Well water in Twin Falls County, Idaho
In Twin Falls County, 49% of 582 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.
655 individual samples from 188 wells, 1956 to 2025. Data refreshed September 9, 2026.
What individual samples found
Each row summarizes every public sample for that contaminant inside Twin Falls 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 | 582 | 95% | 49% | 10 | 107 | 10 ug/L EPA limit | |
| Nitrate 1979 to 2025 | 632 | 98% | 7% | 4.59 | 30.5 | 10 mg/L EPA limit | |
| Fluoride 1981 to 2025 | 570 | 99% | 4% | 0.7 | 32.5 | 4 mg/L EPA limit | |
| Total coliform 2004 to 2014 | 106 | 18% | 2% | 0 | 16 | 0 MPN/100mL EPA limit | |
| Uranium 1956 to 2025 | 120 | 93% | 2% | 8.55 | 45 | 30 ug/L EPA limit | |
| Nitrite 1991 to 2011 | 357 | 4% | 0% | 0 | 0.023 | 1 mg/L EPA limit | |
| Copper 1990 to 2019 | 320 | 83% | 0% | 0.002 | 0.25 | 1.3 mg/L EPA limit | |
| E. coli 1990 to 2008 | 311 | 0% | 0% | 0 | 0 | 0 MPN/100mL EPA limit | |
| Lead 1990 to 2008 | 179 | 21% | 0% | 0 | 2.4 | 15 ug/L EPA limit | |
| Chloride 1982 to 2025 | 655 | 100% | no limit | 44 | 410 | 250 mg/L nuisance level | |
| Sodium 1990 to 2025 | 652 | 100% | no limit | 65 | 271 | 20 mg/L health guideline | |
| Iron 1990 to 2025 | 605 | 24% | no limit | 0 | 8,980 | 300 ug/L nuisance level | |
| Manganese 1990 to 2025 | 604 | 15% | no limit | 0 | 4,340 | 300 ug/L health guideline | |
| Radon 1990 to 1995 | 65 | 100% | no limit | 320 | 4,430 | 2,000 pCi/L health guideline |
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What this means
The contaminants most often above their limit here are arsenic (49% of 582 samples), nitrate (7% of 632 samples), and fluoride (4% of 570 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 Twin Falls County
- Arsenic49% of 582 local samples were above the limit.
- Nitrate7% of 632 local samples were above the limit.
- Fluoride4% of 570 local samples were above the limit.
- Total coliform2% of 106 local samples were above the limit.
- Uranium2% of 120 local samples were above the limit.
- Coliform bacteriaRecommended for every private well, every year.
- RadonMedian local reading is 320 pCi/L.
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
- Fluoride
- Point-of-use reverse osmosis; Activated alumina; 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
Get your Twin Falls 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 Twin Falls County.
Common questions
Is there arsenic in well water in Twin Falls County?
Arsenic was detected in 95% of 582 public well samples in Twin Falls County, and 49% 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 Twin Falls County?
The median radon level across 65 public samples in Twin Falls County was 320 pCi/L, with a high of 4,430 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.
How often should I test a private well in Twin Falls 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): 5,653 samples in Twin Falls 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: 105 samples in Twin Falls 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.