Well water in Cassia County, Idaho
In Cassia County, 11% of 255 tested well samples exceeded the EPA limit for nitrate (10 mg/L), based on public data from Idaho statewide ground water quality monitoring (IDWR EDMS) and USGS Water Quality Portal.
255 individual samples from 82 wells, 1957 to 2025. Data refreshed September 9, 2026.
What individual samples found
Each row summarizes every public sample for that contaminant inside Cassia 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 2004 to 2015 | 29 | 21% | 21% | 0 | 248 | 0 MPN/100mL EPA limit | |
| Nitrate 1990 to 2025 | 255 | 98% | 11% | 4.19 | 17 | 10 mg/L EPA limit | |
| Uranium 1957 to 2025 | 45 | 98% | 2% | 5.8 | 62 | 30 ug/L EPA limit | |
| Arsenic 1990 to 2025 | 184 | 84% | 2% | 2.5 | 12 | 10 ug/L EPA limit | |
| Fluoride 1965 to 2025 | 194 | 89% | 0% | 0.46 | 1.49 | 4 mg/L EPA limit | |
| Nitrite 1991 to 2011 | 121 | 6% | 0% | 0 | 0.03 | 1 mg/L EPA limit | |
| Copper 1990 to 2020 | 117 | 77% | 0% | 0.002 | 0.026 | 1.3 mg/L EPA limit | |
| E. coli 1990 to 2008 | 109 | 0% | 0% | 0 | 0 | 0 MPN/100mL EPA limit | |
| Lead 1990 to 2008 | 58 | 21% | 0% | 0 | 4 | 15 ug/L EPA limit | |
| Chloride 1990 to 2025 | 247 | 100% | no limit | 43.8 | 1,250 | 250 mg/L nuisance level | |
| Sodium 1966 to 2025 | 218 | 100% | no limit | 47.2 | 320 | 20 mg/L health guideline | |
| Iron 1990 to 2025 | 186 | 23% | no limit | 0 | 660 | 300 ug/L nuisance level | |
| Manganese 1990 to 2025 | 183 | 22% | no limit | 0 | 37 | 300 ug/L health guideline | |
| Radon 1990 to 2000 | 35 | 100% | no limit | 490 | 3,880 | 2,000 pCi/L health guideline |
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What this means
The contaminants most often above their limit here are total coliform (21% of 29 samples), nitrate (11% of 255 samples), and uranium (2% of 45 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 Cassia County
- Total coliform21% of 29 local samples were above the limit.
- Nitrate11% of 255 local samples were above the limit.
- Uranium2% of 45 local samples were above the limit.
- Arsenic2% of 184 local samples were above the limit.
- Coliform bacteriaRecommended for every private well, every year.
- RadonMedian local reading is 490 pCi/L.
- ChlorideDetected in 100% of local samples.
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
- Nitrate
- Point-of-use reverse osmosis; Anion exchange; Distillation
- Uranium
- Point-of-use reverse osmosis; Anion exchange; Whole-house anion exchange
- Arsenic
- Point-of-use reverse osmosis; Adsorptive media (iron-based) whole-house; Anion exchange
- Fluoride
- Point-of-use reverse osmosis; Activated alumina; Distillation
Get your Cassia County water tested at home
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Common questions
Is there arsenic in well water in Cassia County?
Arsenic was detected in 84% of 184 public well samples in Cassia 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 Cassia County?
The median radon level across 35 public samples in Cassia County was 490 pCi/L, with a high of 3,880 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.
How often should I test a private well in Cassia 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): 1,928 samples in Cassia 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: 53 samples in Cassia 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.