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Arsenic is widespread in Twin Falls County wells.

1 widespread, 1 worth checking, 3 rare out of 5 health checks.

From 125 wells tested in Twin Falls County since 2000; your own well needs its own test. How to read this

Tested wells over the limit Twin FallsIdaho
Arsenic46%45 of 97Nitrate9%11 of 125Fluoride1%1 of 97Manganese1%1 of 104Lead0%0 of 23
GermsTest every year, whatever the chart says
Coliform bacteria100%21 of 21
StainsNot a health risk
Iron stains1%1 of 104

Rare: under 5%Worth checking: 5 to 15%Widespread: 15%+

Lab results: Water Quality Portal (USGS and EPA)

Who drinks from a well here

1 in 4 people 22,778 people

Nobody else tests these wells. Only you do.

People on wells: EPA researchers' 2020 estimate

Radon risk (a gas from rock)

Possible here. A home air test is inexpensive.

Radon zones: EPA map

Here's how Twin Falls County compares with Idaho and the counties around it.

Each result puts Twin Falls County next to the 6 counties that border it, on the chart's 0 to 50% scale.

This one is widespread in Twin Falls County wells.

Arsenic45 of 97 wells over

About 5 in 10 wells tested here from 2000 to 2011 were over the EPA limit of 10 µg/L, against about 1 in 10 across Idaho. Half the results were under 9.7 µg/L, and the top 1 in 10 were 20 µg/L or more. That's a bigger share than in 41 of the 42 other Idaho counties with 10 or more tested wells, leaving out 1 county whose results are likely overstated.

Twin Falls and its neighbors
Owyhee59%
Twin Falls46%
Elko, NV14%
Elmore14%
Gooding4%
Jerome1%
Cassia0%

Arsenic guide

This one is worth checking in Twin Falls County.

Nitrate11 of 125 wells over

About 1 in 11 wells tested here from 2000 to 2011 were over the EPA limit of 10 mg/L, against about 1 in 24 across Idaho. Half the results were under 4.8 mg/L, and the top 1 in 10 were 8.6 mg/L or more. That's a bigger share than in 38 of the 43 other Idaho counties with 10 or more tested wells.

Twin Falls and its neighbors
Elmore12%
Twin Falls9%
Cassia8%
Owyhee7%
Gooding3%
Jerome2%
Elko, NV0%

Nitrate guide

These 3 are rare in Twin Falls County.

Fluoride1 of 97 wells over

1 of 97 wells tested here from 2000 to 2011 was over the EPA limit of 4 mg/L, against about 1 in 45 across Idaho. Half the results were under 0.77 mg/L, and the top 1 in 10 were 1.3 mg/L or more. That's a bigger share than in 32 of the 42 other Idaho counties with 10 or more tested wells, leaving out 1 county whose results are likely overstated.

Twin Falls and its neighbors
Owyhee11%
Elmore3%
Twin Falls1%
Cassia0%
Elko, NV0%
Gooding0%
Jerome0%
Manganese1 of 104 wells over

1 of 104 wells tested here from 2000 to 2011 was over the EPA health advisory of 0.3 mg/L, against about 1 in 11 across Idaho. That's a smaller share than in 25 of the 43 other Idaho counties with 10 or more tested wells.

Twin Falls and its neighbors
Owyhee20%
Elmore3%
Cassia2%
Twin Falls1%
Elko, NV0%
Gooding0%
Jerome0%

Manganese guide

Lead0 of 23 wells over

None of the 23 wells tested here from 2001 to 2008 were over the EPA action level of 15 µg/L, against about 1 in 9 across Idaho. Half the results were under 0.14 µg/L, and the top 1 in 10 were 0.45 µg/L or more. Twin Falls County had none, while 5 of the 24 other Idaho counties with 10 or more tested wells had at least one.

Twin Falls and its neighbors
Cassia0%
Elmore0%
Gooding0%
Jerome0%
Owyhee0%
Twin Falls0%

Too few tested to show: Elko, NV.

Lead guide

Coliform bacteria turned up in 21 of 21 tested Twin Falls County wells, so test for it every year.

Coliform bacteria21 of 21 wells had it

Almost all wells tested here in 2000 had coliform bacteria, against about 6 in 10 across Idaho.

Twin Falls and its neighbors
Twin Falls100%
Jerome61%
Elmore48%

Too few tested to show: Cassia, Elko, NV, Gooding, Owyhee.

Bacteria guide

Iron reached staining levels in 1 of 104 tested wells, and it isn't a health risk.

Iron stains1 of 104 wells had it

1 of 104 wells tested here from 2000 to 2011 had enough iron to stain (0.3 mg/L), against about 1 in 7 across Idaho.

Twin Falls and its neighbors
Owyhee29%
Elmore14%
Elko, NV8%
Cassia5%
Twin Falls1%
Gooding0%
Jerome0%

Iron guide

Most of the 125 wells tested in Twin Falls County were last tested before 2010.

125 wells had a test in the 2000s and 53 in the 2010s.

An old result shows the water as it was then, so treat these 125 wells as a starting point and test your own.

Most Twin Falls County wells in these records were tested for research or because someone suspected a problem, which can push the shares above what a typical home well would show (how we read the numbers).

One mail-in lab test covers the 3 things to check in Twin Falls County.

  • Bacteria
  • Nitrate
  • Arsenic

What Idaho offers well owners

Here's what we know about radon and PFAS in Twin Falls County.

Twin Falls County is in the EPA's middle radon zone. An indoor air test is cheap and tells you whether to look at your water. Radon guide

Between 2023 and 2025 the EPA had 2 town water systems in Twin Falls County test for PFAS. None found any. No well in Twin Falls County has public PFAS results, so the town systems are the best clue to what's in the groundwater nearby. PFAS guide

Radon zones: EPA Map of Radon Zones. PFAS: EPA tests of town water systems, 2023 to 2025

Other counties in Idaho
AdaCanyonBinghamBonnerKootenaiJefferson

All of Idaho

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Questions people ask about well water in Twin Falls County

Is well water in Twin Falls County safe to drink?

45 of 97 wells tested in Twin Falls County had arsenic over the limit, about 5 in 10. Across Idaho it's about 1 in 10. That's a bigger share than in 41 of the 42 other Idaho counties with 10 or more tested wells, leaving out 1 county whose results are likely overstated. So test your own well for arsenic before you count on it.

Can I get my well tested for free in Twin Falls County?

DHW offers free private well water testing kits through Idaho Health Tools, limited to one kit per household every five years. If not, use a lab Idaho has certified for drinking water. Ask for bacteria, nitrate and arsenic, the tests that matter most in Twin Falls County.

How does Twin Falls County compare with the counties around it?

For arsenic, the share of tested wells over the limit: 1 of the 6 neighboring counties with data had a higher share than Twin Falls County, at 46%. Owyhee had the most, at 59%. Groundwater doesn't stop at county lines, so Owyhee's 59% is worth knowing about if you live near it.

How often should I test my well in Twin Falls County?

In Twin Falls County, 21 of 21 tested wells had coliform bacteria, so the CDC's advice matters here: test for bacteria and nitrate every year, and right away after a flood, a repair, or any change in taste or smell.

By the Tested Water teamData from the USGS, EPA and Department of Health and WelfareUpdated November 8, 2026

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Where the Twin Falls County numbers come from

  • People on wells: 22,778 in Twin Falls County, from EPA estimates built on the 2020 Census (Murray et al.).
  • Lab tests: 1,891 well water samples from 125 wells in the Water Quality Portal, run by the USGS and EPA, 2000 to 2011.
  • Neighboring counties: the 6 counties that share a border with Twin Falls County, from the Census Bureau's county adjacency file.
  • PFAS: EPA tests of 2 town water systems in Twin Falls County under the Fifth Unregulated Contaminant Monitoring Rule, 2023 to 2025.
  • Radon: Twin Falls County is zone 2 on the EPA Map of Radon Zones.
  • State programs and links: Department of Health and Welfare, checked November 1, 2026.
Download this county's data (CSV)How we read these numbersReport an error
One lab test covers what to check in Twin Falls CountyFind a lab test