Montana Private Well Water Risk: Arsenic & Nitrate
These figures are USGS statistical estimates for private (domestic) wells in Montana. They describe the area's modeled risk — not any single well.
Quick answer: Yes — arsenic occurs naturally in Montana groundwater. USGS modeling estimates about 23,270 Montana residents rely on private wells with arsenic above the EPA limit of 10 µg/L. The highest modeled exposure is in Missoula County (~4,888), Ravalli County (~3,257), Lewis and Clark County (~2,763). For nitrate, about 0.1% of Montana's ~282,000 private-well residents are modeled above the 10 mg/L limit. These are area estimates, not a test of your well — arsenic has no taste or smell, so only a certified lab test tells you your level.
This is a modeled probability for the area, not a measurement of your well. USGS statistical models estimate how many private wells in this area are likely to exceed the federal limit — they cannot tell you what is in your water. Two neighbors can have very different results. The only way to know your well is to test it.
Nitrate
0.1%
of the private-well population is modeled above the 10 mg/L nitrate limit
~246
people on wells estimated above the limit
~282,000
total residents on private wells statewide
Source: USGS machine-learning nitrate model, Ransom et al. (2022), domestic-well depth zone.
Find out what's actually in your well
Montana has one of the higher modeled arsenic exposures in the country — but that number is an area estimate, not your well. Arsenic, nitrate and coliform bacteria are the three most common private-well problems, and none of them has a taste or smell. A certified lab test checks all of them at once; the CDC and EPA recommend testing every well at least once a year.
Basic screening only. Confirm arsenic, nitrate and bacteria with a certified lab.
Find a certified drinking-water labAmazon links are affiliate links — we may earn a commission at no extra cost to you.
Arsenic
Across Montana, an estimated ~23,270 people rely on private wells with modeled arsenic above the 10 µg/L EPA limit. The highest modeled exposure is in Missoula County (~4,888 people).
Counties with the highest modeled arsenic exposure
| County | Est. well residents over arsenic limit | 90% range |
|---|---|---|
| Missoula County | ~4,888 | 3,734–6,208 |
| Ravalli County | ~3,257 | 2,543–4,280 |
| Lewis and Clark County | ~2,763 | 2,127–3,535 |
| Flathead County | ~2,633 | 2,070–3,319 |
| Gallatin County | ~1,472 | 1,090–1,974 |
| Jefferson County | ~811 | 632–1,029 |
| Yellowstone County | ~654 | 544–789 |
| Lake County | ~559 | 409–750 |
| Lincoln County | ~430 | 328–568 |
| Beaverhead County | ~415 | 318–534 |
| Richland County | ~404 | 336–483 |
| Cascade County | ~377 | 304–470 |
| Broadwater County | ~336 | 233–475 |
| Stillwater County | ~319 | 258–397 |
| Roosevelt County | ~296 | 242–357 |
| Deer Lodge County | ~237 | 161–381 |
| Carbon County | ~230 | 185–291 |
| Dawson County | ~228 | 178–299 |
| Park County | ~208 | 165–264 |
| Madison County | ~170 | 130–220 |
| Sanders County | ~159 | 120–214 |
| Big Horn County | ~158 | 129–195 |
| Prairie County | ~156 | 128–191 |
| Powell County | ~146 | 91–304 |
| Custer County | ~142 | 115–175 |
| McCone County | ~118 | 97–142 |
| Daniels County | ~118 | 94–144 |
| Mineral County | ~116 | 86–157 |
| Granite County | ~115 | 79–175 |
| Musselshell County | ~105 | 85–130 |
26 additional Montana counties have lower modeled arsenic exposure (fewer than 100 estimated well residents over the limit). Of these, 25 have measured USGS well samples: Blaine, Carter, Chouteau, Fergus, Garfield, Glacier, Golden Valley, Hill, Judith Basin, Liberty, Meagher, Petroleum, Phillips, Pondera, Powder River, Rosebud, Sheridan, Silver Bow, Sweet Grass, Teton, Toole, Treasure, Valley, Wheatland, Wibaux.
Source: USGS domestic-well arsenic model, Lombard et al. (2021); county population product (2017).
Measured well tests by county
The USGS has published laboratory results from wells in 53 Montana counties (at least 5 wells each). These are real measurements of other wells — domestic, public-supply and monitoring wells — not a test of yours. The tables rank counties by the share of sampled wells over each federal threshold.
Nitrate (as N): counties with the most wells > 10 mg/L (EPA MCL)
| County | Wells over | Median | Sampled |
|---|---|---|---|
| Valley County | 45% | 9.2 mg/L | 40 wells |
| Daniels County | 38% | 5.8 mg/L | 8 wells |
| Roosevelt County | 22% | 0.2 mg/L | 67 wells |
| Big Horn County | 14% | 0.2 mg/L | 14 wells |
| Yellowstone County | 9% | 2.2 mg/L | 11 wells |
| Lewis and Clark County | 5% | 0.8 mg/L | 57 wells |
| Jefferson County | 5% | 0.8 mg/L | 19 wells |
| Broadwater County | 5% | 1.4 mg/L | 19 wells |
| Teton County | 4% | 1.2 mg/L | 28 wells |
| Gallatin County | 2% | 0.6 mg/L | 62 wells |
Manganese: counties with the most wells > 300 µg/L (EPA health advisory)
| County | Wells over | Median | Sampled |
|---|---|---|---|
| Chouteau County | 58% | 502 µg/L | 12 wells |
| Sheridan County | 55% | 322 µg/L | 40 wells |
| Liberty County | 50% | 208 µg/L | 6 wells |
| Treasure County | 45% | 73 µg/L | 11 wells |
| Phillips County | 42% | 210 µg/L | 26 wells |
| Roosevelt County | 40% | 211 µg/L | 103 wells |
| Rosebud County | 36% | 166 µg/L | 66 wells |
| Richland County | 36% | 194 µg/L | 58 wells |
| Silver Bow County | 28% | 8 µg/L | 254 wells |
| Dawson County | 25% | 33 µg/L | 16 wells |
Source: USGS ScienceBase 5f36baa782cee144fb3873e8 (manganese, pH, arsenic) and McMahon et al. (2019), Environ. Sci. Technol. 53:29–38 (nitrate, iron); aggregated by county with Census 2020 boundaries. USGS well samples; not a test of your well.
Match the filter to your test result
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Montana well water: FAQ
Is there arsenic in well water in Montana?
Yes. USGS modeling estimates about 23,270 people in Montana use private wells with arsenic above 10 µg/L, across 56 modeled counties. Arsenic comes from natural bedrock and varies well to well, even between neighbors, so a lab test is the only way to know your own level.
Which Montana counties have the most arsenic in well water?
By modeled number of well residents over the arsenic limit: Missoula County (~4,888), Ravalli County (~3,257), Lewis and Clark County (~2,763). Counties with more private-well users rank higher, so a smaller county can still have a high share of affected wells.
How do I remove arsenic from well water in Montana?
Test first. If arsenic is present, a point-of-use reverse-osmosis system under the kitchen sink removes 90%+ of it; boiling does not remove arsenic (it concentrates it), and carbon pitchers and water softeners do not remove it either. Whole-house adsorptive arsenic media is used when levels are high. Re-test after installing and then annually.
Is nitrate a problem in Montana well water?
USGS models about 0.1% of Montana's private-well population (~246 people) above the 10 mg/L nitrate limit. Nitrate is highest in shallow wells near fertilized cropland, septic systems or livestock, and matters most for infants and pregnancy.
Do I need an acid neutralizer for my well in Montana?
Acidic well water is uncommon in Montana, but it does occur. In USGS lab results from 3,351 wells across 53 Montana counties, 5% had pH below 6.5. The most acidic sampled counties are Lincoln County (27% of 11 wells), Silver Bow County (23% of 257 wells), Ravalli County (15% of 279 wells). You only need a neutralizer if your own well tests below about 6.5 — signs include blue-green stains on fixtures and pinhole leaks in copper pipe. A calcite (or calcite/Corosex) neutralizer tank handles mildly acidic water (roughly pH 6–6.5); very acidic water usually needs soda-ash injection. Test first-draw lead and copper too, since acidic water dissolves both from plumbing.
High manganese in Montana well water — what should I do?
In USGS lab results from 2,906 wells across 53 Montana counties, 14% were above the EPA's 300 µg/L health advisory for manganese and 28% were above the 50 µg/L level where it stains fixtures and laundry black or brown. Sampled counties with the highest share over the health advisory: Chouteau County (58% of 12 wells), Sheridan County (55% of 40 wells), Liberty County (50% of 6 wells). If your own test is over 300 µg/L, don't use the water for infant formula and use another source for drinking and cooking until it is treated — high manganese is linked to neurological effects, especially in babies and children. Boiling does not remove it. For the whole house, an oxidizing filter (greensand, Birm or air-injection) handles manganese, usually alongside iron; a water softener can remove low levels. For drinking water, a point-of-use reverse osmosis system reduces manganese at the kitchen tap. Retest after treatment to confirm it worked.