Well water in Gem County, Idaho
In Gem County, 23% of 183 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.
282 individual samples from 99 wells, 1990 to 2024. Data refreshed September 9, 2026.
What individual samples found
Each row summarizes every public sample for that contaminant inside Gem 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 2024 | 183 | 77% | 23% | 5 | 58 | 10 ug/L EPA limit | |
| Total coliform 2010 to 2019 | 127 | 13% | 13% | 0 | 1046 | 0 MPN/100mL EPA limit | |
| Nitrate 1990 to 2024 | 282 | 71% | 7% | 1.17 | 21 | 10 mg/L EPA limit | |
| Uranium 1993 to 2024 | 100 | 64% | 3% | 0.355 | 240 | 30 ug/L EPA limit | |
| Nitrite 1991 to 2023 | 241 | 2% | 0% | 0 | 0.03 | 1 mg/L EPA limit | |
| Fluoride 1990 to 2024 | 195 | 87% | 0% | 0.37 | 3.75 | 4 mg/L EPA limit | |
| E. coli 1990 to 2015 | 135 | 0% | 0% | 0 | 0 | 0 MPN/100mL EPA limit | |
| Copper 1990 to 2022 | 107 | 39% | 0% | 0 | 0.037 | 1.3 mg/L EPA limit | |
| Lead 1990 to 2008 | 55 | 2% | 0% | 0 | 0.22 | 15 ug/L EPA limit | |
| Chloride 1990 to 2024 | 196 | 100% | no limit | 3.6 | 49.4 | 250 mg/L nuisance level | |
| Sodium 1990 to 2024 | 156 | 100% | no limit | 37 | 230 | 20 mg/L health guideline | |
| Iron 1990 to 2024 | 151 | 48% | no limit | 0 | 1,400 | 300 ug/L nuisance level | |
| Manganese 1990 to 2024 | 149 | 56% | no limit | 1 | 490 | 300 ug/L health guideline | |
| Radon 1991 to 1995 | 19 | 100% | no limit | 400 | 860 | 2,000 pCi/L health guideline |
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What this means
The contaminants most often above their limit here are arsenic (23% of 183 samples), total coliform (13% of 127 samples), and nitrate (7% of 282 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 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 Gem County
- Arsenic23% of 183 local samples were above the limit.
- Total coliform13% of 127 local samples were above the limit.
- Nitrate7% of 282 local samples were above the limit.
- Uranium3% of 100 local samples were above the limit.
- Coliform bacteriaRecommended for every private well, every year.
- RadonMedian local reading is 400 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
- 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
- Fluoride
- Point-of-use reverse osmosis; Activated alumina; Distillation
Get your Gem 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 Gem County.
Common questions
Is there arsenic in well water in Gem County?
Arsenic was detected in 77% of 183 public well samples in Gem County, and 23% 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 Gem County?
The median radon level across 19 public samples in Gem County was 400 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 Gem 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): 1,937 samples in Gem 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: 159 samples in Gem 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.