Well water in Madison County, Idaho
In Madison County, <1% of 115 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.
131 individual samples from 86 wells, 1957 to 2024. Data refreshed September 9, 2026.
What individual samples found
Each row summarizes every public sample for that contaminant inside Madison 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 | |
|---|---|---|---|---|---|---|---|
| Total coliform 2013 to 2018 | 19 | 32% | 32% | 0 | 11 | 0 MPN/100mL EPA limit | |
| Arsenic 1991 to 2024 | 115 | 58% | <1% | 0.64 | 15 | 10 ug/L EPA limit | |
| E. coli 1991 to 2008 | 127 | 0% | 0% | 0 | 0 | 0 MPN/100mL EPA limit | |
| Fluoride 1991 to 2024 | 125 | 100% | 0% | 0.41 | 2.98 | 4 mg/L EPA limit | |
| Nitrate 1991 to 2024 | 104 | 98% | 0% | 1.3 | 7.8 | 10 mg/L EPA limit | |
| Copper 1991 to 2019 | 72 | 60% | 0% | 0.001 | 0.01 | 1.3 mg/L EPA limit | |
| Nitrite 1991 to 2011 | 71 | 1% | 0% | 0 | 0.01 | 1 mg/L EPA limit | |
| Lead 1991 to 2004 | 38 | 8% | 0% | 0 | 2 | 15 ug/L EPA limit | |
| Uranium 1957 to 2024 | 32 | 56% | 0% | 0.95 | 4.1 | 30 ug/L EPA limit | |
| Chloride 1991 to 2024 | 131 | 100% | no limit | 10 | 44.9 | 250 mg/L nuisance level | |
| Sodium 1991 to 2024 | 128 | 100% | no limit | 12 | 37.1 | 20 mg/L health guideline | |
| Iron 1991 to 2024 | 101 | 39% | no limit | 0 | 1,150 | 300 ug/L nuisance level | |
| Manganese 1991 to 2024 | 101 | 30% | no limit | 0 | 41.5 | 300 ug/L health guideline | |
| Radon 1991 to 1995 | 17 | 100% | no limit | 590 | 2,500 | 2,000 pCi/L health guideline |
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What this means
The contaminants most often above their limit here are total coliform (32% of 19 samples) and arsenic (<1% of 115 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 Madison County
- Total coliform32% of 19 local samples were above the limit.
- Arsenic<1% of 115 local samples were above the limit.
- Coliform bacteriaRecommended for every private well, every year.
- RadonMedian local reading is 590 pCi/L.
- FluorideDetected in 100% of local samples.
- ChlorideDetected in 100% of local samples.
- NitrateRecommended for every private well, especially near farms or septic systems.
Treatment that fits the local profile
- Total coliform
- Inspect and repair the well cap and casing, then shock chlorinate and retest; UV disinfection whole-house; Chlorination system
- Arsenic
- Point-of-use reverse osmosis; Adsorptive media (iron-based) whole-house; Anion exchange
- Fluoride
- Point-of-use reverse osmosis; Activated alumina; Distillation
- Copper
- Neutralizing filter to raise pH; Point-of-use reverse osmosis
- Chloride
- Point-of-use reverse osmosis; Distillation
Get your Madison 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 Madison County.
Common questions
Is there arsenic in well water in Madison County?
Arsenic was detected in 58% of 115 public well samples in Madison County, and <1% 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 Madison County?
The median radon level across 17 public samples in Madison County was 590 pCi/L, with a high of 2,500 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.
How often should I test a private well in Madison 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): 1,090 samples in Madison 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: 91 samples in Madison 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.