Idaho Private Well Water Risk: Arsenic & Nitrate
These figures are USGS statistical estimates for private (domestic) wells in Idaho. They describe the area's modeled risk — not any single well.
Quick answer: Yes — arsenic occurs naturally in Idaho groundwater. USGS modeling estimates about 47,041 Idaho residents rely on private wells with arsenic above the EPA limit of 10 µg/L. The highest modeled exposure is in Canyon County (~14,043), Twin Falls County (~7,858), Payette County (~2,477). For nitrate, about 3.7% of Idaho's ~316,000 private-well residents are modeled above the 10 mg/L limit. These are area estimates, not a test of your well — arsenic has no taste or smell, so only a certified lab test tells you your level.
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
3.7%
of the private-well population is modeled above the 10 mg/L nitrate limit
~11,800
people on wells estimated above the limit
~316,000
total residents on private wells statewide
Source: USGS machine-learning nitrate model, Ransom et al. (2022), domestic-well depth zone.
Find out what's actually in your well
Idaho has one of the higher modeled arsenic exposures in the country — but that number is an area estimate, not your well. 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.
Arsenic
Across Idaho, an estimated ~47,041 people rely on private wells with modeled arsenic above the 10 µg/L EPA limit. The highest modeled exposure is in Canyon County (~14,043 people).
Counties with the highest modeled arsenic exposure
| County | Est. well residents over arsenic limit | 90% range |
|---|---|---|
| Canyon County | ~14,043 | 11,996–16,346 |
| Twin Falls County | ~7,858 | 6,339–9,411 |
| Payette County | ~2,477 | 2,134–2,828 |
| Gem County | ~2,130 | 1,840–2,452 |
| Jefferson County | ~1,904 | 1,531–2,342 |
| Bingham County | ~1,700 | 1,352–2,133 |
| Bonner County | ~1,664 | 1,213–2,273 |
| Madison County | ~1,647 | 1,365–1,986 |
| Ada County | ~1,475 | 1,244–1,735 |
| Owyhee County | ~1,407 | 1,175–1,667 |
| Bonneville County | ~1,151 | 930–1,423 |
| Kootenai County | ~1,042 | 776–1,384 |
| Cassia County | ~846 | 647–1,083 |
| Gooding County | ~745 | 594–934 |
| Jerome County | ~691 | 552–877 |
| Minidoka County | ~618 | 474–811 |
| Bannock County | ~616 | 466–803 |
| Fremont County | ~531 | 411–698 |
| Elmore County | ~526 | 432–640 |
| Washington County | ~485 | 400–580 |
| Valley County | ~358 | 275–468 |
| Idaho County | ~335 | 246–456 |
| Latah County | ~322 | 249–413 |
| Teton County | ~315 | 245–415 |
| Boise County | ~264 | 192–363 |
| Power County | ~256 | 194–331 |
| Lemhi County | ~243 | 169–347 |
| Boundary County | ~222 | 172–285 |
| Franklin County | ~177 | 129–235 |
| Oneida County | ~129 | 93–178 |
| Custer County | ~121 | 91–162 |
| Shoshone County | ~117 | 87–151 |
| Nez Perce County | ~110 | 84–141 |
11 additional Idaho counties have lower modeled arsenic exposure (fewer than 100 estimated well residents over the limit). Of these, 11 have measured USGS well samples: Adams, Bear Lake, Benewah, Blaine, Butte, Camas, Caribou, Clark, Clearwater, Lewis, Lincoln.
Source: USGS domestic-well arsenic model, Lombard et al. (2021); county population product (2017).
Measured well tests by county
The USGS has published laboratory results from wells in 44 Idaho counties (at least 5 wells each). These are real measurements of other wells — domestic, public-supply and monitoring wells — not a test of yours. The tables rank counties by the share of sampled wells over each federal threshold.
Nitrate (as N): counties with the most wells > 10 mg/L (EPA MCL)
| County | Wells over | Median | Sampled |
|---|---|---|---|
| Washington County | 27% | 1.3 mg/L | 44 wells |
| Lewis County | 17% | 1.6 mg/L | 18 wells |
| Owyhee County | 12% | 0.5 mg/L | 50 wells |
| Canyon County | 9% | 1.9 mg/L | 231 wells |
| Minidoka County | 9% | 3.6 mg/L | 110 wells |
| Nez Perce County | 9% | 0.8 mg/L | 35 wells |
| Elmore County | 8% | 1.7 mg/L | 106 wells |
| Fremont County | 7% | 1.4 mg/L | 58 wells |
| Bingham County | 6% | 1.5 mg/L | 94 wells |
| Teton County | 6% | 1 mg/L | 18 wells |
Manganese: counties with the most wells > 300 µg/L (EPA health advisory)
| County | Wells over | Median | Sampled |
|---|---|---|---|
| Camas County | 35% | 146 µg/L | 20 wells |
| Valley County | 20% | 78 µg/L | 46 wells |
| Owyhee County | 17% | 58 µg/L | 96 wells |
| Washington County | 13% | 15 µg/L | 68 wells |
| Adams County | 13% | 1 µg/L | 31 wells |
| Kootenai County | 9% | 2 µg/L | 103 wells |
| Gem County | 9% | 3 µg/L | 86 wells |
| Bear Lake County | 8% | 1 µg/L | 48 wells |
| Payette County | 7% | 1 µg/L | 75 wells |
| Caribou County | 7% | 1 µg/L | 74 wells |
Source: USGS ScienceBase 5f36baa782cee144fb3873e8 (manganese, pH, arsenic) and McMahon et al. (2019), Environ. Sci. Technol. 53:29–38 (nitrate, iron); aggregated by county with Census 2020 boundaries. USGS well samples; not a test of your well.
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Idaho well water: FAQ
Is there arsenic in well water in Idaho?
Yes. USGS modeling estimates about 47,041 people in Idaho use private wells with arsenic above 10 µg/L, across 44 modeled counties. Arsenic comes from natural bedrock and varies well to well, even between neighbors, so a lab test is the only way to know your own level.
Which Idaho counties have the most arsenic in well water?
By modeled number of well residents over the arsenic limit: Canyon County (~14,043), Twin Falls County (~7,858), Payette County (~2,477). Counties with more private-well users rank higher, so a smaller county can still have a high share of affected wells.
How do I remove arsenic from well water in Idaho?
Test first. If arsenic is present, a point-of-use reverse-osmosis system under the kitchen sink removes 90%+ of it; boiling does not remove arsenic (it concentrates it), and carbon pitchers and water softeners do not remove it either. Whole-house adsorptive arsenic media is used when levels are high. Re-test after installing and then annually.
Is nitrate a problem in Idaho well water?
USGS models about 3.7% of Idaho's private-well population (~11,800 people) above the 10 mg/L nitrate limit. Nitrate is highest in shallow wells near fertilized cropland, septic systems or livestock, and matters most for infants and pregnancy.
Do I need an acid neutralizer for my well in Idaho?
Acidic well water is uncommon in Idaho, but it does occur. In USGS lab results from 6,091 wells across 44 Idaho counties, 2% had pH below 6.5. The most acidic sampled counties are Valley County (38% of 50 wells), Shoshone County (31% of 29 wells), Boundary County (26% of 35 wells). You only need a neutralizer if your own well tests below about 6.5 — signs include blue-green stains on fixtures and pinhole leaks in copper pipe. A calcite (or calcite/Corosex) neutralizer tank handles mildly acidic water (roughly pH 6–6.5); very acidic water usually needs soda-ash injection. Test first-draw lead and copper too, since acidic water dissolves both from plumbing.
High manganese in Idaho well water — what should I do?
In USGS lab results from 4,210 wells across 44 Idaho counties, 4% were above the EPA's 300 µg/L health advisory for manganese and 14% were above the 50 µg/L level where it stains fixtures and laundry black or brown. Sampled counties with the highest share over the health advisory: Camas County (35% of 20 wells), Valley County (20% of 46 wells), Owyhee County (17% of 96 wells). If your own test is over 300 µg/L, don't use the water for infant formula and use another source for drinking and cooking until it is treated — high manganese is linked to neurological effects, especially in babies and children. Boiling does not remove it. For the whole house, an oxidizing filter (greensand, Birm or air-injection) handles manganese, usually alongside iron; a water softener can remove low levels. For drinking water, a point-of-use reverse osmosis system reduces manganese at the kitchen tap. Retest after treatment to confirm it worked.