What is nitrate?
Nitrate (NO₃⁻) is a naturally occurring ion that becomes a groundwater concern when human activity increases its concentration in soil. It is colorless, odorless, and tasteless. Unlike many contaminants, nitrate does not build up in the body — it affects health by interfering with oxygen transport in blood.
How does it get into well water?
Nitrate moves readily through soil into shallow, unconfined aquifers. Primary sources:
- Agricultural fertilizers (corn and soybean fields across the Midwest)
- Concentrated animal feeding operations and manure application
- Septic system leachate — a significant source in suburban and rural areas
- Lawn fertilizers in high-density residential areas
Wells at highest risk are shallow (under 100 ft), located in agricultural watersheds, or near septic systems. Deep bedrock wells are generally lower risk unless there is a direct pathway from the surface.
How common is it?
Nitrate above 10 mg/L is the most frequently detected MCL exceedance in U.S. private wells. It is most prevalent in Iowa, Illinois, Indiana, Nebraska, Kansas, Minnesota, Wisconsin, Ohio, Missouri, and Pennsylvania — the heart of the Corn Belt. USGS surveys suggest roughly 4% of domestic wells nationwide exceed the MCL.
Health effects
Infants under 6 months: methemoglobinemia (blue baby syndrome)
Do not use water above 10 mg/L nitrate (as nitrogen) for infant formula. Infants cannot adequately reduce methemoglobin; nitrate converts hemoglobin to methemoglobin, which cannot carry oxygen. Cases have been fatal. This risk is acute — a single high-nitrate feeding can cause serious harm.
Chronic adult exposure: colorectal cancer
IARC classifies nitrate in drinking water as a Group 2A probable carcinogen when ingested under conditions promoting endogenous nitrosation. The Iowa Women's Health Study and Danish cohort studies show elevated colorectal cancer risk above 5 mg/L with frequent red meat consumption.
Thyroid disruption
Nitrate competitively inhibits iodide uptake in the thyroid gland. Epidemiological studies associate elevated nitrate exposure with hypothyroidism, particularly in iodine-deficient populations.
The EPA limit: MCL = MCLG = 10 mg/L
Nitrate is one of the few health contaminants where the MCL and MCLG are equal — both set at 10 mg/L (as nitrogen). This is unusual: for most carcinogens the MCLG is zero. EPA concluded 10 mg/L is fully protective against methemoglobinemia. The cancer concern from chronic exposure at lower levels was not fully incorporated into the MCL.
Note: results may be reported as nitrate-nitrogen (NO₃-N) or as total nitrate (NO₃). The MCL of 10 mg/L uses the nitrogen form. If your lab reports total nitrate (not as N), divide by 4.4 to convert.
Testing
Test by colorimetric analysis (EPA Method 353.2) at a certified laboratory. Nitrate test strips can screen for gross contamination but are not accurate enough for health decisions near the 10 mg/L limit. Test annually if you are in an agricultural area, and always after flooding or land use changes nearby.
Find a certified lab and learn how to collect a sample
Treatment
- Reverse osmosis (RO) — most practical for point-of-use; removes 85–95% of nitrate. Install at the kitchen tap for drinking and cooking water.
- Ion exchange (nitrate-selective anion resin) — whole-house treatment; specifically designed resins required (standard softener resin is not nitrate-selective).
- Distillation — effective but slow and energy-intensive.
Note: activated carbon filters and standard water softeners do not remove nitrate.