Well water in Colorado
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Testing strongly recommended
tested well samples in Colorado exceeded the EPA limit for nitrate (10 mg/L), based on public data from USGS Water Quality Portal.
67,982 test results across 15 contaminants from at least 5,437 wells, 1946 to 2026; iron has the most at 9,931. Data refreshed September 15, 2026.
Compiled by Nick at Well Water Nerd from public records. How these pages are built
An estimated 286,474 people in Colorado get their drinking water from a private well, per USGS 2015 county water use.
Separately, the USGS national arsenic model estimates that 5% of the 286,474 people on private wells in Colorado draw water above 10 ug/L (the model's range is 4% to 6%), from geology and well depth rather than local tests.
What individual samples found
Each row summarizes every public sample for that contaminant inside Colorado. 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 |
|---|---|---|---|---|---|---|
| Nitrate 1954 to 2025 | 5,831 | 96% | 7.07 | 3,980 | 10 mg/L EPA limit | |
| Uranium 1968 to 2025 | 2,485 | 92% | 9.97 | 5,540 | 30 ug/L EPA limit | |
| PFOA 2021 to 2025 | 85 | 28% | 0 | 52.4 | 4 ng/L EPA limit | |
| PFOS 2021 to 2025 | 85 | 22% | 0 | 25 | 4 ng/L EPA limit | |
| Arsenic 1969 to 2025 | 5,744 | 54% | 0.4 | 2,800 | 10 ug/L EPA limit | |
| Fluoride 1946 to 2026 | 9,605 | 98% | 0.8 | 309 | 4 mg/L EPA limit | |
| Radium (226 + 228) 1974 to 2021 | 300 | 82% | 0.16 | 360 | 5 pCi/L EPA limit | |
| Lead 1968 to 2025 | 5,402 | 35% | 0 | 35,000 | 15 ug/L EPA limit | |
| Copper 1967 to 2025 | 5,187 | 56% | 0 | 14.5 | 1.3 mg/L EPA limit | |
| E. coli 1997 to 2021 | 428 | <1% | 0 | 3 | 0 MPN/100mL EPA limit | |
| Iron 1946 to 2025 | no limit | 9,931 | 73% | 20 | 590,000 | 300 ug/L nuisance level |
| Manganese 1954 to 2025 | no limit | 9,511 | 75% | 20 | 5,100,000 | 300 ug/L health guideline |
| Chloride 1951 to 2025 | no limit | 7,959 | 100% | 22 | 81,200 | 250 mg/L nuisance level |
| Sodium 1954 to 2025 | no limit | 4,968 | 100% | 140 | 3,130 | 20 mg/L health guideline |
| Radon 1993 to 2018 | no limit | 461 | 100% | 1,080 | 25,500 | 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 |
|---|---|---|---|---|
| Fluoride | 4%7,255 samples | 2%1,360 samples | 6%664 samples | 0%326 samples |
| Nitrate | 39%4,733 samples | 39%491 samples | 44%372 samples | 38%235 samples |
| Arsenic | 5%3,321 samples | 12%1,250 samples | 9%735 samples | 11%438 samples |
| Lead | 2%3,374 samples | <1%1,080 samples | 0%571 samples | 0%377 samples |
| Copper | <1%3,012 samples | <1%1,182 samples | 0%647 samples | <1%346 samples |
| Uranium | 23%800 samples | 33%837 samples | 44%571 samples | 40%277 samples |
Fluoride above the limit in Colorado fell from 6% of 664 samples in the 2010s to 0% of 326 in the 2020s.
Nitrate above the limit in Colorado fell from 44% of 372 samples in the 2010s to 38% of 235 in the 2020s.
Arsenic, lead, copper, uranium have held within a few points between the two most recent periods with enough samples.
What this means
The contaminants most often above their limit here are nitrate (40% of 5,831 samples), uranium (33% of 2,485 samples), and pfoa (25% of 85 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.
Iron, Manganese, and Chloride 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 Colorado
- 1Nitrate40% of 5,831 local samples were above the limit.
- 2Uranium33% of 2,485 local samples were above the limit.
- 3PFOA25% of 85 local samples were above the limit.
- 4PFOS18% of 85 local samples were above the limit.
- 5Arsenic8% of 5,744 local samples were above the limit.
- 6Fluoride3% of 9,605 local samples were above the limit.
- 7Radium (226 + 228)3% of 300 local samples were above the limit.
Test your Colorado well for nitrate, uranium and PFOA
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 Colorado: certified labs, state programs and prices.
Already have results back? Get your lab report read against the Colorado numbers, $29. Every line explained, ranked by what matters, with the fix for each.
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Send me the Colorado report
The numbers above, which tests to order and where, and the treatment that fits the local profile, in one email.
The rock under Colorado
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 state centroid. A single well can sit in a different unit, especially near a boundary.
Official help in Colorado
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.
Contamination sources on record in Colorado
CDPHE operating landfills records 55 regulated sites in Colorado, among 55 regulated sites in total. Groundwater does not follow property lines, so a well within a few hundred feet of one of these is worth testing for what that site could release.
- 55
- lined landfills and transfer facilities
Treatment that fits the local profile
Nitrate
Point-of-use reverse osmosis; Anion exchange; Distillation
Uranium
Point-of-use reverse osmosis; Anion exchange; Whole-house anion exchange
PFOA
Granular activated carbon whole-house; Point-of-use reverse osmosis; Anion exchange
PFOS
Granular activated carbon whole-house; Point-of-use reverse osmosis; Anion exchange
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 nitrate
Under-sink reverse osmosis system
under-sink or countertop, about $300
Look for NSF/ANSI 58 certification listing the contaminant you need removed.
For nitrate
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 uranium
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 PFOA
Aquasana Claryum 3-Stage under-sink filter
under-sink or countertop, about $175
Certified to NSF/ANSI 53 to reduce 99% of lead and 99.7% of PFOA and PFOS at the tap. Not a fix for arsenic, nitrate or bacteria.
For PFOA
Whole-house PFAS carbon system
whole-house, at the point of entry, about $3,000
Granular activated carbon certified to NSF/ANSI 53 for PFOA and PFOS; replace media on schedule.
For PFOA
Aquasana HealthShield whole-house filter
whole-house, at the point of entry, about $2,099
Whole-house carbon system the maker certifies to reduce PFAS (PFOA and PFOS) and VOCs at every tap; a UV option adds bacteria protection.
Common questions
Is there arsenic in well water in Colorado?
Arsenic was detected in 54% of 5,744 public well samples in Colorado, and 8% 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 Colorado?
The median radon level across 461 public samples in Colorado was 1,080 pCi/L, with a high of 25,500 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.
How often should I test a private well in Colorado?
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 in Colorado
- Adams19% uranium
- Alamosa16% arsenic
- Arapahoe65% uranium
- Archuleta7% arsenic
- Baca0% fluoride
- Bent41% arsenic
- Boulder9% uranium
- Chaffee10% fluoride
- Cheyenne0% fluoride
- Clear CreekUSGS estimate
- Conejos0% fluoride
- CostillaUSGS estimate
- CrowleyUSGS estimate
- Custer0% fluoride
- Delta3% fluoride
- Denver52% uranium
- DoloresUSGS estimate
- Douglas28% uranium
- Eagle13% arsenic
- Elbert26% uranium
- El Paso46% nitrate
- Fremont47% uranium
- Garfield23% arsenic
- GilpinUSGS estimate
- Grand2% fluoride
- Gunnison2% arsenic
- HinsdaleUSGS estimate
- HuerfanoUSGS estimate
- JacksonUSGS estimate
- Jefferson32% uranium
- Kiowa2% fluoride
- Kit Carson0% fluoride
- Lake0% fluoride
- La Plata7% fluoride
- Larimer0% arsenic
- Las Animas4% fluoride
- LincolnUSGS estimate
- Logan90% uranium
- Mesa49% uranium
- MineralUSGS estimate
- Moffat5% copper
- Montezuma35% arsenic
- Montrose65% uranium
- Morgan75% uranium
- Otero35% arsenic
- OurayUSGS estimate
- Park2% uranium
- PhillipsUSGS estimate
- PitkinUSGS estimate
- Prowers64% arsenic
- Pueblo15% uranium
- Rio Blanco47% fluoride
- Rio Grande0% copper
- Routt1% lead
- Saguache26% arsenic
- San JuanUSGS estimate
- San MiguelUSGS estimate
- Sedgwick31 samples
- Summit3% copper
- TellerUSGS estimate
- Washington0% fluoride
- Weld58% nitrate
- Yuma0% fluoride
1 counties have fewer than 30 public samples and are included in the Colorado totals above rather than shown separately: Broomfield.
Want the numbers themselves? Download the Colorado dataset as CSV, every county and town, with the sources named.
Sources
- USGS Water Quality Portal: 67,982 samples in Colorado. Data from the Water Quality Portal, a service of USGS, EPA and USDA; groundwater samples collected by USGS and cooperators. Retrieved September 3, 2026.
- USGS national arsenic model (Ayotte 2017): estimated share of 286,474 people on private wells in Colorado 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.
- CDPHE operating landfills: 55 regulated sites located in Colorado. Operating commercial landfills from the Colorado Department of Public Health and Environment. Retrieved September 18, 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.