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.
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.
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).
Source: USGS domestic-well arsenic model, Lombard et al. (2021); county population product (2017).
Treating arsenic in Montana
Arsenic is not removed by carbon pitchers or faucet filters. Reverse osmosis is the treatment that brings it below the federal limit at your tap.
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