Well water in Idaho
In Idaho, 18% of 8,340 tested well samples exceeded the EPA limit for arsenic (10 ug/L), based on public data from Idaho statewide ground water quality monitoring (IDWR EDMS) and USGS Water Quality Portal.
10,012 individual samples from 2,928 wells, 1921 to 2026. Data refreshed September 9, 2026.
What individual samples found
Each row summarizes every public sample for that contaminant inside Idaho. 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 | Samples | Detected | Above limit | Median | Highest | Limit | |
|---|---|---|---|---|---|---|---|
| Arsenic 1981 to 2025 | 8,340 | 73% | 18% | 2.3 | 950 | 10 ug/L EPA limit | |
| Total coliform 2003 to 2023 | 2,178 | 17% | 16% | 0 | 2,420 | 0 MPN/100mL EPA limit | |
| Nitrate 1921 to 2025 | 10,012 | 85% | 7% | 1.5 | 130 | 10 mg/L EPA limit | |
| Uranium 1954 to 2026 | 3,125 | 77% | 5% | 2 | 81,600 | 30 ug/L EPA limit | |
| Fluoride 1952 to 2025 | 8,495 | 87% | 1% | 0.36 | 75.9 | 4 mg/L EPA limit | |
| Radium (226 + 228) 1962 to 2016 | 448 | 53% | <1% | 0 | 28.1 | 5 pCi/L EPA limit | |
| E. coli 1990 to 2016 | 5,473 | <1% | <1% | 0 | 200 | 0 MPN/100mL EPA limit | |
| Nitrite 1991 to 2023 | 6,980 | 7% | <1% | 0 | 27.5 | 1 mg/L EPA limit | |
| Lead 1972 to 2023 | 2,749 | 13% | <1% | 0 | 16 | 15 ug/L EPA limit | |
| Copper 1968 to 2022 | 4,969 | 55% | 0% | 0.001 | 0.25 | 1.3 mg/L EPA limit | |
| Chloride 1921 to 2025 | 9,138 | 99% | no limit | 11 | 1,400 | 250 mg/L nuisance level | |
| Sodium 1954 to 2025 | 8,467 | 100% | no limit | 21.4 | 560 | 20 mg/L health guideline | |
| Manganese 1952 to 2025 | 7,608 | 44% | no limit | 0 | 6,090 | 300 ug/L health guideline | |
| Iron 1949 to 2025 | 7,591 | 45% | no limit | 0 | 16,000 | 300 ug/L nuisance level | |
| Radon 1989 to 2016 | 1,617 | 100% | no limit | 470 | 8,040 | 2,000 pCi/L health guideline |
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What this means
The contaminants most often above their limit here are arsenic (18% of 8,340 samples), total coliform (16% of 2,178 samples), and nitrate (7% of 10,012 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 is widespread in local groundwater. It is mainly a staining and taste problem, but high manganese also carries a health advisory for infants.
What to test for in Idaho
- Arsenic18% of 8,340 local samples were above the limit.
- Total coliform16% of 2,178 local samples were above the limit.
- Nitrate7% of 10,012 local samples were above the limit.
- Uranium5% of 3,125 local samples were above the limit.
- Fluoride1% of 8,495 local samples were above the limit.
- Radium (226 + 228)<1% of 448 local samples were above the limit.
- E. coli<1% of 5,473 local samples were above the limit.
Treatment that fits the local profile
- Arsenic
- Point-of-use reverse osmosis; Adsorptive media (iron-based) whole-house; Anion exchange
- Total coliform
- Inspect and repair the well cap and casing, then shock chlorinate and retest; UV disinfection whole-house; Chlorination system
- Nitrate
- Point-of-use reverse osmosis; Anion exchange; Distillation
- Uranium
- Point-of-use reverse osmosis; Anion exchange; Whole-house anion exchange
- Fluoride
- Point-of-use reverse osmosis; Activated alumina; Distillation
Common questions
Is there arsenic in well water in Idaho?
Arsenic was detected in 73% of 8,340 public well samples in Idaho, and 18% 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 Idaho?
The median radon level across 1,617 public samples in Idaho was 470 pCi/L, with a high of 8,040 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.
How often should I test a private well in Idaho?
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 Idaho
- Ada22% total coliform
- Adams0% arsenic
- Bannock8% arsenic
- Bear Lake9% arsenic
- Benewah2% arsenic
- Bingham8% nitrate
- Blaine5% arsenic
- Boise23% fluoride
- Bonner10% arsenic
- Bonneville3% arsenic
- Boundary9% arsenic
- Butte5% e. coli
- Camas2% arsenic
- Canyon40% arsenic
- Caribou0% arsenic
- Cassia11% nitrate
- Clearwater5% nitrate
- Custer2% arsenic
- Elmore18% total coliform
- Franklin10% fluoride
- Fremont28% total coliform
- Gem23% arsenic
- Gooding9% nitrate
- Idaho2% nitrate
- Jefferson16% total coliform
- Jerome1% nitrate
- Kootenai6% arsenic
- Latah2% arsenic
- Lemhi6% fluoride
- Lewis18% nitrate
- Lincoln67% total coliform
- Madison<1% arsenic
- Minidoka25% total coliform
- Nez Perce35% total coliform
- Oneida4% nitrate
- Owyhee60% arsenic
- Payette39% arsenic
- Power6% nitrate
- Shoshone2% arsenic
- Teton9% total coliform
- Twin Falls49% arsenic
- Valley0% arsenic
- Washington53% arsenic
1 counties have fewer than 30 public samples and are included in the Idaho totals above rather than shown separately: Clark.
Sources
- Idaho statewide ground water quality monitoring (IDWR EDMS): 82,145 samples in Idaho. Ground water quality results from the Idaho Department of Water Resources statewide monitoring program and partner agencies (ISDA, IDEQ, INL), EDMS bulk download. Retrieved September 9, 2026.
- USGS Water Quality Portal: 5,045 samples in Idaho. Data from the Water Quality Portal, a service of USGS, EPA and USDA; groundwater samples collected by USGS and cooperators. Retrieved September 3, 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.