Well water in Jefferson County, Montana
1in4
Testing strongly recommended
tested well samples in Jefferson County exceeded the EPA limit for radium (226 + 228) (5 pCi/L), based on public data from USGS Water Quality Portal.
2,878 test results across 12 contaminants from at least 135 wells, 1976 to 2017; chloride has the most at 352. Data refreshed September 15, 2026.
Compiled by Nick at Well Water Nerd from public records. How these pages are built
An estimated 6,647 people in Jefferson County get their drinking water from a private well, per USGS 2015 county water use. That is about 57% of residents.
Separately, the USGS national arsenic model estimates that 12% of the 6,647 people on private wells in Jefferson County draw water above 10 ug/L (the model's range is 10% to 15%), from geology and well depth rather than local tests.
What individual samples found
Each row summarizes every public sample for that contaminant inside Jefferson County. 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 |
|---|---|---|---|---|---|---|
| Radium (226 + 228) 1976 to 2010 | 81 | 96% | 1.45 | 43 | 5 pCi/L EPA limit | |
| Arsenic 1976 to 2017 | 220 | 94% | 0.595 | 19,200 | 10 ug/L EPA limit | |
| Lead 1976 to 2017 | 252 | 48% | 0 | 244 | 15 ug/L EPA limit | |
| Uranium 1976 to 2010 | 116 | 98% | 5.38 | 1,130 | 30 ug/L EPA limit | |
| Nitrate 1976 to 2010 | 200 | 84% | 0.62 | 58.3 | 10 mg/L EPA limit | |
| Fluoride 1976 to 2017 | 323 | 87% | 0.25 | 8.9 | 4 mg/L EPA limit | |
| Copper 1976 to 2017 | 251 | 67% | 0 | 2.29 | 1.3 mg/L EPA limit | |
| Chloride 1976 to 2017 | no limit | 352 | 100% | 1.49 | 161 | 250 mg/L nuisance level |
| Sodium 1976 to 2017 | no limit | 349 | 100% | 6.33 | 350 | 20 mg/L health guideline |
| Iron 1976 to 2017 | no limit | 330 | 88% | 77.5 | 127,000 | 300 ug/L nuisance level |
| Manganese 1976 to 2017 | no limit | 301 | 92% | 102 | 8,400 | 300 ug/L health guideline |
| Radon 1992 to 2010 | no limit | 103 | 100% | 3,860 | 45,000 | 2,000 pCi/L health guideline |
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 | 11%55 samples | <1%166 samples | <1%102 samples | n/a |
| Lead | 4%57 samples | 22%130 samples | 26%65 samples | n/a |
| Copper | 2%56 samples | 0%130 samples | 0%65 samples | n/a |
| Arsenic | 42%24 samples | 30%130 samples | 0%66 samples | n/a |
| Nitrate | 0%54 samples | 5%118 samples | 11%28 samples | n/a |
| Uranium | n/a | 8%75 samples | 31%36 samples | n/a |
Lead above the limit in Jefferson County rose from 22% of 130 samples in the 2000s to 26% of 65 in the 2010s.
Arsenic above the limit in Jefferson County fell from 30% of 130 samples in the 2000s to 0% of 66 in the 2010s.
Nitrate above the limit in Jefferson County rose from 5% of 118 samples in the 2000s to 11% of 28 in the 2010s.
Uranium above the limit in Jefferson County rose from 8% of 75 samples in the 2000s to 31% of 36 in the 2010s.
Fluoride, copper have held within a few points between the two most recent periods with enough samples.
Closer than the county
See the sampled wells near your address
The same records, drawn around one point: which wells within a few miles were sampled and what they found, how deep the wells next to you were drilled, and any leaking tank or cleanup site within a mile.
Check my addressWhat this means
The contaminants most often above their limit here are radium (226 + 228) (23% of 81 samples), arsenic (22% of 220 samples), and lead (19% of 252 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, Iron, and Manganese are widespread in local groundwater. They are mainly a staining and taste problem, but high manganese also carries a health advisory for infants.
The median radon reading (3,860 pCi/L) is above the 2,000 pCi/L level at which treatment is recommended. Radon in water raises indoor air radon when it is used for showers and laundry.
What to test for in Jefferson County
- 1Radium (226 + 228)23% of 81 local samples were above the limit.
- 2Arsenic22% of 220 local samples were above the limit.
- 3Lead19% of 252 local samples were above the limit.
- 4Uranium15% of 116 local samples were above the limit.
- 5Nitrate5% of 200 local samples were above the limit.
- 6Fluoride2% of 323 local samples were above the limit.
- 7Copper<1% of 251 local samples were above the limit.
- 8Volatile organic compounds (VOCs)56 leaking underground storage tank sites on record locally.
Test your Jefferson County well for radium (226 + 228), arsenic and lead
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 Jefferson County numbers, $29. Every line explained, ranked by what matters, with the fix for each.
The rock under Jefferson County
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 county 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 Jefferson County
Montana DEQ records 56 leaking underground storage tank sites in Jefferson County, among 71 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.
- 56
- leaking underground storage tank sitesGasoline and heating oil that leaked from tanks move through groundwater as benzene, MTBE and other volatile organic compounds.
- 7
- registered underground storage tank sitesRegistered tanks are regulated and inspected; they matter most when they are old or near a shallow well.
- 3
- lined landfills and transfer facilities
- 5
- other solid waste facilities
Treatment that fits the local profile
Radium (226 + 228)
Water softener (cation exchange); Point-of-use reverse osmosis
Arsenic
Point-of-use reverse osmosis; Adsorptive media (iron-based) whole-house; Anion exchange
Lead
Point-of-use reverse osmosis; Certified lead-removal carbon filter; Neutralizing filter to raise pH and stop leaching
Uranium
Point-of-use reverse osmosis; Anion exchange; Whole-house anion exchange
Nitrate
Point-of-use reverse osmosis; Anion exchange; Distillation
Systems we would look at first
Some links below are affiliate links. If you buy through one, the merchant pays us a commission at no extra cost to you. It does not change which products we list or what the data says.
For radium (226 + 228)
Under-sink reverse osmosis system
under-sink or countertop, about $300
Look for NSF/ANSI 58 certification listing the contaminant you need removed.
For arsenic
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.
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Common questions
Is there arsenic in well water in Jefferson County?
Arsenic was detected in 94% of 220 public well samples in Jefferson County, and 22% 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 Jefferson County?
The median radon level across 103 public samples in Jefferson County was 3,860 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 Jefferson County?
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 near Jefferson County
Closest first. Groundwater does not stop at a line on a map, so the next county over is a fair second opinion.
Want the numbers themselves? Download the Montana dataset as CSV, every county and town, with the sources named.
Sources
- USGS Water Quality Portal: 2,878 samples in Jefferson County. 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 6,647 people on private wells in Jefferson County 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: 71 regulated sites located in Jefferson County. 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.