Well water in Lemhi County, Idaho
In Lemhi County, 6% of 119 tested well samples exceeded the EPA limit for fluoride (4 mg/L), based on public data from Idaho statewide ground water quality monitoring (IDWR EDMS) and USGS Water Quality Portal.
122 individual samples from 28 wells, 1971 to 2025. Data refreshed September 9, 2026.
What individual samples found
Each row summarizes every public sample for that contaminant inside Lemhi 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 | Samples | Detected | Above limit | Median | Highest | Limit | |
|---|---|---|---|---|---|---|---|
| Fluoride 1990 to 2025 | 119 | 85% | 6% | 0.217 | 11 | 4 mg/L EPA limit | |
| Arsenic 1990 to 2025 | 117 | 71% | 3% | 2 | 15 | 10 ug/L EPA limit | |
| Nitrate 1990 to 2025 | 119 | 87% | 0% | 0.36 | 3.4 | 10 mg/L EPA limit | |
| Nitrite 1991 to 2011 | 81 | 4% | 0% | 0 | 0.01 | 1 mg/L EPA limit | |
| Copper 1990 to 2020 | 80 | 80% | 0% | 0.003 | 0.12 | 1.3 mg/L EPA limit | |
| E. coli 1990 to 2008 | 67 | 0% | 0% | 0 | 0 | 0 MPN/100mL EPA limit | |
| Lead 1990 to 2008 | 43 | 12% | 0% | 0 | 1.62 | 15 ug/L EPA limit | |
| Uranium 2016 to 2025 | 26 | 73% | 0% | 2 | 16 | 30 ug/L EPA limit | |
| Chloride 1990 to 2025 | 122 | 100% | no limit | 5.97 | 115 | 250 mg/L nuisance level | |
| Sodium 1978 to 2025 | 122 | 100% | no limit | 12 | 560 | 20 mg/L health guideline | |
| Iron 1978 to 2025 | 120 | 53% | no limit | 4.5 | 4,900 | 300 ug/L nuisance level | |
| Manganese 1971 to 2025 | 115 | 40% | no limit | 0 | 540 | 300 ug/L health guideline | |
| Radon 1991 to 1994 | 23 | 100% | no limit | 890 | 2,400 | 2,000 pCi/L health guideline |
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What this means
The contaminants most often above their limit here are fluoride (6% of 119 samples) and arsenic (3% of 117 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 and Iron are widespread in local groundwater. They are mainly a staining and taste problem, but high manganese also carries a health advisory for infants.
What to test for in Lemhi County
- Fluoride6% of 119 local samples were above the limit.
- Arsenic3% of 117 local samples were above the limit.
- Coliform bacteriaRecommended for every private well, every year.
- RadonMedian local reading is 890 pCi/L.
- ChlorideDetected in 100% of local samples.
- NitrateRecommended for every private well, especially near farms or septic systems.
- CopperDetected in 80% of local samples.
Treatment that fits the local profile
- Fluoride
- Point-of-use reverse osmosis; Activated alumina; Distillation
- Arsenic
- Point-of-use reverse osmosis; Adsorptive media (iron-based) whole-house; Anion exchange
- Copper
- Neutralizing filter to raise pH; Point-of-use reverse osmosis
- Chloride
- Point-of-use reverse osmosis; Distillation
- Iron
- Oxidizing filter whole-house; Water softener (low levels); Aeration plus filtration
Get your Lemhi County water tested at home
A licensed local water treatment company tests your tap water in your home, walks you through the numbers, and quotes treatment only if something needs fixing. Free, about 30 minutes, no obligation. Tell us what you need and we match you with a company that serves Lemhi County.
Common questions
Is there arsenic in well water in Lemhi County?
Arsenic was detected in 71% of 117 public well samples in Lemhi County, and 3% 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 Lemhi County?
The median radon level across 23 public samples in Lemhi County was 890 pCi/L, with a high of 2,400 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.
How often should I test a private well in Lemhi 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.
Sources
- Idaho statewide ground water quality monitoring (IDWR EDMS): 1,116 samples in Lemhi County. 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: 38 samples in Lemhi 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.
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.