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Arsenic in Idaho well water, by county

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

Arsenic is widespread, over the limit in 15% or more of tested wells, in 8 of the 44 Idaho counties with 10 or more tested wells. 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 Idaho compares with the rest of the country

Across Idaho, about 1 in 10 tested wells had arsenic over the EPA limit of 10 µg/L (398 of 2,610). Across all tested wells in the US it's about 1 in 9. So arsenic turns up somewhat more often in Idaho 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
Owyhee County 56
59% 33 of 56
 Widespread
Twin Falls County 97
46% 45 of 97
 Widespread
Washington County 36
44% 16 of 36
 Widespread
Payette County 32
41% 13 of 32
 Widespread
Canyon County 163
40% 65 of 163
 Widespread
Shoshone County 492
25% 124 of 492
 Widespread
Gem County 64
20% 13 of 64
 Widespread
Boise County 20
20% 4 of 20
 Widespread
Ada County 151
15% 22 of 151
 Worth checking
Elmore County 73
14% 10 of 73
 Worth checking
Boundary County 17
12% 2 of 17
 Worth checking
Clark County 18
11% 2 of 18
 Worth checking
Bonner County 57
11% 6 of 57
 Worth checking
Kootenai County 52
10% 5 of 52
 Worth checking
Bear Lake County 22
9% 2 of 22
 Worth checking
Camas County 11
9% 1 of 11
 Worth checking
Jefferson County 58
9% 5 of 58
 Worth checking
Franklin County 13
8% 1 of 13
 Worth checking
Bannock County 56
7% 4 of 56
 Worth checking
Minidoka County 78
6% 5 of 78
 Worth checking
Blaine County 34
6% 2 of 34
 Worth checking
Idaho County 36
6%* 2 of 36
 Worth checking
Power County 42
5% 2 of 42
 Rare
Lemhi County 24
4% 1 of 24
 Rare
Gooding County 56
4% 2 of 56
 Rare
Caribou County 113
4% 4 of 113
 Rare
Custer County 32
3% 1 of 32
 Rare
Benewah County 36
3% 1 of 36
 Rare
Latah County 37
3% 1 of 37
 Rare
Butte County 143
1% 2 of 143
 Rare
Jerome County 78
1% 1 of 78
 Rare
Bingham County 85
1% 1 of 85
 Rare
Cassia County 60
0% 0 of 60
 Rare
Fremont County 43
0% 0 of 43
 Rare
Lincoln County 37
0% 0 of 37
 Rare
Bonneville County 34
0% 0 of 34
 Rare
Nez Perce County 28
0% 0 of 28
 Rare
Madison County 23
0% 0 of 23
 Rare
Oneida County 22
0% 0 of 22
 Rare
Valley County 21
0% 0 of 21
 Rare
Lewis County 17
0% 0 of 17
 Rare
Adams County 15
0% 0 of 15
 Rare
Teton County 15
0% 0 of 15
 Rare
Clearwater 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:

  • Idaho County: All of these tests come from the US Geological Survey, mostly before 2013 in one small part of the county, so this likely overstates it.

Download this table (CSV) · Free to reuse under CC BY 4.0

Where to get your well tested for arsenic in Idaho

1

Free state testing

Idaho sends well owners a free test kit, one per household every 5 years. Check which tests it includes before you count on it for arsenic.

Idaho's private well page
2

A lab Idaho certifies

Ask for arsenic by name. Results from a certified lab count for a home sale or a loan.

Labs certified by Idaho
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 Idaho

Questions people ask about arsenic in Idaho

Is there arsenic in Idaho well water?

Arsenic is widespread, over the limit in 15% or more of tested wells, in 8 of the 44 Idaho counties with 10 or more tested wells. Across Idaho, about 1 in 10 tested wells had arsenic over the EPA limit of 10 µg/L (398 of 2,610). Across all tested wells in the US it's about 1 in 9. So arsenic turns up somewhat more often in Idaho than in the country as a whole. Check your county below.

Where can I get my well tested for arsenic in Idaho?

Idaho sends well owners a free test kit, one per household every 5 years. If that doesn't cover you, use a lab certified by Idaho for drinking water, or a mail-in kit that includes arsenic from a certified lab.

See Idaho'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 Health and Welfare.