Well water in Custer County, Idaho
In Custer County, 2% of 90 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.
104 individual samples from 45 wells, 1978 to 2025. Data refreshed September 9, 2026.
What individual samples found
Each row summarizes every public sample for that contaminant inside Custer 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 | |
|---|---|---|---|---|---|---|---|
| Arsenic 1990 to 2025 | 90 | 57% | 2% | 0.29 | 16 | 10 ug/L EPA limit | |
| Fluoride 1978 to 2025 | 96 | 74% | 2% | 0.201 | 6.71 | 4 mg/L EPA limit | |
| Nitrate 1990 to 2025 | 98 | 98% | 2% | 0.33 | 12 | 10 mg/L EPA limit | |
| Nitrite 1991 to 2011 | 64 | 11% | 0% | 0 | 0.02 | 1 mg/L EPA limit | |
| Copper 1990 to 2020 | 64 | 64% | 0% | 0.001 | 0.04 | 1.3 mg/L EPA limit | |
| E. coli 1990 to 2008 | 51 | 0% | 0% | 0 | 0 | 0 MPN/100mL EPA limit | |
| Lead 1990 to 2008 | 38 | 26% | 0% | 0 | 2 | 15 ug/L EPA limit | |
| Uranium 2016 to 2025 | 19 | 68% | 0% | 1.3 | 18 | 30 ug/L EPA limit | |
| Sodium 1990 to 2025 | 104 | 100% | no limit | 7.1 | 310 | 20 mg/L health guideline | |
| Chloride 1990 to 2025 | 103 | 99% | no limit | 4.03 | 353 | 250 mg/L nuisance level | |
| Iron 1990 to 2025 | 91 | 31% | no limit | 0 | 310 | 300 ug/L nuisance level | |
| Manganese 1990 to 2025 | 88 | 24% | no limit | 0 | 84.3 | 300 ug/L health guideline | |
| Radon 1991 to 1996 | 54 | 100% | no limit | 1,480 | 8,000 | 2,000 pCi/L health guideline |
Sampled fewer than 10 times, too few to summarize: Total coliform (5).
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What this means
The contaminants most often above their limit here are arsenic (2% of 90 samples), fluoride (2% of 96 samples), and nitrate (2% of 98 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 Custer County
- Arsenic2% of 90 local samples were above the limit.
- Fluoride2% of 96 local samples were above the limit.
- Nitrate2% of 98 local samples were above the limit.
- Coliform bacteriaRecommended for every private well, every year.
- RadonMedian local reading is 1,480 pCi/L.
- ChlorideDetected in 99% of local samples.
- CopperDetected in 64% of local samples.
Treatment that fits the local profile
- Arsenic
- Point-of-use reverse osmosis; Adsorptive media (iron-based) whole-house; Anion exchange
- Fluoride
- Point-of-use reverse osmosis; Activated alumina; Distillation
- Nitrate
- Point-of-use reverse osmosis; Anion exchange; Distillation
- Copper
- Neutralizing filter to raise pH; Point-of-use reverse osmosis
- Chloride
- Point-of-use reverse osmosis; Distillation
Get your Custer 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 Custer County.
Common questions
Is there arsenic in well water in Custer County?
Arsenic was detected in 57% of 90 public well samples in Custer County, and 2% 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 Custer County?
The median radon level across 54 public samples in Custer County was 1,480 pCi/L, with a high of 8,000 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.
How often should I test a private well in Custer 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): 896 samples in Custer 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: 69 samples in Custer 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.