Arsenic has turned up often in Gallatin County wells, mostly from one small area.
1 likely overstated, 2 rare out of 3 health checks.
From 271 wells tested in Gallatin County since 2009; your own well needs its own test. How to read this
*Arsenic: Almost all of these tests come from a single agency in one small part of the county, so this likely overstates it.
Not charted, because fewer than 10 wells were tested: fluoride (6).
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 3 people 34,439 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)
Likely here. Test your home's air first.
Radon zones: EPA map
Here's how Gallatin County compares with Montana and the counties around it.
Each result puts Gallatin County next to the 8 counties that border it, on the chart's 0 to 50% scale.
This one has turned up often here, mostly from one small area.
Arsenic13 of 25 wells over
* Likely overstated, so we don't rank it.
Too few tested to show: Broadwater, Madison, Meagher, Park.
Almost all of these tests come from a single agency in one small part of the county, so this likely overstates it.
These 2 are rare in Gallatin County.
Manganese0 of 209 wells over
* Likely overstated, so we don't rank it.
Too few tested to show: Broadwater, Madison, Meagher, Park.
Nitrate0 of 36 wells over
Too few tested to show: Broadwater, Madison, Meagher, Park.
Too few Gallatin County wells were tested for this one.
Fluoride6 wells tested
None of the 9 Gallatin County wells tested for bacteria had it, but every well needs a yearly test.
Coliform bacteria0 of 9 wells had it
E. coli0 of 9 wells had it
E. coli means human or animal waste is getting in, so if a test of your Gallatin County well finds it, boil drinking water until the well is fixed.
Iron reached staining levels in 3 of 226 tested wells, and it isn't a health risk.
Iron stains3 of 226 wells had it
Too few tested to show: Broadwater, Madison, Meagher, Park.
Gallatin County has 271 tested wells, and the busiest decade for testing was the 2010s.
Only 28 of the 271 wells have a test from 2020 or later, so a fresh test of your own well tells you more than any of these.
Most Gallatin 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 Gallatin County.
- Bacteria
- Nitrate
- Arsenic
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 82 feet.
That comes from 14,166 household wells in Montana's drilling records. Most wells of every kind here are 55 to 153 feet deep. Wells as deep as Gallatin 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 are shallower than in 38 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 Gallatin County.
Gallatin 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 5 town water systems in Gallatin County test for PFAS. None found any. No well in Gallatin 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 Gallatin County
Is well water in Gallatin County safe to drink?
13 of 25 wells tested in Gallatin County had arsenic over the limit, about 5 in 10. Across Montana it's about 2 in 10. We don't rank this against other Montana counties, because most of these tests come from one small area. Almost all of these tests come from a single agency in one small part of the county, so this likely overstates it. So test your own well for arsenic before you count on it.
Can I get my well tested for free in Gallatin 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 and arsenic, the tests that matter most in Gallatin County.
How deep are wells in Gallatin County?
Half the household wells here are shallower than 82 feet. Typical household wells here are shallower than in 38 of the 55 other Montana counties with enough drilling records.
How does Gallatin County compare with the counties around it?
For arsenic, the share of tested wells over the limit: 1 of the 4 neighboring counties with data had a higher share than Gallatin County, at 52%. Jefferson had the most, at 62%. Groundwater doesn't stop at county lines, so Jefferson's 62% is worth knowing about if you live near it.
How often should I test my well in Gallatin County?
None of the 9 wells tested for bacteria in Gallatin 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.
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Where the Gallatin County numbers come from
- People on wells: 34,439 in Gallatin County, from EPA estimates built on the 2020 Census (Murray et al.).
- Lab tests: 549 well water samples from 271 wells in the Water Quality Portal, run by the USGS and EPA, 2009 to 2023.
- Well depths: 14,166 Montana drilling records for Gallatin County, gathered in the US Groundwater Well Database (Lin and others, 2024).
- Neighboring counties: the 8 counties that share a border with Gallatin County, from the Census Bureau's county adjacency file.
- Results marked likely overstated: our own check of who tested the wells, when and where.
- PFAS: EPA tests of 5 town water systems in Gallatin County under the Fifth Unregulated Contaminant Monitoring Rule, 2023 to 2025.
- Radon: Gallatin County is zone 1 on the EPA Map of Radon Zones.
- State programs and links: Department of Environmental Quality, checked November 1, 2026.