Manganese is widespread in Yellowstone County wells.
1 widespread, 3 worth checking, 2 rare out of 6 health checks.
From 14 wells tested in Yellowstone County since 2009; your own well needs its own test. How to read this
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 6 people 26,528 people
Nobody else tests these wells. Only you do.
People on wells: EPA researchers' 2020 estimate
How deep wells go here
Shallow wells: watch for fertilizer and germs from above.
Well depths: US Groundwater Well Database
Radon risk (a gas from rock)
Possible here. A home air test is inexpensive.
Radon zones: EPA map
Here's how Yellowstone County compares with Montana and the counties around it.
Each result puts Yellowstone County next to the 7 counties that border it, on the chart's 0 to 50% scale.
This one is widespread in Yellowstone County wells.
Manganese3 of 14 wells over
* Likely overstated, so we don't rank it.
Too few tested to show: Golden Valley, Musselshell, Stillwater, Treasure.
These 3 are worth checking in Yellowstone County.
Arsenic1 of 12 wells over
* Likely overstated, so we don't rank it.
Too few tested to show: Golden Valley, Musselshell, Stillwater, Treasure.
Uranium1 of 12 wells over
Too few tested to show: Golden Valley, Musselshell, Rosebud, Stillwater, Treasure.
Nitrate1 of 12 wells over
Too few tested to show: Golden Valley, Musselshell, Stillwater, Treasure.
These 2 are rare in Yellowstone County.
Fluoride0 of 14 wells over
Too few tested to show: Golden Valley, Musselshell, Stillwater, Treasure.
Lead0 of 12 wells over
Too few tested to show: Golden Valley, Musselshell, Stillwater, Treasure.
Coliform bacteria turned up in 1 of 2 tested Yellowstone County wells, so test for it every year.
Coliform bacteria1 of 2 wells had it
E. coli0 of 2 wells had it
E. coli means human or animal waste is getting in, so if a test of your Yellowstone County well finds it, boil drinking water until the well is fixed.
Iron reached staining levels in 1 of 12 tested wells, and it isn't a health risk.
Iron stains1 of 12 wells had it
Too few tested to show: Golden Valley, Musselshell, Stillwater, Treasure.
Yellowstone County has 14 tested wells, and the busiest decade for testing was the 2010s.
None of the 14 wells has a test from 2020 or later, so a fresh test of your own well tells you more than any of these.
Most Yellowstone 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 5 things to check in Yellowstone County.
- Bacteria
- Nitrate
- Manganese
- Arsenic
- Uranium
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 38 feet.
That comes from 8,523 household wells in Montana's drilling records. Most wells of every kind here are 27 to 90 feet deep. Wells as shallow as Yellowstone County's pick up bacteria and nitrate from the surface more easily, so the yearly test matters even more here. Typical household wells here are shallower than in 54 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 Yellowstone County.
Yellowstone 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 7 town water systems in Yellowstone County test for PFAS. None found any. No well in Yellowstone 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
Questions people ask about well water in Yellowstone County
Is well water in Yellowstone County safe to drink?
3 of 14 wells tested in Yellowstone County had manganese over the limit, about 2 in 10. Across Montana it's about 1 in 3. That's a bigger 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 Yellowstone 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 and uranium, the tests that matter most in Yellowstone County.
How deep are wells in Yellowstone County?
Half the household wells here are shallower than 38 feet. Typical household wells here are shallower than in 54 of the 55 other Montana counties with enough drilling records.
How does Yellowstone County compare with the counties around it?
For manganese, the share of tested wells over the limit: 2 of the 3 neighboring counties with data had a higher share than Yellowstone County, at 21%. Rosebud had the most, at 46%. Groundwater doesn't stop at county lines, so Rosebud's 46% is worth knowing about if you live near it.
How often should I test my well in Yellowstone County?
In Yellowstone County, 1 of 2 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.
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Where the Yellowstone County numbers come from
- People on wells: 26,528 in Yellowstone County, from EPA estimates built on the 2020 Census (Murray et al.).
- Lab tests: 113 well water samples from 14 wells in the Water Quality Portal, run by the USGS and EPA, 2009 to 2012.
- Well depths: 8,523 Montana drilling records for Yellowstone County, gathered in the US Groundwater Well Database (Lin and others, 2024).
- Neighboring counties: the 7 counties that share a border with Yellowstone County, from the Census Bureau's county adjacency file.
- PFAS: EPA tests of 7 town water systems in Yellowstone County under the Fifth Unregulated Contaminant Monitoring Rule, 2023 to 2025.
- Radon: Yellowstone County is zone 2 on the EPA Map of Radon Zones.
- State programs and links: Department of Environmental Quality, checked November 1, 2026.