Colorado well water by county: arsenic and nitrate
Modelled arsenic and nitrate for private-well users in every Colorado county, what to test for, and where to find a certified lab. Private wells aren’t covered by the Safe Drinking Water Act.
All 64 counties
- El Paso CountyWell users 33,500Arsenic about 3 in 100Nitrate 0.7 mg/L
- Jefferson CountyWell users 28,300Arsenic about 3 in 100Nitrate 0.7 mg/L
- Douglas CountyWell users 19,200Arsenic about 3 in 100Nitrate 0.2 mg/L
- Weld CountyWell users 18,900Arsenic about 6 in 100Nitrate 2.2 mg/L
- La Plata CountyWell users 18,400Arsenic about 3 in 100Nitrate 0.4 mg/L
- Elbert CountyWell users 15,200Arsenic about 4 in 100Nitrate 0.2 mg/L
- Adams CountyWell users 15,200Arsenic about 5 in 100Nitrate 0.8 mg/L
- Boulder CountyWell users 13,700Arsenic about 3 in 100Nitrate 1.6 mg/L
- Arapahoe CountyWell users 10,900Arsenic about 5 in 100Nitrate 0.5 mg/L
- Park CountyWell users 10,700Arsenic about 2 in 100Nitrate 0.2 mg/L
- Larimer CountyWell users 10,000Arsenic about 4 in 100Nitrate 1.4 mg/L
- Garfield CountyWell users 9,500Arsenic about 5 in 100Nitrate 0.4 mg/L
- Logan CountyWell users 8,900Arsenic about 6 in 100Nitrate 3.4 mg/L
- Teller CountyWell users 8,700Arsenic about 3 in 100Nitrate 0.4 mg/L
- Pueblo CountyWell users 7,300Arsenic about 2 in 100Nitrate 1.1 mg/L
- Fremont CountyWell users 6,000Arsenic about 6 in 100Nitrate 0.5 mg/L
- Chaffee CountyWell users 6,000Arsenic about 9 in 100Nitrate 0.2 mg/L
- Grand CountyWell users 5,400Arsenic about 3 in 100Nitrate 0.1 mg/L
- Alamosa CountyWell users 5,300Arsenic about 64 in 100Nitrate 0.2 mg/L
- Rio Grande CountyWell users 5,000Arsenic about 30 in 100Nitrate 0.5 mg/L
- Morgan CountyWell users 5,000Arsenic about 6 in 100Nitrate 4.1 mg/L
- Clear Creek CountyWell users 4,600Arsenic about 2 in 100Nitrate 0.1 mg/L
- Eagle CountyWell users 4,300Arsenic about 3 in 100Nitrate 0.2 mg/L
- Pitkin CountyWell users 4,200Arsenic about 3 in 100Nitrate 0.2 mg/L
- Gunnison CountyWell users 4,100Arsenic about 4 in 100Nitrate 0.2 mg/L
- Routt CountyWell users 4,100Arsenic about 3 in 100Nitrate 0.2 mg/L
- Gilpin CountyWell users 3,700Arsenic about 2 in 100Nitrate 0.2 mg/L
- Conejos CountyWell users 3,400Arsenic about 29 in 100Nitrate 0.6 mg/L
- Yuma CountyWell users 3,400Arsenic about 5 in 100Nitrate 3.6 mg/L
- Kit Carson CountyWell users 3,200Arsenic about 5 in 100Nitrate 3.1 mg/L
- Archuleta CountyWell users 3,200Arsenic about 3 in 100Nitrate 0.2 mg/L
- Summit CountyWell users 2,900Arsenic about 2 in 100Nitrate 0.1 mg/L
- Custer CountyWell users 2,600Arsenic about 4 in 100Nitrate 0.4 mg/L
- Saguache CountyWell users 2,500Arsenic about 23 in 100Nitrate 0.3 mg/L
- Washington CountyWell users 2,400Arsenic about 5 in 100Nitrate 3.6 mg/L
- Mesa CountyWell users 2,300Arsenic about 5 in 100Nitrate 0.4 mg/L
- Las Animas CountyWell users 2,200Arsenic about 3 in 100Nitrate 0.7 mg/L
- Moffat CountyWell users 2,200Arsenic about 5 in 100Nitrate 0.2 mg/L
- Bent CountyWell users 2,100Arsenic about 4 in 100Nitrate 1.8 mg/L
- Montrose CountyWell users 2,000Arsenic about 7 in 100Nitrate 0.6 mg/L
- Lake CountyWell users 1,800Arsenic about 4 in 100Nitrate 0.1 mg/L
- Denver CountyWell users 1,700Arsenic about 6 in 100Nitrate 1.1 mg/L
- Lincoln CountyWell users 1,600Arsenic about 3 in 100Nitrate 2.5 mg/L
- Montezuma CountyWell users 1,500Arsenic about 3 in 100Nitrate 0.4 mg/L
- Baca CountyWell users 1,400Arsenic about 5 in 100Nitrate 4.5 mg/L
- Huerfano CountyWell users 1,200Arsenic about 2 in 100Nitrate 0.6 mg/L
- San Miguel CountyWell users 1,200Arsenic about 3 in 100Nitrate 0.1 mg/L
- Costilla CountyWell users 1,200Arsenic about 16 in 100Nitrate 0.4 mg/L
- Phillips CountyWell users 1,100Arsenic about 5 in 100Nitrate 3.4 mg/L
- Otero CountyWell users 1,000Arsenic about 5 in 100Nitrate 2.7 mg/L
- Rio Blanco CountyWell users 1,000Arsenic about 5 in 100Nitrate 0.2 mg/L
- Ouray CountyWell users 760Arsenic about 4 in 100Nitrate 0.1 mg/L
- Delta CountyWell users 620Arsenic about 4 in 100Nitrate 0.6 mg/L
- Cheyenne CountyWell users 540Arsenic about 4 in 100Nitrate 3.8 mg/L
- Prowers CountyWell users 540Arsenic about 4 in 100Nitrate 2.7 mg/L
- Sedgwick CountyWell users 530Arsenic about 6 in 100Nitrate 2.4 mg/L
- Crowley CountyWell users 510Arsenic about 3 in 100Nitrate 3.0 mg/L
- Jackson CountyWell users 470Arsenic about 3 in 100Nitrate 0.1 mg/L
- Dolores CountyWell users 440Arsenic about 3 in 100Nitrate 0.2 mg/L
- Kiowa CountyWell users 410Arsenic about 4 in 100Nitrate 7.8 mg/L
- Mineral CountyWell users 290Arsenic about 3 in 100Nitrate 0.1 mg/L
- Broomfield CountyWell users 270Arsenic about 4 in 100Nitrate 1.0 mg/L
- Hinsdale CountyWell users 240Arsenic about 3 in 100Nitrate 0.1 mg/L
- San Juan CountyWell users 20Arsenic about 3 in 100Nitrate 0.1 mg/L
Each mark = 1 in 100 well users the model places above 10 µg/L arsenic (scale 0–10)10 mg/L nitrate: the federal limit (MCL) for public systems
- These figures describe the area, not your well. Only a lab test shows what’s in your water.
- CDC says the Well Water Index figures “do not describe risk to individual wells.” (CDC Tracking Network)
- The arsenic model is better at spotting low-arsenic areas than at confirming high ones.
- Well-user counts are 2010 USGS estimates; tract values are averages over each census tract.
- Total coliform bacteriaEvery year
- NitrateEvery year
- Total dissolved solidsEvery year
- pHEvery year
- ArsenicAsk your local health department
EPA: Private Drinking Water Wells · CDC: Guidelines for Testing Well Water
Use a state-certified lab
Colorado Department of Public Health & Environment lists the laboratories certified to test drinking water in Colorado.
Screening tells you if something is worth a closer look
A lab result is the number you can act on.
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Lab-certified results
ETR Labs Basic Water Quality Test Kit
ETR Laboratories
50+ substances, including lead, copper, arsenic, nitrate, hardness and coliform bacteria
- Analyzed by Environmental Testing and Research Laboratories (ETR Labs), Leominster, MA · ISO/IEC 17025:2017, ANAB certificate AT-2638
- Turnaround: 2–4 business days after the lab receives it
What it can't tell you: It does not test PFAS, and not every result on the panel falls under the lab's accredited scope.
Screening: quick check, not lab-grade
Watersafe Drinking Water Test Kit
Silver Lake Research
Lead, bacteria, nitrate, copper, iron, chlorine, hardness and pH
- Turnaround: Minutes; the bacteria test can take up to 48 hours
What it can't tell you: Each test gives a yes/no or a rough range, not a lab number, and it cannot detect PFAS or arsenic.
A mail-in kit’s lab is accredited (ISO/IEC 17025) but may not be certified by your state. For a home sale, a loan, or a health-department follow-up, use a state-certified lab.
Colorado well water
Sources: CDC Well Water Index (2024) · USGS arsenic and nitrate models · USGS domestic well populations · EPA and CDC testing guidance. Data last updated 2026-10-08. All sources →
How many people use private wells in Colorado?
About 340,700 people across 64 Colorado counties, by the USGS 2010 estimate. USGS models place about 19,400 of them in areas where wells are likely to be above the 10 µg/L arsenic limit. These are area estimates, not well results.
Are private wells regulated in Colorado?
The quality and safety of drinking water from private domestic wells are not regulated by the Federal Government under the Safe Drinking Water Act nor by most state governments and laws. (EPA) Testing and treatment are the owner's responsibility.
Where can I get my well tested in Colorado?
Colorado Department of Public Health & Environment keeps the list of laboratories certified to test drinking water in Colorado. Only use laboratories that are certified to do drinking water testing. (EPA)