Manganese is widespread in Lewis and Clark County wells.

1 widespread, 4 worth checking, 2 rare out of 7 health checks.

From 110 wells tested in Lewis and Clark County since 2001; your own well needs its own test. How to read this

Tested wells over the limit Lewis and ClarkMontana

Manganese19%5 of 27 Arsenic14%12 of 86 Uranium12%3 of 24 Nitrate12%3 of 24 Radon10%2 of 21 Lead3%2 of 70 Fluoride0%0 of 24

GermsTest every year, whatever the chart says

Coliform bacteria0%0 of 4 E. coli0%0 of 3

StainsNot a health risk

Iron stains16%16 of 103

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

Lab results: Water Quality Portal (USGS and EPA)

Show bacteria and iron

Who drinks from a well here

1 in 3 people 27,963 people

Nobody else tests these wells. Only you do.

People on wells: EPA researchers' 2020 estimate

How deep wells go here

Middle depth: some shelter from the surface, not complete.

Well depths: US Groundwater Well Database

Radon risk (a gas from rock)

LowMiddleHigh

Likely here. Test your home's air first.

Radon zones: EPA map

Here's how Lewis and Clark County compares with Montana and the counties around it.

Each result puts Lewis and Clark County next to the 7 counties that border it, on the chart's 0 to 50% scale.

This one is widespread in Lewis and Clark County wells.

Manganese5 of 27 wells over

About 2 in 10 wells tested here from 2001 to 2017 were over the EPA health advisory of 0.3 mg/L, against about 1 in 3 across Montana. Half the results were under 0.063 mg/L, and the top 1 in 10 were 0.37 mg/L or more. That's a smaller share than in 7 of the 13 other Montana counties with 10 or more tested wells, leaving out 4 counties whose results are likely overstated.

Lewis and Clark and its neighbors

Powell44%

Jefferson22%

Lewis and Clark19%

Flathead16%

Cascade0%

* Likely overstated, so we don't rank it.

Too few tested to show: Broadwater, Meagher, Teton.

Manganese guide

These 4 are worth checking in Lewis and Clark County.

Arsenic12 of 86 wells over

About 1 in 7 wells tested here from 2001 to 2017 were over the EPA limit of 10 µg/L, against about 2 in 10 across Montana. At least half the results were too low for the lab to measure, and the top 1 in 10 were 9.0 µg/L or more. That's a bigger share than in 8 of the 9 other Montana counties with 10 or more tested wells, leaving out 7 counties whose results are likely overstated.

Lewis and Clark and its neighbors

Jefferson62%

Powell43%

Cascade18%

Lewis and Clark14%

Flathead0%

* Likely overstated, so we don't rank it.

Too few tested to show: Broadwater, Meagher, Teton.

Arsenic guide

Uranium3 of 24 wells over

3 of 24 wells tested here from 2008 to 2010 were over the EPA limit of 30 µg/L, against about 1 in 12 across Montana. Half the results were under 3.5 µg/L, and the top 1 in 10 were 36 µg/L or more. That's a bigger share than in 9 of the 10 other Montana counties with 10 or more tested wells, leaving out 1 county whose results are likely overstated.

Lewis and Clark and its neighbors

Lewis and Clark12%

Jefferson11%

Too few tested to show: Broadwater, Cascade, Flathead, Meagher, Powell, Teton.

Uranium guide

Nitrate3 of 24 wells over

3 of 24 wells tested here from 2001 to 2010 were over the EPA limit of 10 mg/L, against about 1 in 13 across Montana. Half the results were under 0.58 mg/L, and the top 1 in 10 were 11 mg/L or more. That's a bigger share than in 10 of the 13 other Montana counties with 10 or more tested wells, leaving out 1 county whose results are likely overstated.

Lewis and Clark and its neighbors

Cascade18%

Lewis and Clark12%

Jefferson1%

Powell0%

* Likely overstated, so we don't rank it.

Too few tested to show: Broadwater, Flathead, Meagher, Teton.

Nitrate guide

Radon in water2 of 21 wells over

2 of 21 wells tested here from 2009 to 2010 were over 4,000 pCi/L, a level the EPA proposed, against about 2 in 10 across Montana. Half the results were under 1,700 pCi/L, and the top 1 in 10 were 4,800 pCi/L or more. That's a bigger share than in 2 of the 4 other Montana counties with 10 or more tested wells.

Lewis and Clark and its neighbors

Jefferson36%

Lewis and Clark10%

Too few tested to show: Broadwater, Cascade, Flathead, Meagher, Powell, Teton.

Radon guide

These 2 are rare in Lewis and Clark County.

Lead2 of 70 wells over

2 of 70 wells tested here from 2001 to 2017 were over the EPA action level of 15 µg/L, against about 1 in 24 across Montana. That's a bigger share than in 12 of the 15 other Montana counties with 10 or more tested wells, leaving out 1 county whose results are likely overstated.

Lewis and Clark and its neighbors

Jefferson19%

Lewis and Clark3%

Cascade0%

Flathead0%

Powell0%

Too few tested to show: Broadwater, Meagher, Teton.

Lead guide

Fluoride0 of 24 wells over

None of the 24 wells tested here from 2001 to 2017 were over the EPA limit of 4 mg/L, against about 1 in 19 across Montana. Half the results were under 0.28 mg/L, and the top 1 in 10 were 0.36 mg/L or more. Lewis and Clark County had none, while 6 of the 14 other Montana counties with 10 or more tested wells had at least one, leaving out 1 county whose results are likely overstated.

Lewis and Clark and its neighbors

Jefferson1%

Cascade0%

Lewis and Clark0%

Powell0%

Too few tested to show: Broadwater, Flathead, Meagher, Teton.

None of the 4 Lewis and Clark County wells tested for bacteria had it, but every well needs a yearly test.

Coliform bacteria0 of 4 wells had it

None of the 4 wells tested here from 2009 to 2014 had coliform bacteria, against about 3 in 10 across Montana.

Bacteria guide

E. coli0 of 3 wells had it

None of the 3 wells tested here from 2009 to 2010 had E. coli, against about 1 in 30 across Montana.

E. coli means human or animal waste is getting in, so if a test of your Lewis and Clark County well finds it, boil drinking water until the well is fixed.

Bacteria guide

Iron reached staining levels in 16 of 103 tested wells, and it isn't a health risk.

Iron stains16 of 103 wells had it

About 2 in 10 wells tested here from 2001 to 2017 had enough iron to stain (0.3 mg/L), against about 1 in 3 across Montana.

Lewis and Clark and its neighbors

Powell32%

Cascade27%

Jefferson23%

Flathead17%

Lewis and Clark16%

Too few tested to show: Broadwater, Meagher, Teton.

Iron guide

Lewis and Clark County has 110 tested wells, and the busiest decade for testing was the 2010s.

40 wells had a test in the 2000s and 104 in the 2010s.

None of the 110 wells has a test from 2020 or later, so a fresh test of your own well tells you more than any of these.

Most Lewis and Clark 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 6 things to check in Lewis and Clark County.

Bacteria
Nitrate
Manganese
Arsenic
Uranium
Radon in water

What Montana offers well owners

Free testingDPHHS offers free well water testing to prospective or licensed childcare facilities. See the program
Certified labsMontana's list of certified labs
Well recordsWe didn't find an online search.

Half the household wells here are shallower than 111 feet.

That comes from 12,870 household wells in Montana's drilling records. Most wells of every kind here are 60 to 195 feet deep. Wells as deep as Lewis and Clark County's are usually better protected from fertilizer and bacteria at the surface, but they can pick up more of what's in the rock, like arsenic and uranium. Typical household wells here go deeper than in 31 of the 55 other Montana counties with enough drilling records.

How to find your own well's depth

Well depths: US Groundwater Well Database

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

Lewis and Clark County is in the EPA's highest radon zone. Radon from well water adds to the radon in your indoor air, so test your air first, and your water too if the air is high. Radon guide

Between 2023 and 2025 the EPA had 1 town water system in Lewis and Clark County test for PFAS. None found any. No well in Lewis and Clark 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 Lewis and Clark County

Flathead

Elsewhere in Montana

Gallatin Ravalli Yellowstone

All of Montana

Questions people ask about well water in Lewis and Clark County

Is well water in Lewis and Clark County safe to drink?

5 of 27 wells tested in Lewis and Clark County had manganese over the limit, about 2 in 10. Across Montana it's about 1 in 3. That's a smaller share than in 7 of the 13 other Montana counties with 10 or more tested wells, leaving out 4 counties whose results are likely overstated. So test your own well for manganese before you count on it.

Can I get my well tested for free in Lewis and Clark County?

DPHHS offers free well water testing to prospective or licensed childcare facilities. If not, use a lab Montana has certified for drinking water. Ask for bacteria, nitrate, manganese, arsenic, uranium and radon in water, the tests that matter most in Lewis and Clark County.

How deep are wells in Lewis and Clark County?

Half the household wells here are shallower than 111 feet. Typical household wells here go deeper than in 31 of the 55 other Montana counties with enough drilling records.

How does Lewis and Clark County compare with the counties around it?

For manganese, the share of tested wells over the limit: 2 of the 4 neighboring counties with data had a higher share than Lewis and Clark County, at 19%. Powell had the most, at 44%. Groundwater doesn't stop at county lines, so Powell's 44% is worth knowing about if you live near it.

How often should I test my well in Lewis and Clark County?

None of the 4 wells tested for bacteria in Lewis and Clark County had it, but the CDC still says to 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 Environmental QualityUpdated November 8, 2026

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Where the Lewis and Clark County numbers come from

People on wells: 27,963 in Lewis and Clark County, from EPA estimates built on the 2020 Census (Murray et al.).
Lab tests: 1,285 well water samples from 110 wells in the Water Quality Portal, run by the USGS and EPA, 2001 to 2017.
Well depths: 12,870 Montana drilling records for Lewis and Clark County, gathered in the US Groundwater Well Database (Lin and others, 2024).
Neighboring counties: the 7 counties that share a border with Lewis and Clark County, from the Census Bureau's county adjacency file.
PFAS: EPA tests of 1 town water systems in Lewis and Clark County under the Fifth Unregulated Contaminant Monitoring Rule, 2023 to 2025.
Radon: Lewis and Clark County is zone 1 on the EPA Map of Radon Zones.
State programs and links: Department of Environmental Quality, checked November 1, 2026.

Download this county's data (CSV)How we read these numbers Report an error