Well water in Twin Falls County, Idaho
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Testing strongly recommended
tested well samples in Twin Falls County exceeded the EPA limit for arsenic (10 ug/L), based on public data from USGS Water Quality Portal and Idaho statewide ground water quality monitoring (IDWR EDMS).
13,045 test results across 14 contaminants from at least 537 wells, 1952 to 2025; nitrate has the most at 2,937. Data refreshed September 15, 2026.
Compiled by Nick at Well Water Nerd from public records. How these pages are built
An estimated 24,149 people in Twin Falls County get their drinking water from a private well, per USGS 2015 county water use. That is about 29% of residents.
Separately, the USGS national arsenic model estimates that 33% of the 24,149 people on private wells in Twin Falls County draw water above 10 ug/L (the model's range is 26% to 39%), from geology and well depth rather than local tests.
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 | Above limit | Samples | Detected | Median | Highest | Limit |
|---|---|---|---|---|---|---|
| Arsenic 1977 to 2025 | 1,183 | 97% | 10 | 130 | 10 ug/L EPA limit | |
| Nitrate 1952 to 2025 | 2,937 | 98% | 4.9 | 270 | 10 mg/L EPA limit | |
| Fluoride 1952 to 2025 | 1,312 | 99% | 0.71 | 44 | 4 mg/L EPA limit | |
| Total coliform 2004 to 2014 | 106 | 18% | 0 | 16 | 0 MPN/100mL EPA limit | |
| Uranium 1956 to 2025 | 120 | 93% | 8.55 | 45 | 30 ug/L EPA limit | |
| Copper 1982 to 2019 | 695 | 82% | 0.002 | 0.25 | 1.3 mg/L EPA limit | |
| Lead 1977 to 2008 | 460 | 19% | 0 | 2.4 | 15 ug/L EPA limit | |
| Nitrite 1991 to 2011 | 357 | 4% | 0 | 0.023 | 1 mg/L EPA limit | |
| E. coli 1990 to 2008 | 311 | 0% | 0 | 0 | 0 MPN/100mL EPA limit | |
| Sodium 1952 to 2025 | no limit | 1,518 | 100% | 61 | 290 | 20 mg/L health guideline |
| Chloride 1952 to 2025 | no limit | 1,507 | 100% | 43.6 | 430 | 250 mg/L nuisance level |
| Iron 1952 to 2025 | no limit | 1,246 | 34% | 0 | 8,980 | 300 ug/L nuisance level |
| Manganese 1952 to 2025 | no limit | 1,228 | 23% | 0 | 4,340 | 300 ug/L health guideline |
| Radon 1990 to 1995 | no limit | 65 | 100% | 320 | 4,430 | 2,000 pCi/L health guideline |
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 | 14%1,471 samples | 12%1,160 samples | 12%228 samples | 6%78 samples |
| Fluoride | 6%624 samples | 5%416 samples | 2%194 samples | 1%78 samples |
| Arsenic | 50%486 samples | 53%452 samples | 40%184 samples | 39%61 samples |
| Copper | 0%418 samples | 0%110 samples | 0%167 samples | n/a |
| Lead | 0%421 samples | 0%39 samples | n/a | n/a |
| Nitrite | 0%155 samples | 0%161 samples | 0%41 samples | n/a |
Nitrate above the limit in Twin Falls County fell from 12% of 228 samples in the 2010s to 6% of 78 in the 2020s.
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 arsenic (49% of 1,183 samples), nitrate (13% of 2,937 samples), and fluoride (5% of 1,312 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
- 1Arsenic49% of 1,183 local samples were above the limit.
- 2Nitrate13% of 2,937 local samples were above the limit.
- 3Fluoride5% of 1,312 local samples were above the limit.
- 4Total coliform2% of 106 local samples were above the limit.
- 5Uranium2% of 120 local samples were above the limit.
- 6Coliform bacteriaRecommended for every private well, every year.
- 7RadonMedian local reading is 320 pCi/L.
Test your Twin Falls County well for arsenic, nitrate and fluoride
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 Twin Falls County numbers, $29. Every line explained, ranked by what matters, with the fix for each.
The rock under Twin Falls County
Snake River Plain basaltic-rock aquifers
Igneous and metamorphic-rock. Granite and similar crystalline rock hold water only in fractures. Wells are deep and yields modest, and the rock itself is the source of arsenic, radon and uranium, which is why those three head the test list in bedrock country.
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
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
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 arsenic
Under-sink reverse osmosis system
under-sink or countertop, about $300
Look for NSF/ANSI 58 certification listing the contaminant you need removed.
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.
For arsenic
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 total coliform
Aquasana Rhino Well Water with UV
whole-house, at the point of entry, about $2,871
Whole-house system built for wells: sediment pre-filter, KDF and carbon, and a UV lamp rated to kill 99.99% of bacteria and viruses. It does not remove arsenic, nitrate, uranium or radon.
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Common questions
Is there arsenic in well water in Twin Falls County?
Arsenic was detected in 97% of 1,183 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.
Counties near Twin Falls 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: 7,392 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.
- 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 national arsenic model (Ayotte 2017): estimated share of 24,149 people on private wells in Twin Falls 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.