Radon in Colorado Well Water

Colorado is classified Zone 1 for radon — the highest risk category. About half of the homes tested exceed the EPA action level of 4 pCi/L (CDPHE). For well owners, radon in water is an additional exposure pathway that most people never test for.

Uranium in tested wells, county by county

Radon is what uranium turns into as it decays, so uranium-bearing rock like granite is a common radon source. CDPHE's private well lab results (2011–2020) show where tested wells went over the federal uranium limit of 30 µg/L. Boulder and Jefferson, the foothills counties where many wells are drilled into granite and gneiss, top the list.

People usually test because they have a reason to, so these rates describe tested wells, not every well in the county. CDPHE's private well data doesn't include radon in water. Uranium results show where the rock carries uranium, but they can't tell you your radon level, so test for it.

The Scale of the Problem

Where Does Radon Come From?

Radon is produced by the radioactive decay of uranium and thorium, which occur naturally in rocks and soil. Much of Colorado's mountain and foothill country is granite and other crystalline rock that carries uranium, so radon is common across the state.

The Pikes Peak Granite

The Pikes Peak granite — a billion-year-old formation underlying Teller, western El Paso, and parts of Park and Fremont counties — contains unusually large concentrations of uranium. Sediments weathered from this granite have been spreading for millions of years and are one source of uranium and radon across the entire Denver Basin aquifer system.

Wells drilled straight into granite sit at the source. In one USGS study of wells in southeastern Park County, the median radon level in water from granite wells was 6,300 pCi/L, against 795 pCi/L in wells in volcanic rock (USGS Fact Sheet 2004-3066).

Front Range Crystalline Rock

The entire Front Range uplift exposes Precambrian crystalline rocks — granites, gneisses, and schists — that contain varying levels of uranium. Communities from Estes Park to Canon City, and all the foothills in between, sit on rock that produces radon.

Radon in Water vs. Radon in Air

Most radon discussions focus on soil gas seeping into basements. But for well owners, there's a second pathway: radon dissolved in groundwater.

What Are the Levels?

Based on testing by GeoWater Services and other Colorado well specialists:

Area	Typical Range (pCi/L in water)
Pikes Peak region (Woodland Park, Divide, Florissant)	1,000 - 5,000+ pCi/L
Front Range foothills (Evergreen, Conifer, Bailey)	1,000 - 3,000 pCi/L
Denver Basin wells	Variable, generally lower

EPA Proposed Standards

The EPA has proposed two standards for radon in drinking water:

• 300 pCi/L — the proposed MCL
• 4,000 pCi/L — an alternative MCL for states with programs to address indoor air radon

Neither standard has been finalized. Most Colorado mountain wells exceed the 300 pCi/L level. Many exceed 4,000 pCi/L. If you're on a well in the mountains, assume your water has radon until you've tested and confirmed otherwise.

Health Effects

Radon is the second leading cause of lung cancer in the United States, responsible for an estimated 21,000 deaths per year. The risk is additive with smoking — if you smoke and have radon exposure, your risk multiplies.

Unlike many environmental hazards, radon's health effects are well established by extensive epidemiological research, including studies of uranium miners. There is no known safe level of radon exposure.

Testing for Radon in Water

Testing your well water for radon requires a specific sample collection method — the water must be collected without agitation to prevent radon from off-gassing before it reaches the lab. Most labs provide special sample containers and instructions.

See our testing guide for labs that test water for radon in Colorado.

Treatment

The good news: radon treatment for well water is highly effective.

Treatment	Effectiveness	Notes
Aeration (point-of-entry)	95-99% removal	The standard treatment. Sprays or bubbles air through water before it enters the house. Radon gas is vented outside. Most effective for high levels.
Granular activated carbon (GAC)	85-99% removal	Works well for lower radon levels. Carbon becomes radioactive over time and requires special disposal. Not recommended for very high levels.

The Double Exposure

If you're in a Colorado mountain home on a well, you likely have two radon exposure pathways: soil gas seeping through your foundation AND radon dissolved in your water. Testing and addressing both is important.

A sub-slab depressurization system handles the soil gas pathway. An aeration or GAC system handles the water pathway. Together, they can reduce your total radon exposure dramatically.