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Arsenic in Montana well water, by county

By the Tested Water teamLab data from the USGS and EPA Water Quality Portal, 2000 to 2025Updated November 8, 2026

Arsenic is widespread, over the limit in 15% or more of tested wells, in 1 of the 17 Montana counties with 10 or more tested wells. 6 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 an arsenic test See every county

How Montana compares with the rest of the country

Across Montana, about 2 in 10 tested wells had arsenic over the EPA limit of 10 µg/L (131 of 693). Across all tested wells in the US it's about 1 in 9. So arsenic turns up somewhat more often in Montana than in the country as a whole. Check your county below.

Why it matters: Drinking it for years raises the risk of skin, bladder and lung cancer.

Arsenic dissolves out of certain rocks and old sediments, so it can be high even where nobody has polluted anything.

Arsenic in tested wells, county by county

Share of each county's tested wells with at least one result over the EPA limit of 10 µg/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
Jefferson County 21
62%* 13 of 21
 Widespread
Roosevelt County 32
59%* 19 of 32
 Widespread
Gallatin County 25
52%* 13 of 25
 Widespread
Silver Bow County 28
46% 13 of 28
 Widespread
Powell County 53
43%* 23 of 53
 Widespread
Powder River County 73
19%* 14 of 73
 Widespread
Cascade County 11
18%* 2 of 11
 Widespread
Lewis and Clark County 86
14% 12 of 86
 Worth checking
Richland County 16
12% 2 of 16
 Worth checking
Yellowstone County 12
8% 1 of 12
 Worth checking
Rosebud County 191
6%* 11 of 191
 Worth checking
Glacier County 31
3% 1 of 31
 Rare
Flathead County 18
0% 0 of 18
 Rare
Ravalli County 18
0% 0 of 18
 Rare
Carbon County 14
0% 0 of 14
 Rare
Valley County 13
0% 0 of 13
 Rare
Big Horn County 12
0% 0 of 12
 Rare

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

  • Jefferson County: All of these tests come from the US Geological Survey in one small part of the county, so this likely overstates it.
  • Roosevelt County: All of these tests come from the US Geological Survey in one small part of the county, so this likely overstates it.
  • Gallatin County: Almost all of these tests come from a single agency in one small part of the county, so this likely overstates it.
  • Powell County: All of these tests come from a single agency between 2000 and 2000 in one small part of the county, so this likely overstates it.
  • Powder River 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.
  • 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.
  • Rosebud County: Almost 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.

11 more Montana counties have 1 to 9 tested wells for arsenic, too few to rate.

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Where to get your well tested for arsenic 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 arsenic.

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 arsenic.

Compare kits

Keep reading about wells in Montana

Questions people ask about arsenic in Montana

Is there arsenic in Montana well water?

Arsenic is widespread, over the limit in 15% or more of tested wells, in 1 of the 17 Montana counties with 10 or more tested wells. 6 more look high, but those results likely overstate it. Across Montana, about 2 in 10 tested wells had arsenic over the EPA limit of 10 µg/L (131 of 693). Across all tested wells in the US it's about 1 in 9. So arsenic turns up somewhat more often in Montana than in the country as a whole. Check your county below.

Where can I get my well tested for arsenic 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 arsenic 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 µg/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.