Well water in Custer County, Idaho
In Custer County, 3% of 315 tested well samples exceeded the EPA limit for fluoride (4 mg/L), based on public data from USGS Water Quality Portal and Idaho statewide ground water quality monitoring (IDWR EDMS).
2,752 test results across 14 contaminants from at least 110 wells, 1967 to 2025; nitrate has the most at 490. Data refreshed September 15, 2026.
Compiled by Nick at Well Water Nerd from public records. How these pages are built
An estimated 2,453 people in Custer County get their drinking water from a private well, per USGS 2015 county water use. That is about 60% of residents.
Separately, the USGS national arsenic model estimates that 5% of the 2,453 people on private wells in Custer County draw water above 10 ug/L (the model's range is 4% to 7%), from geology and well depth rather than local tests.
What individual samples found
Each row summarizes every public sample for that contaminant inside Custer 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 | Above limit | Samples | Detected | Median | Highest | Limit |
|---|---|---|---|---|---|---|
| Fluoride 1967 to 2025 | 315 | 84% | 0.22 | 9.6 | 4 mg/L EPA limit | |
| Nitrate 1967 to 2025 | 490 | 95% | 0.41 | 53.1 | 10 mg/L EPA limit | |
| Arsenic 1984 to 2025 | 251 | 66% | 0.53 | 16 | 10 ug/L EPA limit | |
| Copper 1990 to 2020 | 173 | 61% | 0.001 | 0.051 | 1.3 mg/L EPA limit | |
| Lead 1990 to 2008 | 126 | 17% | 0 | 2 | 15 ug/L EPA limit | |
| Nitrite 1991 to 2011 | 64 | 11% | 0 | 0.02 | 1 mg/L EPA limit | |
| E. coli 1990 to 2008 | 51 | 0% | 0 | 0 | 0 MPN/100mL EPA limit | |
| Uranium 2016 to 2025 | 19 | 68% | 1.3 | 18 | 30 ug/L EPA limit | |
| Chloride 1967 to 2025 | no limit | 330 | 99% | 4.1 | 353 | 250 mg/L nuisance level |
| Sodium 1967 to 2025 | no limit | 319 | 100% | 7 | 310 | 20 mg/L health guideline |
| Iron 1968 to 2025 | no limit | 299 | 52% | 3 | 440 | 300 ug/L nuisance level |
| Manganese 1971 to 2025 | no limit | 256 | 36% | 0 | 84.3 | 300 ug/L health guideline |
| Radon 1991 to 1996 | no limit | 54 | 100% | 1,480 | 8,000 | 2,000 pCi/L health guideline |
Sampled fewer than 10 times, too few to summarize: Total coliform (5).
Over time
Share of samples above the limit by period. A cell needs ten samples to show; a comparison needs twenty in each period. Later periods sample fewer wells, and often the ones already known to have a problem.
| Contaminant | before 2000 | 2000s | 2010s | 2020s |
|---|---|---|---|---|
| Nitrate | 3%243 samples | <1%188 samples | 0%46 samples | 0%13 samples |
| Fluoride | 4%158 samples | 2%108 samples | 6%36 samples | 0%13 samples |
| Arsenic | 2%101 samples | 2%102 samples | 0%36 samples | 0%12 samples |
| Copper | 0%101 samples | 0%36 samples | 0%35 samples | n/a |
| Lead | 0%104 samples | 0%22 samples | n/a | n/a |
| Nitrite | 0%25 samples | 0%33 samples | n/a | n/a |
Nitrate, fluoride, arsenic, copper, lead, nitrite have held within a few points between the two most recent periods with enough samples.
Closer than the county
See the sampled wells near your address
The same records, drawn around one point: which wells within a few miles were sampled and what they found, how deep the wells next to you were drilled, and any leaking tank or cleanup site within a mile.
Check my addressWhat this means
The contaminants most often above their limit here are fluoride (3% of 315 samples), nitrate (2% of 490 samples), and arsenic (2% of 251 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 Iron 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 Custer County
- 1Fluoride3% of 315 local samples were above the limit.
- 2Nitrate2% of 490 local samples were above the limit.
- 3Arsenic2% of 251 local samples were above the limit.
- 4Coliform bacteriaRecommended for every private well, every year.
- 5RadonMedian local reading is 1,480 pCi/L.
- 6ChlorideDetected in 99% of local samples.
- 7CopperDetected in 61% of local samples.
Test your Custer County well for fluoride, nitrate and arsenic
Certified lab analysis of your own tap water, bottles and prepaid return shipping included. The Essential well panel covers arsenic, uranium, nitrate, lead, copper, manganese, iron, sodium and bacteria; radon and PFAS are add-ons.
Order the Tap Score well water test kit, from $209Already tested? Upload the report
Just want bacteria checked first? Tap Score coliform and E. coli test, $99. Coliform and E. coli are the tests every well needs every year.
Want a quick look before paying for a lab? Home well water test strips on Amazon check hardness, iron, pH and nitrate in minutes. They are a screen, not a result: arsenic, bacteria, uranium and PFAS need a certified lab.
Affiliate links: the merchant pays us a commission and the price is the same for you. It does not change what the data above says. As an Amazon Associate we earn from qualifying purchases.
Where to get a well tested in Idaho: certified labs, state programs and prices.
Already have results back? Get your lab report read against the Custer County numbers, $29. Every line explained, ranked by what matters, with the fix for each.
The rock under Custer County
Not a principal aquifer
The USGS map places this area outside any principal aquifer, which usually means wells here draw from fractured bedrock or thin local deposits rather than a large regional aquifer. Bedrock wells of that kind are where arsenic, radon and uranium turn up, and yields vary from well to well.
Classification from the USGS Principal Aquifers of the United States (national map, 2003 edition, GIS release 2015), read at the county centroid. A single well can sit in a different unit, especially near a boundary.
Official help in Idaho
Two places to start that are not selling anything: the state program that answers private well questions, and its list of labs certified to run drinking water tests. A certified lab is the difference between a result you can act on and a number.
- Wells, Idaho DEQ
- Drinking Water Testing and Certification, Idaho Department of Health and Welfare
Treatment that fits the local profile
Fluoride
Point-of-use reverse osmosis; Activated alumina; Distillation
Nitrate
Point-of-use reverse osmosis; Anion exchange; Distillation
Arsenic
Point-of-use reverse osmosis; Adsorptive media (iron-based) whole-house; Anion exchange
Systems we would look at first
Some links below are affiliate links. If you buy through one, the merchant pays us a commission at no extra cost to you. It does not change which products we list or what the data says.
For fluoride
Under-sink reverse osmosis system
under-sink or countertop, about $300
Look for NSF/ANSI 58 certification listing the contaminant you need removed.
For fluoride
Aquasana SmartFlow reverse osmosis
under-sink or countertop, about $225
Certified to NSF/ANSI 58 and 53 to reduce arsenic (96%, pentavalent), fluoride (90%), lead (99%), nitrate, nitrite and PFOA/PFOS at the kitchen tap, with a remineralizer.
For arsenic
Whole-house arsenic and uranium filter
whole-house, at the point of entry, about $2,500
Adsorptive iron-based media; sized to your well yield.
Free, no obligation
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Common questions
Is there arsenic in well water in Custer County?
Arsenic was detected in 66% of 251 public well samples in Custer 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 Custer County?
The median radon level across 54 public samples in Custer County was 1,480 pCi/L, with a high of 8,000 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.
How often should I test a private well in Custer 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.
Counties near Custer County
Closest first. Groundwater does not stop at a line on a map, so the next county over is a fair second opinion.
Want the numbers themselves? Download the Idaho dataset as CSV, every county and town, with the sources named.
Sources
- USGS Water Quality Portal: 1,856 samples in Custer 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.
- Idaho statewide ground water quality monitoring (IDWR EDMS): 896 samples in Custer 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 national arsenic model (Ayotte 2017): estimated share of 2,453 people on private wells in Custer County above 10 ug/L of arsenic, from geology, well depth and land use. County estimates of domestic well users with arsenic above 10 ug/L from the USGS national logistic regression model (Ayotte, Medalie and Qi, 2017), with private well population from the USGS 2015 county water-use compilation (Dieter and others, 2018). Retrieved September 15, 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.