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Nitrate in Montana well water, by county

By the Tested Water teamLab data from the USGS and EPA Water Quality Portal, 2000 to 2023Updated November 8, 2026

Nitrate is widespread, over the limit in 15% or more of tested wells, in 3 of the 15 Montana counties with 10 or more tested wells. 1 more look high, but those results likely overstate it. A county's share tells you how likely a problem is nearby. Only a test of your own well tells you what's in it.

Find a nitrate test See every county

How Montana compares with the rest of the country

Across Montana, about 1 in 13 tested wells had nitrate over the EPA limit of 10 mg/L (44 of 588). Across all tested wells in the US it's about 1 in 12. Montana is about average, so the county you live in tells you more than the state does.

Why it matters: It's dangerous for babies under 6 months, because it keeps their blood from carrying enough oxygen.

Nitrate soaks down into groundwater from fertilizer, manure and septic systems.

Nitrate in tested wells, county by county

Share of each county's tested wells with at least one result over the EPA limit of 10 mg/L. Counties need 10 or more tested wells to be listed. Widespread means 15% or more, worth checking 5 to 15%, rare under 5%.

CountyWells testedWells over the limitLevel
Valley County 53
32% 17 of 53
 Widespread
Cascade County 11
18%* 2 of 11
 Widespread
Big Horn County 12
17% 2 of 12
 Widespread
Roosevelt County 68
16% 11 of 68
 Widespread
Lewis and Clark County 24
12% 3 of 24
 Worth checking
Yellowstone County 12
8% 1 of 12
 Worth checking
Glacier County 31
3% 1 of 31
 Rare
Jefferson County 67
1% 1 of 67
 Rare
Rosebud County 98
1% 1 of 98
 Rare
Powell County 63
0% 0 of 63
 Rare
Gallatin County 36
0% 0 of 36
 Rare
Ravalli County 21
0% 0 of 21
 Rare
Carbon County 14
0% 0 of 14
 Rare
Richland County 14
0% 0 of 14
 Rare
Silver Bow County 13
0% 0 of 13
 Rare

* Likely overstated. These shares come from a narrow set of wells, so they probably run high for the county as a whole:

  • Cascade County: All of these tests come from a single agency in one small part of the county, so it may not match the rest of the county.

13 more Montana counties have 1 to 9 tested wells for nitrate, too few to rate.

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Where to get your well tested for nitrate in Montana

1

Free testing, with limits

Montana's free well testing is only for childcare facilities. Check which tests it includes before you count on it for nitrate.

Montana's private well page
3

A mail-in kit

Fill the bottles at your kitchen tap and mail them back. We mark the kits that include nitrate.

Compare kits

Keep reading about wells in Montana

Questions people ask about nitrate in Montana

Is there nitrate in Montana well water?

Nitrate is widespread, over the limit in 15% or more of tested wells, in 3 of the 15 Montana counties with 10 or more tested wells. 1 more look high, but those results likely overstate it. Across Montana, about 1 in 13 tested wells had nitrate over the EPA limit of 10 mg/L (44 of 588). Across all tested wells in the US it's about 1 in 12. Montana is about average, so the county you live in tells you more than the state does.

Where can I get my well tested for nitrate in Montana?

Montana's free well testing is only for childcare facilities. If that doesn't cover you, use a lab certified by Montana for drinking water, or a mail-in kit that includes nitrate from a certified lab.

See Montana's list of certified labs, or compare mail-in kits.

Where these numbers come from

  • Lab tests: Water Quality Portal (USGS and EPA), well samples since 2000. A well counts as over if any of its tests was over the EPA limit of 10 mg/L. Wells tested for studies and monitoring, not a random sample of homes.
  • Likely overstated: our own check of who tested the wells, when and where. How we check.
  • State programs and links: Department of Environmental Quality.