Montana well water by county: arsenic and nitrate
Modelled arsenic and nitrate for private-well users in every Montana county, what to test for, and where to find a certified lab. Private wells aren’t covered by the Safe Drinking Water Act.
All 56 counties
- Flathead CountyWell users 32,700Arsenic about 12 in 100Nitrate 0.5 mg/L
- Missoula CountyWell users 26,900Arsenic about 8 in 100Nitrate 0.3 mg/L
- Ravalli CountyWell users 25,600Arsenic about 7 in 100Nitrate 0.4 mg/L
- Lewis and Clark CountyWell users 20,200Arsenic about 12 in 100Nitrate 0.8 mg/L
- Gallatin CountyWell users 19,700Arsenic about 12 in 100Nitrate 1.4 mg/L
- Yellowstone CountyWell users 16,300Arsenic about 8 in 100Nitrate 1.1 mg/L
- Lake CountyWell users 13,000Arsenic about 15 in 100Nitrate 0.4 mg/L
- Lincoln CountyWell users 9,500Arsenic about 8 in 100Nitrate 0.1 mg/L
- Cascade CountyWell users 8,700Arsenic about 5 in 100Nitrate 0.9 mg/L
- Jefferson CountyWell users 5,900Arsenic about 8 in 100Nitrate 0.5 mg/L
- Park CountyWell users 5,600Arsenic about 7 in 100Nitrate 0.4 mg/L
- Sanders CountyWell users 5,500Arsenic about 13 in 100Nitrate 0.1 mg/L
- Stillwater CountyWell users 5,100Arsenic about 6 in 100Nitrate 0.7 mg/L
- Hill CountyWell users 5,000Arsenic about 6 in 100Nitrate 0.9 mg/L
- Carbon CountyWell users 4,900Arsenic about 5 in 100Nitrate 0.8 mg/L
- Big Horn CountyWell users 4,900Arsenic about 6 in 100Nitrate 0.7 mg/L
- Madison CountyWell users 4,500Arsenic about 6 in 100Nitrate 0.5 mg/L
- Richland CountyWell users 4,400Arsenic about 5 in 100Nitrate 0.8 mg/L
- Beaverhead CountyWell users 4,100Arsenic about 6 in 100Nitrate 0.5 mg/L
- Silver Bow CountyWell users 4,000Arsenic about 6 in 100Nitrate 0.5 mg/L
- Fergus CountyWell users 3,200Arsenic about 4 in 100Nitrate 0.5 mg/L
- Glacier CountyWell users 3,200Arsenic about 6 in 100Nitrate 0.5 mg/L
- Broadwater CountyWell users 3,100Arsenic about 11 in 100Nitrate 0.6 mg/L
- Teton CountyWell users 3,000Arsenic about 5 in 100Nitrate 1.8 mg/L
- Rosebud CountyWell users 2,900Arsenic about 5 in 100Nitrate 0.4 mg/L
- Mineral CountyWell users 2,800Arsenic about 8 in 100Nitrate 0.0 mg/L
- Deer Lodge CountyWell users 2,700Arsenic about 7 in 100Nitrate 0.3 mg/L
- Custer CountyWell users 2,500Arsenic about 6 in 100Nitrate 0.1 mg/L
- Dawson CountyWell users 2,400Arsenic about 5 in 100Nitrate 0.6 mg/L
- Musselshell CountyWell users 2,200Arsenic about 5 in 100Nitrate 0.2 mg/L
- Roosevelt CountyWell users 2,200Arsenic about 6 in 100Nitrate 1.7 mg/L
- Powell CountyWell users 2,200Arsenic about 8 in 100Nitrate 0.4 mg/L
- Blaine CountyWell users 1,900Arsenic about 5 in 100Nitrate 0.4 mg/L
- Valley CountyWell users 1,900Arsenic about 14 in 100Nitrate 1.9 mg/L
- Granite CountyWell users 1,700Arsenic about 9 in 100Nitrate 0.2 mg/L
- Chouteau CountyWell users 1,700Arsenic about 6 in 100Nitrate 0.8 mg/L
- Phillips CountyWell users 1,500Arsenic about 6 in 100Nitrate 0.4 mg/L
- Sweet Grass CountyWell users 1,400Arsenic about 8 in 100Nitrate 0.4 mg/L
- Pondera CountyWell users 1,300Arsenic about 5 in 100Nitrate 0.7 mg/L
- Judith Basin CountyWell users 1,300Arsenic about 3 in 100Nitrate 0.6 mg/L
- Powder River CountyWell users 1,100Arsenic about 5 in 100Nitrate 0.3 mg/L
- Sheridan CountyWell users 1,000Arsenic about 5 in 100Nitrate 1.3 mg/L
- Wheatland CountyWell users 1,000Arsenic about 3 in 100Nitrate 0.6 mg/L
- McCone CountyWell users 1,000Arsenic about 7 in 100Nitrate 0.4 mg/L
- Prairie CountyWell users 900Arsenic about 5 in 100Nitrate 0.2 mg/L
- Garfield CountyWell users 770Arsenic about 6 in 100Nitrate 0.1 mg/L
- Fallon CountyWell users 720Arsenic about 5 in 100Nitrate 0.4 mg/L
- Meagher CountyWell users 700Arsenic about 5 in 100Nitrate 0.2 mg/L
- Carter CountyWell users 670Arsenic about 5 in 100Nitrate 0.4 mg/L
- Daniels CountyWell users 620Arsenic about 5 in 100Nitrate 2.7 mg/L
- Liberty CountyWell users 570Arsenic about 8 in 100Nitrate 0.8 mg/L
- Golden Valley CountyWell users 530Arsenic about 4 in 100Nitrate 0.5 mg/L
- Wibaux CountyWell users 420Arsenic about 4 in 100Nitrate 0.4 mg/L
- Treasure CountyWell users 310Arsenic about 5 in 100Nitrate 0.3 mg/L
- Petroleum CountyWell users 260Arsenic about 5 in 100Nitrate 0.1 mg/L
- Toole CountyWell users 190Arsenic about 7 in 100Nitrate 1.5 mg/L
Each mark = 1 in 100 well users the model places above 10 µg/L arsenic (scale 0–10)10 mg/L nitrate: the federal limit (MCL) for public systems
- These figures describe the area, not your well. Only a lab test shows what’s in your water.
- CDC says the Well Water Index figures “do not describe risk to individual wells.” (CDC Tracking Network)
- The arsenic model is better at spotting low-arsenic areas than at confirming high ones.
- Well-user counts are 2010 USGS estimates; tract values are averages over each census tract.
- Total coliform bacteriaEvery year
- NitrateEvery year
- Total dissolved solidsEvery year
- pHEvery year
- ArsenicAsk your local health department
EPA: Private Drinking Water Wells · CDC: Guidelines for Testing Well Water
Use a state-certified lab
Montana Department of Public Health & Human Services lists the laboratories certified to test drinking water in Montana.
Screening tells you if something is worth a closer look
A lab result is the number you can act on.
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Lab-certified results
ETR Labs Basic Water Quality Test Kit
ETR Laboratories
50+ substances, including lead, copper, arsenic, nitrate, hardness and coliform bacteria
- Analyzed by Environmental Testing and Research Laboratories (ETR Labs), Leominster, MA · ISO/IEC 17025:2017, ANAB certificate AT-2638
- Turnaround: 2–4 business days after the lab receives it
What it can't tell you: It does not test PFAS, and not every result on the panel falls under the lab's accredited scope.
Screening: quick check, not lab-grade
Watersafe Drinking Water Test Kit
Silver Lake Research
Lead, bacteria, nitrate, copper, iron, chlorine, hardness and pH
- Turnaround: Minutes; the bacteria test can take up to 48 hours
What it can't tell you: Each test gives a yes/no or a rough range, not a lab number, and it cannot detect PFAS or arsenic.
A mail-in kit’s lab is accredited (ISO/IEC 17025) but may not be certified by your state. For a home sale, a loan, or a health-department follow-up, use a state-certified lab.
Montana well water
Sources: CDC Well Water Index (2024) · USGS arsenic and nitrate models · USGS domestic well populations · EPA and CDC testing guidance. Data last updated 2026-10-08. All sources →
How many people use private wells in Montana?
About 282,300 people across 56 Montana counties, by the USGS 2010 estimate. USGS models place about 24,300 of them in areas where wells are likely to be above the 10 µg/L arsenic limit. These are area estimates, not well results.
Are private wells regulated in Montana?
The quality and safety of drinking water from private domestic wells are not regulated by the Federal Government under the Safe Drinking Water Act nor by most state governments and laws. (EPA) Testing and treatment are the owner's responsibility.
Where can I get my well tested in Montana?
Montana Department of Public Health & Human Services keeps the list of laboratories certified to test drinking water in Montana. Only use laboratories that are certified to do drinking water testing. (EPA)