Well water in Montana
1in9
Testing recommended
tested well samples in Montana exceeded the EPA limit for nitrate (10 mg/L), based on public data from USGS Water Quality Portal.
122,778 test results across 15 contaminants from at least 7,583 wells, 1908 to 2025; chloride has the most at 29,009. Data refreshed September 15, 2026.
Compiled by Nick at Well Water Nerd from public records. How these pages are built
An estimated 304,630 people in Montana get their drinking water from a private well, per USGS 2015 county water use.
Separately, the USGS national arsenic model estimates that 8% of the 304,630 people on private wells in Montana draw water above 10 ug/L (the model's range is 6% to 10%), from geology and well depth rather than local tests.
What individual samples found
Each row summarizes every public sample for that contaminant inside Montana. Above limit means above the EPA maximum contaminant level; where EPA has no enforceable limit, the row shows a health guideline or nuisance level instead.
| Contaminant | Above limit | Samples | Detected | Median | Highest | Limit |
|---|---|---|---|---|---|---|
| Nitrate 1910 to 2024 | 8,286 | 90% | 0.605 | 1,770 | 10 mg/L EPA limit | |
| Arsenic 1960 to 2025 | 5,666 | 52% | 0.2 | 19,200 | 10 ug/L EPA limit | |
| Uranium 1975 to 2024 | 530 | 89% | 2.17 | 1,130 | 30 ug/L EPA limit | |
| Radium (226 + 228) 1975 to 2021 | 1,885 | 99% | 1.6 | 44 | 5 pCi/L EPA limit | |
| Lead 1960 to 2025 | 6,939 | 24% | 0 | 50,000 | 15 ug/L EPA limit | |
| Fluoride 1947 to 2024 | 11,094 | 95% | 0.33 | 20 | 4 mg/L EPA limit | |
| Copper 1971 to 2025 | 5,265 | 44% | 0 | 142 | 1.3 mg/L EPA limit | |
| E. coli 2001 to 2021 | 134 | 0% | 0 | 0 | 0 MPN/100mL EPA limit | |
| Chloride 1908 to 2025 | no limit | 29,009 | 100% | 26 | 200,000 | 250 mg/L nuisance level |
| Sodium 1910 to 2025 | no limit | 26,309 | 100% | 256 | 114,000 | 20 mg/L health guideline |
| Manganese 1960 to 2025 | no limit | 14,772 | 82% | 65 | 130,000 | 300 ug/L health guideline |
| Iron 1916 to 2025 | no limit | 12,546 | 74% | 63.5 | 664,000 | 300 ug/L nuisance level |
| Radon 1992 to 2014 | no limit | 339 | 100% | 1,120 | 45,000 | 2,000 pCi/L health guideline |
Sampled fewer than 10 times, too few to summarize: PFOA (2), PFOS (2).
Over time
Share of samples above the limit by period. A cell needs ten samples to show; a comparison needs twenty in each period. Later periods sample fewer wells, and often the ones already known to have a problem.
| Contaminant | before 2000 | 2000s | 2010s | 2020s |
|---|---|---|---|---|
| Fluoride | 3%7,609 samples | 4%419 samples | 2%2,446 samples | 0%620 samples |
| Nitrate | 10%6,849 samples | 1%572 samples | 10%592 samples | 54%273 samples |
| Lead | 8%2,616 samples | 6%606 samples | 1%2,877 samples | <1%840 samples |
| Arsenic | 15%1,317 samples | 19%603 samples | 6%2,935 samples | 10%811 samples |
| Copper | <1%2,654 samples | 0%576 samples | <1%1,618 samples | 21%417 samples |
| Radium (226 + 228) | 14%44 samples | 4%48 samples | 7%1,248 samples | <1%545 samples |
Nitrate above the limit in Montana rose from 10% of 592 samples in the 2010s to 54% of 273 in the 2020s.
Copper above the limit in Montana rose from <1% of 1618 samples in the 2010s to 21% of 417 in the 2020s.
Radium (226 + 228) above the limit in Montana fell from 7% of 1248 samples in the 2010s to <1% of 545 in the 2020s.
Fluoride, lead, arsenic have held within a few points between the two most recent periods with enough samples.
What this means
The contaminants most often above their limit here are nitrate (11% of 8,286 samples), arsenic (10% of 5,666 samples), and uranium (9% of 530 samples). A well next door can differ sharply from yours because bedrock fractures, well depth and plumbing all matter, so these rates describe the area, not your tap.
Chloride, Manganese, and Iron are widespread in local groundwater. They are mainly a staining and taste problem, but high manganese also carries a health advisory for infants.
What to test for in Montana
- 1Nitrate11% of 8,286 local samples were above the limit.
- 2Arsenic10% of 5,666 local samples were above the limit.
- 3Uranium9% of 530 local samples were above the limit.
- 4Radium (226 + 228)5% of 1,885 local samples were above the limit.
- 5Lead4% of 6,939 local samples were above the limit.
- 6Fluoride3% of 11,094 local samples were above the limit.
- 7Copper2% of 5,265 local samples were above the limit.
- 8Volatile organic compounds (VOCs)4,516 leaking underground storage tank sites on record locally.
Test your Montana well for nitrate, arsenic and uranium
Certified lab analysis of your own tap water, bottles and prepaid return shipping included. The Essential well panel covers arsenic, uranium, nitrate, lead, copper, manganese, iron, sodium and bacteria; radon and PFAS are add-ons.
Order the Tap Score well water test kit, from $209Already tested? Upload the report
Just want bacteria checked first? Tap Score coliform and E. coli test, $99. Coliform and E. coli are the tests every well needs every year.
Want a quick look before paying for a lab? Home well water test strips on Amazon check hardness, iron, pH and nitrate in minutes. They are a screen, not a result: arsenic, bacteria, uranium and PFAS need a certified lab.
Affiliate links: the merchant pays us a commission and the price is the same for you. It does not change what the data above says. As an Amazon Associate we earn from qualifying purchases.
Where to get a well tested in Montana: certified labs, state programs and prices.
Already have results back? Get your lab report read against the Montana numbers, $29. Every line explained, ranked by what matters, with the fix for each.
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The numbers above, which tests to order and where, and the treatment that fits the local profile, in one email.
The rock under Montana
Not a principal aquifer
The USGS map places this area outside any principal aquifer, which usually means wells here draw from fractured bedrock or thin local deposits rather than a large regional aquifer. Bedrock wells of that kind are where arsenic, radon and uranium turn up, and yields vary from well to well.
Classification from the USGS Principal Aquifers of the United States (national map, 2003 edition, GIS release 2015), read at the state centroid. A single well can sit in a different unit, especially near a boundary.
Official help in Montana
Two places to start that are not selling anything: the state program that answers private well questions, and its list of labs certified to run drinking water tests. A certified lab is the difference between a result you can act on and a number.
- Water Resources, Montana DEQ
- Water Laboratory Certification Program, Montana DPHHS
Contamination sources on record in Montana
Montana DEQ records 4,516 leaking underground storage tank sites in Montana, among 5,949 regulated sites in total. Groundwater does not follow property lines, so a well within a few hundred feet of one of these is worth testing for what that site could release.
- 4,516
- leaking underground storage tank sitesGasoline and heating oil that leaked from tanks move through groundwater as benzene, MTBE and other volatile organic compounds.
- 1,133
- registered underground storage tank sitesRegistered tanks are regulated and inspected; they matter most when they are old or near a shallow well.
- 73
- lined landfills and transfer facilities
- 227
- other solid waste facilities
Treatment that fits the local profile
Nitrate
Point-of-use reverse osmosis; Anion exchange; Distillation
Arsenic
Point-of-use reverse osmosis; Adsorptive media (iron-based) whole-house; Anion exchange
Uranium
Point-of-use reverse osmosis; Anion exchange; Whole-house anion exchange
Radium (226 + 228)
Water softener (cation exchange); Point-of-use reverse osmosis
Lead
Point-of-use reverse osmosis; Certified lead-removal carbon filter; Neutralizing filter to raise pH and stop leaching
Systems we would look at first
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For nitrate
Under-sink reverse osmosis system
under-sink or countertop, about $300
Look for NSF/ANSI 58 certification listing the contaminant you need removed.
For nitrate
Aquasana SmartFlow reverse osmosis
under-sink or countertop, about $225
Certified to NSF/ANSI 58 and 53 to reduce arsenic (96%, pentavalent), fluoride (90%), lead (99%), nitrate, nitrite and PFOA/PFOS at the kitchen tap, with a remineralizer.
For arsenic
Whole-house arsenic and uranium filter
whole-house, at the point of entry, about $2,500
Adsorptive iron-based media; sized to your well yield.
For lead
Aquasana Claryum 3-Stage under-sink filter
under-sink or countertop, about $175
Certified to NSF/ANSI 53 to reduce 99% of lead and 99.7% of PFOA and PFOS at the tap. Not a fix for arsenic, nitrate or bacteria.
For lead
Acid neutralizing filter
whole-house, at the point of entry, about $1,200
Raises pH so plumbing stops leaching lead and copper.
Common questions
Is there arsenic in well water in Montana?
Arsenic was detected in 52% of 5,666 public well samples in Montana, and 10% were above the EPA limit of 10 ug/L. Arsenic has no taste or smell; a lab test is the only way to know your level.
How high is radon in well water in Montana?
The median radon level across 339 public samples in Montana was 1,120 pCi/L, with a high of 45,000 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.
How often should I test a private well in Montana?
Test for bacteria and nitrate every year. Test once for arsenic, uranium, radon, fluoride, manganese and PFAS, then again every three to five years or after any change in taste, color, or plumbing.
Counties in Montana
- Beaverhead14% arsenic
- Big Horn5% lead
- Blaine3% lead
- Broadwater0% fluoride
- Carbon4% nitrate
- Carter7% nitrate
- Cascade25% nitrate
- Chouteau40% nitrate
- Custer13% fluoride
- Daniels42% nitrate
- Dawson14% nitrate
- Deer Lodge17% arsenic
- Fallon2% fluoride
- Fergus9% nitrate
- Flathead8% nitrate
- Gallatin52% arsenic
- Garfield10% nitrate
- Glacier6% arsenic
- Golden ValleyUSGS estimate
- GraniteUSGS estimate
- Hill14% nitrate
- Jefferson23% radium (226 + 228)
- Judith Basin6% nitrate
- Lake15% fluoride
- Lewis and Clark13% uranium
- Liberty15% nitrate
- Lincoln4% nitrate
- Madison23% arsenic
- McCone13% nitrate
- MeagherUSGS estimate
- MineralUSGS estimate
- Missoula2% nitrate
- Musselshell6% nitrate
- ParkUSGS estimate
- Petroleum8% fluoride
- Phillips13% arsenic
- PonderaUSGS estimate
- Powder River6% arsenic
- Powell28% arsenic
- Prairie3% nitrate
- Ravalli4% arsenic
- Richland19% fluoride
- Roosevelt46% arsenic
- Rosebud5% lead
- Sanders14% fluoride
- Sheridan16% arsenic
- Silver Bow23% arsenic
- Stillwater33% nitrate
- Sweet Grass4% fluoride
- Teton20% nitrate
- Toole12% nitrate
- Treasure12% nitrate
- Valley49% nitrate
- WheatlandUSGS estimate
- Wibaux3% fluoride
- Yellowstone7% nitrate
Want the numbers themselves? Download the Montana dataset as CSV, every county and town, with the sources named.
Sources
- USGS Water Quality Portal: 122,778 samples in Montana. Data from the Water Quality Portal, a service of USGS, EPA and USDA; groundwater samples collected by USGS and cooperators. Retrieved September 3, 2026.
- USGS national arsenic model (Ayotte 2017): estimated share of 304,630 people on private wells in Montana above 10 ug/L of arsenic, from geology, well depth and land use. County estimates of domestic well users with arsenic above 10 ug/L from the USGS national logistic regression model (Ayotte, Medalie and Qi, 2017), with private well population from the USGS 2015 county water-use compilation (Dieter and others, 2018). Retrieved September 15, 2026.
- Montana DEQ regulated sites: 5,949 regulated sites located in Montana. Petroleum tank releases, underground storage tank facilities and licensed solid waste facilities from the Montana Department of Environmental Quality. Retrieved September 18, 2026.
Non-detects count as samples and are treated as zero when computing medians. Exactly at the limit counts as not above it. A sample belongs to the town containing the well's reported coordinates.