Yellowstone County, Montana: Private Well Water Quality
USGS models estimate the private-well arsenic exposure across Yellowstone County, and USGS laboratory samples from at least 55 wells in the county show measured nitrate, manganese, iron and pH. The arsenic figure is an area estimate; the measurements are of other wells — neither is a test of your own well.
Quick answer: Arsenic is the headline concern for Yellowstone County well water: USGS modeling estimates about 654 residents on private wells with arsenic above the EPA limit of 10 µg/L. Of the 51 Yellowstone County wells the USGS actually tested for arsenic, 4% were over the limit (median 2 µg/L). The most common measured contaminant in local wells is nitrate: 9% of 11 sampled wells were > 10 mg/L. These figures describe wells across the Montana county — not your tap — so the only way to know your own water is a certified lab test.
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.
~654
people estimated to rely on private wells with modeled arsenic above the 10 µg/L EPA limit
544–789
90% confidence range for that estimate
9% of wells
sampled by USGS in the county measured > 10 mg/L for nitrate (11 wells)
Find out what's actually in your well
USGS models flag arsenic in Yellowstone County, but two wells on the same road can test very differently. 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.
Measured well tests in Yellowstone County
Unlike the arsenic estimate above, these are real laboratory results: the USGS has published chemistry from at least 55 wells in Yellowstone County, sampled 2005–2013. They describe other wells in the county — domestic, public-supply and monitoring wells — not your well.
| Parameter | Median | Wells over threshold | What the threshold means |
|---|---|---|---|
| Nitrate (as N)11 wells | 2.2 mg/L90th pct: 5 mg/L | 9%Some | > 10 mg/L (EPA MCL). Federal drinking-water limit. Nitrate from fertilizer, manure and septic systems; unsafe for infants and in pregnancy. |
| Manganese52 wells | 5 µg/L90th pct: 242 µg/L | 8%Some | > 300 µg/L (EPA health advisory). Lifetime health advisory; above 50 µg/L it already stains fixtures and tastes metallic. Neurological concern for infants. |
| Iron11 wells | 0 µg/L90th pct: 11 µg/L | 0%Low | > 300 µg/L (EPA secondary standard). Aesthetic standard — rust-colored staining, metallic taste, clogged fixtures. Not a health limit. |
| pH55 wells | 7.5 90th pct: 8.1 | 4%Low | outside 6.5–8.5 (EPA secondary range). Low pH is corrosive and leaches lead and copper from plumbing; high pH causes scale and bitter taste. |
What the samples show
- Nitrate (as N): Nitrate shows up in some wells: 9% of sampled wells were over the limit, and the top 10% reached 5 mg/L.
- Manganese: Manganese is common at nuisance levels: 25% of sampled wells were over the 50 µg/L staining standard; 8% over the health advisory.
- Iron: Iron was low in most sampled wells (median 0 µg/L; 0% over the staining standard).
- pH: pH is in the normal range for most sampled wells (median 7.5).
- Arsenic (measured, 51 wells): Measured arsenic was low in most sampled wells (median 2 µg/L; 4% over the limit).
What to do about the biggest finding (nitrate): Test for nitrate; if it is over 10 mg/L, use reverse osmosis or an anion-exchange unit for drinking water — boiling makes nitrate worse. Read more: Nitrate in well water.
Source: USGS well samples aggregated to Yellowstone County, Montana — manganese, pH and arsenic from USGS ScienceBase 5f36baa782cee144fb3873e8 (site medians, 1988–2019); nitrate and iron from McMahon et al. (2019), Environ. Sci. Technol. 53:29–38 (NWIS wells). County assignment by Census 2020 boundaries. Statistics shown only where ≥ 5 wells were sampled. USGS well samples; not a test of your well.
Arsenic in Yellowstone County drinking water: what the estimate means
Arsenic occurs naturally in bedrock and can dissolve into groundwater, so it shows up in private wells with no color, taste or smell. The USGS estimate above combines a statistical model of arsenic probability with the number of people on domestic wells in Yellowstone County. A higher number means more of your neighbors are likely drinking water over the federal limit — but your own well could be higher or lower. Long-term arsenic exposure is linked to skin, bladder and lung cancers, so the EPA sets the limit at 10 µg/L.
Because arsenic is tasteless and invisible, a certified lab test is the only way to detect it — and unlike bacteria, boiling does not remove arsenic (it slightly concentrates it as water evaporates). A lab reports total arsenic; if yours is elevated, reverse osmosis or an adsorptive arsenic filter reduces it, while carbon pitchers and water softeners do not.
Statewide, Montana also has a modeled nitrate exceedance of 0.1% of private-well residents — worth testing for at the same time.
What to do
- Test your well for arsenic, nitrate, manganese and iron — it's the only way to know your actual level.
- If arsenic or nitrate is present, install a reverse-osmosis system — carbon pitchers do not remove either.
- For manganese or iron staining, a whole-house oxidizing filter or softener is the fix; RO for the drinking tap.
- Re-test annually; well chemistry changes over time.
Yellowstone County well water: frequently asked questions
Is there arsenic in Yellowstone County drinking water?
Arsenic occurs naturally in the bedrock under Yellowstone County, Montana, and dissolves into some private wells with no color, taste or smell. USGS modeling estimates about 654 residents (range 544–789) on private wells with arsenic above the EPA limit of 10 µg/L. USGS also sampled 51 wells in the county for arsenic directly: the median was 2 µg/L and 4% were over the limit. These are area and other-well figures, not a test of your own well — the only way to know your arsenic level is a certified lab test of your tap.
Yellowstone County, MT water contamination: what have well tests found?
USGS laboratory samples from at least 55 wells in Yellowstone County (2005–2013) measured: nitrate 9% of 11 wells > 10 mg/L; arsenic 4% of 51 wells > 10 µg/L; manganese 8% of 52 wells > 300 µg/L; iron 0% of 11 wells > 300 µg/L; pH 4% of 55 wells outside 6.5–8.5. The biggest measured issue is nitrate. These are tests of other wells in the county, not a public water system report — private wells are unregulated, so your own well needs its own lab test for arsenic, nitrate, manganese and iron.
How do I remove arsenic from Yellowstone County well water?
Test first so you know your level and form of arsenic. If arsenic is present, a point-of-use reverse-osmosis system under the kitchen sink removes 90%+ of it; carbon pitchers and standard "filter" jugs do not remove arsenic or nitrate. Whole-house treatment (adsorptive arsenic media) is used when the level is high. Re-test after installing and then annually.
Match the filter to your test result
Carbon pitchers and faucet filters do not remove arsenic or nitrate. Reverse osmosis is the point-of-use fix; a whole-house system handles the rest of the home.
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