Quick answer
About 341,000 people in Colorado (6.8% of residents) got their water from private wells in 2010, according to USGS estimates. Colorado Department of Public Health and Environment runs the state’s private well guidance. Jefferson County Public Health offers free bacteria and nitrate test kits to private well owners in Jefferson County while supplies last.
- People on private wells
- 340,655
- Share of residents
- 6.8%
- Rank among states
- 34th
- Well users modeled above 10 µg/L arsenic
- 5.7%
Where to get your well tested in Colorado
- Colorado Department of Public Health and Environment: Drinking Water: Private Wells, the state’s guidance for private well owners.
- Well water testing in Colorado: what to test for and how to collect a sample.
- Certified laboratories in Colorado. Use a lab certified for drinking water, so results are reliable.
Free or low-cost testing
Jefferson County Public Health offers free bacteria and nitrate test kits to private well owners in Jefferson County while supplies last. (source)
Mail-in lab kits are another option when no local lab is convenient. See well water test kits for what home kits can and can’t tell you.
What to test for
- Every year: total coliform bacteria and nitrate.
- Before buying equipment: pH, hardness, iron, manganese and, if there’s a smell, hydrogen sulfide.
- Based on location: arsenic, uranium and radon, PFAS, and lead in older plumbing.
USGS modeling estimates that about 5.7% of Colorado’s private well users have water above the 10 µg/L arsenic limit. Arsenic has no taste or smell, so include it in your first test.
Private wells by county
Estimated people relying on private wells in each Colorado county in 2010, the share of residents that represents, and the modeled share of well users with arsenic above 10 µg/L. These are USGS model estimates (2010 population), not measurements of any well. Arsenic is hidden for counties with fewer than 500 well users.
| County | People on wells | Share | Arsenic >10 µg/L |
|---|---|---|---|
| El Paso County | 33,485 | 5.4% | 3.0% |
| Jefferson County | 28,300 | 5.3% | 2.6% |
| Douglas County | 19,151 | 6.7% | 3.3% |
| Weld County | 18,851 | 7.5% | 6.2% |
| La Plata County | 18,363 | 35.8% | 3.5% |
| Elbert County | 15,205 | 65.9% | 4.3% |
| Adams County | 15,181 | 3.4% | 5.5% |
| Boulder County | 13,744 | 4.7% | 2.6% |
| Arapahoe County | 10,934 | 1.9% | 5.1% |
| Park County | 10,734 | 66.2% | 2.5% |
| Larimer County | 10,036 | 3.3% | 4.0% |
| Garfield County | 9,536 | 16.9% | 5.3% |
Show all 64 counties
| Logan County | 8,903 | 39.2% | 5.4% |
| Teller County | 8,744 | 37.4% | 3.4% |
| Pueblo County | 7,309 | 4.6% | 2.5% |
| Fremont County | 6,009 | 12.8% | 6.0% |
| Chaffee County | 5,984 | 33.6% | 8.6% |
| Grand County | 5,368 | 36.2% | 3.0% |
| Alamosa County | 5,293 | 34.3% | 63.8% |
| Rio Grande County | 4,983 | 41.6% | 29.5% |
| Morgan County | 4,952 | 17.6% | 6.2% |
| Clear Creek County | 4,645 | 51.1% | 2.0% |
| Eagle County | 4,320 | 8.3% | 2.6% |
| Pitkin County | 4,229 | 24.7% | 2.5% |
| Gunnison County | 4,116 | 26.9% | 4.0% |
| Routt County | 4,075 | 17.3% | 2.9% |
| Gilpin County | 3,716 | 68.3% | 2.0% |
| Conejos County | 3,423 | 41.5% | 28.7% |
| Yuma County | 3,358 | 33.4% | 4.4% |
| Kit Carson County | 3,199 | 38.7% | 5.1% |
| Archuleta County | 3,151 | 26.1% | 3.0% |
| Summit County | 2,892 | 10.3% | 2.0% |
| Custer County | 2,602 | 61.1% | 4.0% |
| Saguache County | 2,460 | 40.3% | 22.7% |
| Washington County | 2,375 | 49.3% | 4.7% |
| Mesa County | 2,322 | 1.6% | 5.3% |
| Las Animas County | 2,214 | 14.3% | 2.7% |
| Moffat County | 2,199 | 15.9% | 5.2% |
| Bent County | 2,142 | 33.0% | 4.0% |
| Montrose County | 1,989 | 4.8% | 7.0% |
| Lake County | 1,787 | 24.5% | 3.9% |
| Denver County | 1,707 | 0.3% | 5.3% |
| Lincoln County | 1,583 | 29.0% | 3.0% |
| Montezuma County | 1,469 | 5.8% | 3.7% |
| Baca County | 1,352 | 35.7% | 4.6% |
| Huerfano County | 1,246 | 18.6% | 2.0% |
| San Miguel County | 1,157 | 15.7% | 3.0% |
| Costilla County | 1,151 | 32.7% | 16.0% |
| Phillips County | 1,066 | 24.0% | 4.4% |
| Otero County | 1,017 | 5.4% | 4.4% |
| Rio Blanco County | 1,011 | 15.2% | 5.2% |
| Ouray County | 763 | 17.2% | 4.0% |
| Delta County | 616 | 2.0% | 4.4% |
| Cheyenne County | 537 | 29.2% | 4.0% |
| Prowers County | 535 | 4.3% | 4.3% |
| Sedgwick County | 528 | 22.2% | 6.0% |
| Crowley County | 508 | 8.7% | 3.0% |
| Jackson County | 468 | 33.6% | – |
| Dolores County | 441 | 21.4% | – |
| Kiowa County | 408 | 29.2% | – |
| Mineral County | 288 | 40.5% | – |
| Broomfield County | 271 | 0.5% | – |
| Hinsdale County | 235 | 27.9% | – |
| San Juan County | 19 | 2.8% | – |
Counties with the highest modeled arsenic share: Alamosa County (63.8%), Rio Grande County (29.5%), Conejos County (28.7%), Saguache County (22.7%), Costilla County (16.0%).
After you get results
Enter your numbers in the test results tool to see what needs treating and in what order. The guide to reading a well water test explains units and limits.
Common questions
Does Colorado offer free well water testing?
Jefferson County Public Health offers free bacteria and nitrate test kits to private well owners in Jefferson County while supplies last.
Who oversees private wells in Colorado?
Colorado Department of Public Health and Environment (Drinking Water: Private Wells) publishes guidance for private well owners. Private wells aren’t covered by the federal Safe Drinking Water Act, so testing and treatment are up to the owner.
How many people use private wells in Colorado?
USGS estimates about 341,000 people, or 6.8% of the population in 2010. That ranks 34th of the states covered by the estimate.
What should I test my well for in Colorado?
Test every year for total coliform bacteria and nitrate, plus pH, hardness, iron and manganese before buying equipment. Arsenic is a known concern in parts of Colorado, so add it to the first test.
Sources
- Colorado Department of Public Health and Environment: Drinking Water: Private Wells
- Colorado Department of Public Health and Environment: well water testing
- Colorado certified laboratories
- Colorado free or low-cost testing
- USGS data release: domestic well locations and populations served, 2000 and 2010 (Johnson, Belitz and Lombard, 2019)
- CDC Environmental Public Health Tracking: private well water data (from USGS)