measured as nitrate ion, which works out to about 11 mg/L as nitrogen. Both standards were designed primarily to protect bottle-fed infants.
These limits apply to public water systems. If you have a private well, no agency is testing your water for you.
Testing Your Well Water
If you rely on a private well, testing is the only way to know whether nitrates are present. Health departments recommend testing for nitrate at least once per year. You should also test before becoming pregnant or before an infant will be using the water for formula or drinking.
Home test strips exist, but for reliable results, use an accredited laboratory. Your county health or environmental services department can point you to one and may even provide sample containers and instructions. Because nitrate levels can fluctuate with seasons, rainfall, and nearby land use, a single clean test doesn’t guarantee your water will stay safe indefinitely.
Removing Nitrates From Water
Boiling water does not remove nitrates. It actually makes the problem worse. As water evaporates, the nitrate stays behind, increasing its concentration in whatever liquid remains. Standard carbon filters, like those in most pitcher-style systems, also do not remove nitrate.
The treatment methods that do work include reverse osmosis, ion exchange systems, and distillation. Reverse osmosis units designed for under-sink installation are the most common household option and can reduce nitrate levels by 80% or more. Ion exchange systems work similarly to water softeners, swapping nitrate ions for chloride. If your well consistently tests above 10 mg/L, switching to bottled water for drinking and cooking (especially for infants and pregnant women) is the simplest immediate step while you arrange a longer-term solution.