Arsenic is worth checking in Jefferson County wells.

1 worth checking, 4 rare out of 5 health checks.

From 67 wells tested in Jefferson County since 2000; your own well needs its own test. How to read this

Tested wells over the limit JeffersonIdaho

Arsenic9%5 of 58 Manganese3%2 of 62 Nitrate3%2 of 67 Lead0%0 of 22 Fluoride0%0 of 60

StainsNot a health risk

Iron stains3%2 of 60

Not charted, because fewer than 10 wells were tested: uranium (4).

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

Lab results: Water Quality Portal (USGS and EPA)

Show iron

Who drinks from a well here

7 in 10 people 20,622 people

Nobody else tests these wells. Only you do.

People on wells: EPA researchers' 2020 estimate

Radon risk (a gas from rock)

LowMiddleHigh

Possible here. A home air test is inexpensive.

Radon zones: EPA map

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

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

This one is worth checking in Jefferson County.

Arsenic5 of 58 wells over

About 1 in 12 wells tested here from 2000 to 2022 were over the EPA limit of 10 µg/L, against about 1 in 10 across Idaho. Half the results were under 2.1 µg/L, and the top 1 in 10 were 4.5 µg/L or more. That's a bigger share than in 26 of the 42 other Idaho counties with 10 or more tested wells, leaving out 1 county whose results are likely overstated.

Jefferson and its neighbors

Clark11%

Jefferson9%

Butte1%

Bingham1%

Bonneville0%

Fremont0%

Madison0%

Arsenic guide

These 4 are rare in Jefferson County.

Manganese2 of 62 wells over

2 of 62 wells tested here from 2000 to 2022 were over the EPA health advisory of 0.3 mg/L, against about 1 in 11 across Idaho. At least half the results were too low for the lab to measure, and the top 1 in 10 were 0.0098 mg/L or more. That's a bigger share than in 24 of the 43 other Idaho counties with 10 or more tested wells.

Jefferson and its neighbors

Butte9%

Fremont5%

Jefferson3%

Bingham0%

Bonneville0%

Clark0%

Madison0%

Manganese guide

Nitrate2 of 67 wells over

2 of 67 wells tested here from 2000 to 2023 were over the EPA limit of 10 mg/L, against about 1 in 24 across Idaho. Half the results were under 1.5 mg/L, and the top 1 in 10 were 4.3 mg/L or more. That's a bigger share than in 23 of the 43 other Idaho counties with 10 or more tested wells.

Jefferson and its neighbors

Bingham9%

Fremont7%

Jefferson3%

Butte1%

Bonneville0%

Clark0%

Madison0%

Nitrate guide

Lead0 of 22 wells over

None of the 22 wells tested here from 2000 to 2022 were over the EPA action level of 15 µg/L, against about 1 in 9 across Idaho. Jefferson County had none, while 5 of the 24 other Idaho counties with 10 or more tested wells had at least one.

Jefferson and its neighbors

Bingham5%

Butte3%

Clark0%

Fremont0%

Jefferson0%

Too few tested to show: Bonneville, Madison.

Lead guide

Fluoride0 of 60 wells over

None of the 60 wells tested here from 2000 to 2018 were over the EPA limit of 4 mg/L, against about 1 in 45 across Idaho. Half the results were under 0.43 mg/L, and the top 1 in 10 were 0.95 mg/L or more. Jefferson County had none, while 14 of the 42 other Idaho counties with 10 or more tested wells had at least one, leaving out 1 county whose results are likely overstated.

Jefferson and its neighbors

Clark7%

Butte1%

Bingham0%

Bonneville0%

Fremont0%

Jefferson0%

Madison0%

Too few Jefferson County wells were tested for this one.

Uranium4 wells tested

Only 4 wells here were tested from 2000 to 2022, too few to say how common it is. Across Idaho it's under 1 in 100.

Uranium guide

Every well needs a bacteria test every year.

No bacteria tests from Jefferson County wells are in the public data. Test every year anyway.

Iron reached staining levels in 2 of 60 tested wells, and it isn't a health risk.

Iron stains2 of 60 wells had it

2 of 60 wells tested here from 2000 to 2018 had enough iron to stain (0.3 mg/L), against about 1 in 7 across Idaho.

Jefferson and its neighbors

Butte20%

Fremont9%

Madison4%

Clark4%

Jefferson3%

Bingham1%

Bonneville0%

Iron guide

Most of the 67 wells tested in Jefferson County were last tested before 2010.

61 wells had a test in the 2000s, 31 in the 2010s and 4 since 2020.

An old result shows the water as it was then, so treat these 67 wells as a starting point and test your own.

Most Jefferson 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 Jefferson County.

Bacteria
Nitrate
Arsenic

What Idaho offers well owners

Free testingDHW offers free private well water testing kits through Idaho Health Tools, limited to one kit per household every five years. See the program
Certified labsIdaho's list of certified labs
Well recordsIdaho's well record search

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

Jefferson 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 1 town water system in Jefferson County test for PFAS. None found any. No well in Jefferson 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

Next door to Jefferson County

Bingham

Elsewhere in Idaho

Ada Canyon Bonner Twin Falls Kootenai

All of Idaho

Questions people ask about well water in Jefferson County

Is well water in Jefferson County safe to drink?

5 of 58 wells tested in Jefferson County had arsenic over the limit, about 1 in 12. Across Idaho it's about 1 in 10. That's a bigger share than in 26 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 Jefferson 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 Jefferson County.

How does Jefferson 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 Jefferson County, at 9%. Clark had the most, at 11%. Groundwater doesn't stop at county lines, so Clark's 11% is worth knowing about if you live near it.

How often should I test my well in Jefferson County?

Jefferson County has no public bacteria results for wells, so follow the CDC's advice: 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 Jefferson County numbers come from

People on wells: 20,622 in Jefferson County, from EPA estimates built on the 2020 Census (Murray et al.).
Lab tests: 1,035 well water samples from 67 wells in the Water Quality Portal, run by the USGS and EPA, 2000 to 2023.
Neighboring counties: the 6 counties that share a border with Jefferson County, from the Census Bureau's county adjacency file.
PFAS: EPA tests of 1 town water systems in Jefferson County under the Fifth Unregulated Contaminant Monitoring Rule, 2023 to 2025.
Radon: Jefferson 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 numbers Report an error