Private wells
Montana well water: who's on a well and what to test for
People on private wells
392,500
Homes on wells
36.8%
Est. above arsenic limit
23,300
Every Montana county
Sorted by people on private wells. Arsenic and nitrate are modeled for typical wells in the county, not measured at yours; depth is the median of domestic well records.
| Flathead County | 47,100 | 44% | 12.3% | 0.5 | 184 ft |
| Missoula County | 37,900 | 31% | 8.4% | 0.3 | 110 ft |
| Gallatin County | 34,400 | 30% | 11.6% | 1.4 | 90 ft |
| Ravalli County | 33,400 | 75% | 6.5% | 0.4 | 80 ft |
| Lewis and Clark County | 28,000 | 40% | 11.8% | 0.8 | 120 ft |
| Yellowstone County | 26,500 | 15% | 7.9% | 1.1 | 41 ft |
| Lake County | 16,000 | 51% | 14.7% | 0.4 | 178 ft |
| Cascade County | 13,300 | 16% | 5.1% | 0.9 | 143 ft |
| Lincoln County | 12,600 | 65% | 8.1% | 0.1 | 160 ft |
| Jefferson County | 8,400 | 69% | 7.8% | 0.5 | 170 ft |
| Silver Bow County | 8,100 | 21% | 5.6% | 0.5 | 116 ft |
| Park County | 7,600 | 47% | 6.9% | 0.4 | 125 ft |
| Sanders County | 7,500 | 60% | 13.4% | 0.1 | 180 ft |
| Carbon County | 7,000 | 64% | 5.2% | 0.8 | 60 ft |
| Big Horn County | 6,400 | 47% | 6.0% | 0.7 | 54 ft |
| Stillwater County | 6,200 | 72% | 6.3% | 0.7 | 60 ft |
| Madison County | 5,900 | 61% | 6.5% | 0.5 | 109 ft |
| Hill County | 5,200 | 30% | 6.4% | 0.9 | 120 ft |
| Roosevelt County | 5,000 | 47% | 5.5% | 1.7 | 77 ft |
| Beaverhead County | 5,000 | 55% | 5.8% | 0.5 | 81 ft |
| Richland County | 4,500 | 39% | 5.3% | 0.8 | 65 ft |
| Fergus County | 4,400 | 38% | 3.6% | 0.5 | 140 ft |
| Broadwater County | 4,300 | 61% | 11.5% | 0.6 | 138 ft |
| Glacier County | 3,800 | 30% | 6.2% | 0.5 | 102 ft |
| Rosebud County | 3,200 | 37% | 5.1% | 0.4 | 120 ft |
| Valley County | 3,200 | 42% | 14.3% | 1.9 | 72 ft |
| Teton County | 3,200 | 52% | 4.6% | 1.8 | 35 ft |
| Blaine County | 3,000 | 43% | 5.0% | 0.4 | 80 ft |
| Custer County | 2,900 | 24% | 5.9% | 0.1 | 198 ft |
| Sheridan County | 2,600 | 72% | 4.8% | 1.3 | 120 ft |
| Deer Lodge County | 2,600 | 31% | 6.8% | 0.3 | 62 ft |
| Chouteau County | 2,500 | 42% | 5.6% | 0.8 | 75 ft |
| Musselshell County | 2,500 | 55% | 4.6% | 0.2 | 207 ft |
| Mineral County | 2,500 | 56% | 8.4% | 0.0 | 140 ft |
| Dawson County | 2,500 | 29% | 4.8% | 0.6 | 96 ft |
| Powell County | 2,200 | 45% | 8.4% | 0.4 | 83 ft |
| Granite County | 2,100 | 69% | 9.0% | 0.2 | 98 ft |
| Sweet Grass County | 1,900 | 52% | 7.6% | 0.4 | 80 ft |
| Pondera County | 1,900 | 31% | 4.8% | 0.7 | 90 ft |
| Phillips County | 1,900 | 43% | 6.2% | 0.4 | 73 ft |
| Judith Basin County | 1,400 | 71% | 3.0% | 0.7 | 100 ft |
| Powder River County | 1,200 | 70% | 4.8% | 0.3 | 200 ft |
| Daniels County | 1,100 | 61% | 5.4% | 2.7 | 80 ft |
| McCone County | 1,100 | 61% | 6.6% | 0.4 | 115 ft |
| Toole County | 1,100 | 25% | 6.7% | 1.5 | 100 ft |
| Carter County | 990 | 66% | 5.0% | 0.4 | 150 ft |
| Fallon County | 970 | 30% | 4.6% | 0.4 | 130 ft |
| Meagher County | 870 | 47% | 4.6% | 0.2 | 111 ft |
| Garfield County | 790 | 67% | 5.9% | 0.1 | 157 ft |
| Wheatland County | 740 | 39% | 3.5% | 0.6 | 110 ft |
| Liberty County | 620 | 38% | 8.0% | 0.8 | 132 ft |
| Golden Valley County | 580 | 71% | 3.8% | 0.5 | 100 ft |
| Treasure County | 510 | 63% | 5.0% | 0.3 | 65 ft |
| Prairie County | 500 | 48% | 4.9% | 0.2 | 100 ft |
| Wibaux County | 500 | 49% | 4.4% | 0.4 | 140 ft |
| Petroleum County | 330 | 61% | 4.5% | 0.1 | 100 ft |
Shading: red is among the highest counties nationally, amber the middle, green the lower range. Click a column to sort.
Most well users above the arsenic limit
- Missoula County: about 4,900 people
- Ravalli County: about 3,300 people
- Lewis and Clark County: about 2,800 people
- Flathead County: about 2,600 people
- Gallatin County: about 1,500 people
Test first, then treat what the test finds
We may earn a commission from links below, at no cost to you. Picks follow the testing order, not who pays. How we make money.
1. Certified lab well test
Start here, every year
A mail-in kit analyzed by a certified lab measures bacteria, nitrate, arsenic, metals and more. Test strips cannot measure arsenic or uranium reliably.
2. Sediment pre-filter
Sand, silt or cloudy water
A 5-micron cartridge after the pressure tank catches sand, silt and rust before they reach fixtures or other filters. Cartridges are cheap and swap every few months.
3. Iron and sulfur filter
Orange stains or rotten-egg smell
Air-injection or oxidizing-media filters remove orange staining and rotten-egg smell. Size one from a lab result for iron, manganese and hydrogen sulfide.
4. UV disinfection system
Coliform bacteria in a test
Inactivates bacteria and viruses; usually for private wells or after boil notices.
5. Under-sink reverse osmosis
Arsenic, nitrate or uranium in a test
The most thorough option for drinking water: PFAS, lead, nitrate, arsenic and more. Look for NSF/ANSI 58.
6. Whole-house well filter
Several problems at once
Multi-stage sediment and carbon for every tap. Add UV if bacteria shows up, and choose stages from your lab results, not a guess.
Questions
Which Montana counties rely most on wells?+
By share of homes: Ravalli County (75%), Stillwater County (72%), Sheridan County (72%), Golden Valley County (71%), Judith Basin County (71%). By number of people: Flathead County (47,100), Missoula County (37,900), Gallatin County (34,400).
Is arsenic a problem in Montana wells?+
USGS estimates about 23,300 Montana well users have water above the 10 microgram per liter arsenic limit that applies to public systems, the largest numbers in Missoula County, Ravalli County, Lewis and Clark County. Arsenic has no taste or smell, so only a lab test finds it.
Do I have to test my well when I sell a house in Montana?+
We could not confirm a statewide rule for Montana. Check with your county health department. Buyers can still make a lab test a condition of the offer.
How often should I test my well?+
No federal law covers private wells. EPA's drinking water limits apply to public water systems, so testing and treating a private well is up to the owner. EPA and CDC recommend testing at least once a year for coliform bacteria, nitrate, pH and total dissolved solids, and testing for arsenic, uranium, radon and other local concerns based on where you live.
Well-user counts, arsenic and nitrate figures are each a modeled estimate built from national datasets, not a count of tested wells, and well depths summarize drilling records rather than every well. Your own well can be very different from the county average, which is why the only answer for your home is a lab test. Sources: EPA Office of Research and Development, Water Source 2020 (private well estimates by census block); CDC Environmental Public Health Tracking, Well Water Index (from USGS nitrate and arsenic models); USGS, county-level domestic well population with arsenic above 10 micrograms per liter (Ayotte and others, 2017); USGS Synthesized Domestic Well Database for the Conterminous United States (SynDWell-US), McCarthy and others, 2025 (depth, where a county has 30+ usable records).