Nitrate in well water
Nitrate soaks into wells from fertilizer, manure and septic systems. The EPA limit is 10 mg/L measured as nitrogen. Babies under 6 months are the ones at real risk, so test every year and before a new baby comes home. Boiling makes nitrate stronger. A reverse osmosis filter at the kitchen sink removes most of it.
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What you need to know about nitrate
| The limit | 10 mg/L measured as nitrogen (mg/L is milligrams per liter, the same as parts per million). If your lab reports the whole nitrate ion instead, the same limit is 45 mg/L. |
|---|---|
| Who it hurts | Babies under 6 months who drink formula made with the water. Pregnant women should avoid water over the limit too. |
| Where it comes from | Fertilizer, manure, feedlots and septic systems. Shallow wells near farm fields tend to have the highest levels. |
| How often to test | Every year, and again before a new baby comes home. |
| What removes it | Reverse osmosis at the kitchen sink, anion exchange for the whole house, or a countertop distiller. |
| What doesn't | Boiling, which concentrates it. Carbon filters and water softeners aren't built to remove nitrate. |
These counties see the most nitrate in their wells
Ranked by the share of tested wells with a result over the limit. We only list counties with at least 25 tested wells, so a few bad results can't skew the ranking.
| County | Wells tested | Wells over the limit |
|---|---|---|
| Malheur County, OR | 49 | |
| Iowa County, WI | 49 | |
| Adams County, WI | 114 | |
| Whatcom County, WA | 104 | |
| Stearns County, MN | 56 | |
| Rock County, WI | 99 | |
| Kent County, MD | 47 | |
| Dunn County, WI | 77 | |
| Waushara County, WI | 113 | |
| Lancaster County, PA | 90 |
* Likely overstated, because of who tested these wells, when or where.
See the full rankings and download the data
Nitrate soaks down from fields, feedlots and septic systems
Nitrate is a form of nitrogen. A little of it occurs naturally, and the Minnesota Department of Health says natural levels in water are usually under 3 mg/L. Higher levels come from fertilizer, manure, feedlots, septic systems and wastewater soaking down through the soil.
Farm country has the most. In a national USGS study, about 4% of wells in drinking-water aquifers were over the EPA limit. Under farmland, 22% of shallow groundwater samples were over it.
More than 1 in 3 wells in that study had over 1 mg/L, which USGS says is a sign that people's activities are reaching the water.
Some wells are easier for nitrate to reach. Minnesota lists shallow wells, dug wells, wells with cracked or leaking casings, and wells in sand, fractured rock or karst (limestone full of cracks and channels). Older wells near septic systems or fields are at higher risk.
Babies under 6 months are the ones in real danger
The EPA says babies under 6 months who drink water over the limit could become seriously ill, and without treatment could die. The signs are shortness of breath and "blue baby syndrome."
Here's what happens. In a young baby's stomach, nitrate turns into nitrite, which keeps the blood from carrying oxygen. Doctors call it methemoglobinemia. Penn State Extension says the clearest sign is bluish skin, especially around the eyes and mouth. Caught early, it's readily diagnosed and reversed with treatment.
If a baby looks blue or gray around the mouth or is struggling to breathe, get medical care right away.
Pregnant women should avoid water over the limit too. Penn State notes that low birth weights have been linked to high nitrate. The Minnesota Department of Health says newer studies tie long-term nitrate below the limit to thyroid problems, pregnancy problems and some cancers, mainly colorectal, and that more research is needed to confirm them.
For healthy adults, Penn State cites the World Health Organization and the National Academy of Sciences: nitrate in drinking water isn't considered a significant risk.
Test every year, and before a baby comes home
The CDC says to test a private well every year for nitrate, total coliform bacteria, pH and total dissolved solids. Minnesota adds one more: test before you bring a new baby home.
A result over 1 mg/L means nitrate from people is probably reaching your well. Minnesota suggests more testing at that point, since pesticides can travel the same path.
Check the units on your report. Most labs report nitrate "as N" (nitrogen), where the limit is 10 mg/L. Some report the whole nitrate ion, where the same limit is 45 mg/L.
Some states pay for the test. Iowa's Private Well Grants program, for example, tests private wells for bacteria, nitrate, arsenic and manganese for free.
Reverse osmosis at the kitchen sink is the usual fix
If your result is over 10 mg/L, switch to bottled water for any baby's formula right away. Penn State calls bottled water an effective, low-cost option for infants and pregnant women.
Then look for the source. Have a licensed well contractor check the casing and cap, and look at what's near the well: septic system, manure, fertilized lawn or fields. For treatment, nitrate only matters in water you drink or cook with, so most people treat 1 tap.
| Option | What it treats | Typical equipment cost | Things to know |
|---|---|---|---|
| Reverse osmosis | 1 tap, usually the kitchen sink | $300 to $2,000 | Removes about 83% to 92% of nitrate, so very high levels can still come out over 10 mg/L |
| Anion exchange | The whole house | $500 to $1,500 | Raises chloride and can make water more corrosive |
| Distiller | 1 countertop unit | $300 to $2,000 | Slow, about 1 gallon every 4 to 5 hours |
Costs are equipment ranges from Penn State Extension. Installation and filter changes add to them. University of Georgia Extension says carbon filters and standard water softeners don't remove nitrate.
Minnesota recommends that babies under 6 months keep drinking bottled water even after you install treatment, in case the system stops working.
Questions people ask about nitrate
Can I boil the water to get rid of nitrate?
No. Boiling evaporates some of the water and leaves the nitrate behind, so the level goes up. Penn State and the Minnesota Department of Health both warn against it.
Can I breastfeed if our well is high in nitrate?
Penn State Extension says nursing mothers don't pass nitrite to babies through breast milk. The concern is formula or water given straight to the baby.
My report says 30 mg/L nitrate. Is that over the limit?
It depends on the units. If it says "nitrate as N" or "NO3-N," 30 is 3 times the limit. If it's the whole nitrate ion, 30 mg/L works out to about 6.8 mg/L as nitrogen, which is under 10. Call the lab if the report doesn't say.
Would a deeper well have less nitrate?
Often. USGS found nitrate much higher in shallow groundwater under farmland than in the deeper water most wells draw from. Shallow water can move down over time, though, so a deep well still needs a yearly test.
Where this comes from
- U.S. EPA, National Primary Drinking Water Regulations
- CDC, Guidelines for Testing Well Water
- Penn State Extension, Nitrates in Drinking Water
- Penn State Extension, Tips for Buying Water Treatment Equipment
- Minnesota Department of Health, Nitrate in Drinking Water
- USGS Circular 1360, Water Quality in Principal Aquifers of the United States, 1991 to 2010
- University of Georgia Extension, Nitrate and Nitrite in Water
- Iowa DNR, Private Well Testing