Well water in Washington County, Idaho
In Washington County, 53% of 120 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.
263 individual samples from 133 wells, 1972 to 2025. Data refreshed September 9, 2026.
What individual samples found
Each row summarizes every public sample for that contaminant inside Washington 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 1991 to 2025 | 120 | 93% | 53% | 11 | 950 | 10 ug/L EPA limit | |
| Nitrate 1991 to 2025 | 263 | 76% | 35% | 5.25 | 92.7 | 10 mg/L EPA limit | |
| Total coliform 2014 to 2019 | 118 | 15% | 15% | 0 | 219 | 0 MPN/100mL EPA limit | |
| Uranium 2016 to 2025 | 19 | 42% | 5% | 0 | 35 | 30 ug/L EPA limit | |
| Nitrite 1991 to 2019 | 217 | 7% | 0% | 0 | 0.02 | 1 mg/L EPA limit | |
| Fluoride 1972 to 2025 | 150 | 91% | 0% | 0.385 | 2.77 | 4 mg/L EPA limit | |
| Copper 1991 to 2019 | 82 | 49% | 0% | 0 | 0.075 | 1.3 mg/L EPA limit | |
| E. coli 1991 to 2008 | 75 | 0% | 0% | 0 | 0 | 0 MPN/100mL EPA limit | |
| Lead 1991 to 2010 | 42 | 10% | 0% | 0 | 3 | 15 ug/L EPA limit | |
| Chloride 1991 to 2025 | 148 | 100% | no limit | 9.1 | 220 | 250 mg/L nuisance level | |
| Sodium 1991 to 2025 | 130 | 100% | no limit | 79.5 | 521 | 20 mg/L health guideline | |
| Iron 1991 to 2025 | 130 | 69% | no limit | 240 | 4,400 | 300 ug/L nuisance level | |
| Manganese 1991 to 2025 | 126 | 78% | no limit | 206 | 935 | 300 ug/L health guideline | |
| Radon 1991 to 1995 | 13 | 100% | no limit | 610 | 860 | 2,000 pCi/L health guideline |
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What this means
The contaminants most often above their limit here are arsenic (53% of 120 samples), nitrate (35% of 263 samples), and total coliform (15% of 118 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, Iron, and Manganese 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 Washington County
- Arsenic53% of 120 local samples were above the limit.
- Nitrate35% of 263 local samples were above the limit.
- Total coliform15% of 118 local samples were above the limit.
- Uranium5% of 19 local samples were above the limit.
- Coliform bacteriaRecommended for every private well, every year.
- RadonMedian local reading is 610 pCi/L.
- ChlorideDetected in 100% of local samples.
Treatment that fits the local profile
- Arsenic
- Point-of-use reverse osmosis; Adsorptive media (iron-based) whole-house; Anion exchange
- Nitrate
- Point-of-use reverse osmosis; Anion exchange; Distillation
- Total coliform
- Inspect and repair the well cap and casing, then shock chlorinate and retest; UV disinfection whole-house; Chlorination system
- Uranium
- Point-of-use reverse osmosis; Anion exchange; Whole-house anion exchange
- Fluoride
- Point-of-use reverse osmosis; Activated alumina; Distillation
Get your Washington 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 Washington County.
Common questions
Is there arsenic in well water in Washington County?
Arsenic was detected in 93% of 120 public well samples in Washington County, and 53% 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 Washington County?
The median radon level across 13 public samples in Washington County was 610 pCi/L, with a high of 860 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.
How often should I test a private well in Washington 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): 1,613 samples in Washington 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: 20 samples in Washington 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.