Private wells
Colorado well water: who's on a well and what to test for
People on private wells
512,400
Homes on wells
10.2%
Est. above arsenic limit
14,300
Every Colorado county
Sorted by people on private wells. Arsenic and nitrate are modeled for typical wells in the county, not measured at yours; depth is the median of domestic well records.
| El Paso County | 59,900 | 7% | 3.0% | 0.7 | 300 ft |
| Jefferson County | 45,500 | 8% | 2.7% | 0.7 | 226 ft |
| Weld County | 31,700 | 10% | 6.2% | 2.2 | 97 ft |
| Douglas County | 26,200 | 7% | 3.4% | 0.2 | 323 ft |
| La Plata County | 22,000 | 40% | 3.5% | 0.4 | 154 ft |
| Larimer County | 20,200 | 8% | 4.1% | 1.4 | 200 ft |
| Elbert County | 19,700 | 76% | 4.5% | 0.2 | 341 ft |
| Boulder County | 19,400 | 7% | 2.7% | 1.6 | 180 ft |
| Mesa County | 16,300 | 11% | 5.2% | 0.4 | 150 ft |
| Park County | 15,100 | 87% | 2.3% | 0.2 | 220 ft |
| Adams County | 14,200 | 3% | 5.5% | 0.8 | 310 ft |
| Arapahoe County | 13,000 | 2% | 5.2% | 0.5 | 323 ft |
| Pueblo County | 12,300 | 8% | 2.4% | 1.1 | 56 ft |
| Teller County | 12,200 | 52% | 3.5% | 0.4 | 250 ft |
| Garfield County | 11,300 | 19% | 5.3% | 0.4 | 120 ft |
| Las Animas County | 8,900 | 61% | 2.7% | 0.7 | 142 ft |
| Montezuma County | 8,400 | 33% | 3.5% | 0.4 | 105 ft |
| Montrose County | 8,200 | 20% | 7.0% | 0.6 | 123 ft |
| Chaffee County | 8,200 | 45% | 8.7% | 0.2 | 110 ft |
| Fremont County | 7,500 | 23% | 6.1% | 0.5 | 180 ft |
| Delta County | 7,400 | 25% | 4.3% | 0.7 | 97 ft |
| Morgan County | 6,900 | 24% | 6.3% | 4.1 | 90 ft |
| Grand County | 6,600 | 39% | 2.7% | 0.1 | 166 ft |
| Summit County | 6,400 | 18% | 2.0% | 0.1 | 125 ft |
| Routt County | 6,000 | 21% | 3.0% | 0.2 | 180 ft |
| Eagle County | 5,900 | 10% | 2.6% | 0.2 | 120 ft |
| Logan County | 5,700 | 28% | 5.6% | 3.4 | 98 ft |
| Alamosa County | 5,500 | 34% | 64.1% | 0.2 | 105 ft |
| Rio Grande County | 5,400 | 50% | 29.7% | 0.5 | 103 ft |
| Clear Creek County | 5,400 | 55% | 1.9% | 0.1 | 223 ft |
| Gunnison County | 5,300 | 38% | 4.0% | 0.2 | 68 ft |
| Archuleta County | 4,600 | 34% | 2.8% | 0.2 | 125 ft |
| Gilpin County | 4,600 | 77% | 2.1% | 0.2 | 240 ft |
| Pitkin County | 4,500 | 22% | 2.6% | 0.2 | 100 ft |
| Yuma County | 4,100 | 43% | 4.7% | 3.6 | 220 ft |
| Conejos County | 4,000 | 57% | 29.1% | 0.6 | 80 ft |
| Custer County | 3,800 | 84% | 4.1% | 0.4 | 184 ft |
| Saguache County | 3,300 | 55% | 22.8% | 0.3 | 135 ft |
| Otero County | 3,300 | 17% | 4.5% | 2.7 | 37 ft |
| Moffat County | 3,000 | 24% | 5.2% | 0.2 | 240 ft |
| Bent County | 2,800 | 37% | 4.1% | 1.8 | 181 ft |
| Washington County | 2,500 | 52% | 4.6% | 3.6 | 115 ft |
| Huerfano County | 2,300 | 39% | 2.4% | 0.6 | 120 ft |
| Kit Carson County | 2,200 | 30% | 5.3% | 3.1 | 235 ft |
| Lake County | 2,100 | 32% | 4.2% | 0.1 | 103 ft |
| San Miguel County | 2,000 | 22% | 3.3% | 0.1 | 182 ft |
| Prowers County | 2,000 | 17% | 4.2% | 2.7 | 116 ft |
| Costilla County | 1,800 | 61% | 16.0% | 0.4 | 160 ft |
| Rio Blanco County | 1,600 | 25% | 5.3% | 0.2 | 79 ft |
| Lincoln County | 1,600 | 34% | 3.3% | 2.5 | 60 ft |
| Ouray County | 1,300 | 28% | 4.4% | 0.1 | 200 ft |
| Baca County | 1,300 | 35% | 4.9% | 4.5 | 250 ft |
| Dolores County | 1,200 | 44% | 2.9% | 0.2 | 156 ft |
| Phillips County | 1,100 | 25% | 4.5% | 3.4 | 260 ft |
| Crowley County | 1,100 | 34% | 2.7% | 3.0 | 50 ft |
| Jackson County | 660 | 62% | 2.6% | 0.1 | 100 ft |
| Sedgwick County | 650 | 27% | 5.7% | 2.4 | 120 ft |
| Broomfield County | 600 | 1% | 4.1% | 1.0 | 150 ft |
| Kiowa County | 580 | 37% | 3.6% | 7.8 | 75 ft |
| Cheyenne County | 580 | 32% | 4.0% | 3.8 | 92 ft |
| Mineral County | 540 | 71% | 2.9% | 0.1 | 80 ft |
| Hinsdale County | 330 | 54% | 3.5% | 0.1 | 80 ft |
| San Juan County | 70 | 26% | 2.9% | 0.1 | 140 ft |
| Denver County | 40 | 0% | 5.6% | 1.1 | 60 ft |
Shading: red is among the highest counties nationally, amber the middle, green the lower range. Click a column to sort.
Most well users above the arsenic limit
- Alamosa County: about 2,800 people
- El Paso County: about 1,500 people
- Rio Grande County: about 1,400 people
- Elbert County: about 1,300 people
- Douglas County: about 1,000 people
Highest modeled nitrate
- Kiowa County: 7.8 mg/L average (limit 10)
- Baca County: 4.5 mg/L average (limit 10)
- Morgan County: 4.1 mg/L average (limit 10)
- Cheyenne County: 3.8 mg/L average (limit 10)
- Washington County: 3.6 mg/L average (limit 10)
Test first, then treat what the test finds
We may earn a commission from links below, at no cost to you. Picks follow the testing order, not who pays. How we make money.
1. Certified lab well test
Start here, every year
A mail-in kit analyzed by a certified lab measures bacteria, nitrate, arsenic, metals and more. Test strips cannot measure arsenic or uranium reliably.
2. Sediment pre-filter
Sand, silt or cloudy water
A 5-micron cartridge after the pressure tank catches sand, silt and rust before they reach fixtures or other filters. Cartridges are cheap and swap every few months.
3. Iron and sulfur filter
Orange stains or rotten-egg smell
Air-injection or oxidizing-media filters remove orange staining and rotten-egg smell. Size one from a lab result for iron, manganese and hydrogen sulfide.
4. UV disinfection system
Coliform bacteria in a test
Inactivates bacteria and viruses; usually for private wells or after boil notices.
5. Under-sink reverse osmosis
Arsenic, nitrate or uranium in a test
The most thorough option for drinking water: PFAS, lead, nitrate, arsenic and more. Look for NSF/ANSI 58.
6. Whole-house well filter
Several problems at once
Multi-stage sediment and carbon for every tap. Add UV if bacteria shows up, and choose stages from your lab results, not a guess.
Questions
Which Colorado counties rely most on wells?+
By share of homes: Park County (87%), Custer County (84%), Gilpin County (77%), Elbert County (76%), Mineral County (71%). By number of people: El Paso County (59,900), Jefferson County (45,500), Weld County (31,700).
Is arsenic a problem in Colorado wells?+
USGS estimates about 14,300 Colorado well users have water above the 10 microgram per liter arsenic limit that applies to public systems, the largest numbers in Alamosa County, El Paso County, Rio Grande County. Arsenic has no taste or smell, so only a lab test finds it.
Do I have to test my well when I sell a house in Colorado?+
We could not confirm a statewide rule for Colorado. Check with your county health department. Buyers can still make a lab test a condition of the offer.
How often should I test my well?+
No federal law covers private wells. EPA's drinking water limits apply to public water systems, so testing and treating a private well is up to the owner. EPA and CDC recommend testing at least once a year for coliform bacteria, nitrate, pH and total dissolved solids, and testing for arsenic, uranium, radon and other local concerns based on where you live.
Well-user counts, arsenic and nitrate figures are each a modeled estimate built from national datasets, not a count of tested wells, and well depths summarize drilling records rather than every well. Your own well can be very different from the county average, which is why the only answer for your home is a lab test. Sources: EPA Office of Research and Development, Water Source 2020 (private well estimates by census block); CDC Environmental Public Health Tracking, Well Water Index (from USGS nitrate and arsenic models); USGS, county-level domestic well population with arsenic above 10 micrograms per liter (Ayotte and others, 2017); USGS Synthesized Domestic Well Database for the Conterminous United States (SynDWell-US), McCarthy and others, 2025 (depth, where a county has 30+ usable records).