Radon Water Test: What Homeowners Need to Know
Most homeowners think of radon as an air problem, so a radon water test can seem unfamiliar. Radon can dissolve in groundwater and enter a home through the water supply, where it escapes into the air during showering, dishwashing, and laundry. The concern applies almost entirely to homes on private wells rather than municipal water. This guide summarizes EPA and Colorado Department of Public Health and Environment guidance current as of 2026. It is educational only and does not diagnose any condition; testing decisions belong with a certified professional, and health questions belong with your physician.
What is a radon water test and who needs one?
A radon water test measures the concentration of radon dissolved in a home’s water supply, reported in picocuries per liter of water. It matters most for households that draw water from a private well, because groundwater can pick up radon as it moves through uranium-bearing rock and soil. Homes connected to a public water system that uses surface water rarely need a water test, since radon largely escapes from open reservoirs before the water reaches the tap.
The reason waterborne radon matters is indirect. When radon-laden water is agitated, such as in a hot shower or a running washing machine, the gas is released into indoor air, where it can be inhaled. A smaller portion of the risk comes from ingesting radon in drinking water directly. The inhalation pathway, driven by radon escaping from agitated water into room air, is generally considered the larger contributor to overall risk from waterborne radon. For private-well households, a water test complements an air test rather than replacing it. The broader context of how radon enters and accumulates in homes is covered in the Colorado radon testing guide.
How does radon get into water?
Radon is produced continuously by the radioactive decay of uranium in soil and rock. Groundwater flowing through uranium-rich geology dissolves some of that radon and carries it into a well. Because the water moves underground in a confined system with little chance to off-gas, it can arrive at the wellhead still carrying dissolved radon that would otherwise have escaped in open air. The same Colorado geology that produces high indoor-air radon also creates the potential for elevated radon in well water in some areas, which is why well owners in radon-prone parts of the state should not overlook the water pathway.
Surface water behaves differently. Lakes, rivers, and reservoirs are exposed to open air, so most dissolved radon escapes before the water enters a treatment and distribution system. By the time surface water reaches a tap, it has had ample opportunity to off-gas, leaving little radon behind. That is why municipal customers, especially those on surface-water supplies, rarely face a waterborne radon concern. The distinction between groundwater and surface water is the single biggest factor in deciding whether a radon water test is warranted. A household that knows it draws from a private well in a radon-prone area has the clearest reason to consider testing the water alongside the air.
How is the water test performed?
A radon water test involves collecting a water sample carefully to prevent the gas from escaping before analysis. Because radon off-gasses easily, the sampling procedure is specific: the sample is usually taken in a sealed container filled with no air gap, often from an indoor tap run briefly first, and then sent promptly to a qualified laboratory. Mishandling the sample can let radon escape and produce a falsely low reading that understates the true concentration in the supply.
Lab analysis
The laboratory measures the dissolved radon concentration and reports it in picocuries per liter of water, a different unit basis than the air measurement even though both use picocuries per liter. Homeowners should use a lab experienced in radon-in-water analysis and follow the sampling kit instructions exactly, including any requirement to return the sample promptly so the result reflects the true concentration. Because of the off-gassing issue, a do-it-yourself water test is more error-prone than an air test, so many homeowners arrange the water test through a radon professional who knows the proper sampling technique. Careful collection is the difference between a meaningful result and a falsely reassuring one.
How do water levels relate to the air action level?
The EPA action level for radon in indoor air is 4 picocuries per liter, and that air standard remains the central reference point. Waterborne radon matters chiefly because of how much it contributes to indoor air levels. A widely cited rule of thumb holds that roughly 10,000 pCi/L of radon in water adds about 1 pCi/L to indoor air, though the exact transfer depends on water use and ventilation. A home with heavy water use and tight ventilation may see a larger contribution, while a home with modest use and good air exchange may see less, which is why the figure is a guideline rather than a precise conversion.
The EPA has not finalized a single enforceable national standard for radon in drinking water, and proposed approaches have linked water limits to whether a state runs a program to reduce radon in air. Under those proposals, states with active indoor-air radon programs would have a higher allowable water limit, reflecting the idea that the air pathway is where most of the risk concentrates. The practical takeaway for homeowners is to interpret a water result in terms of its contribution to indoor air and to address both together rather than fixating on the water number in isolation. If air radon already exceeds 4 pCi/L, an air mitigation system, described in the guide on how to fix radon in a home, is the priority, while a high water reading may call for separate water treatment.
How do air and water radon tests work together?
For a private-well household, the air test and the water test answer different questions, and the most complete picture comes from doing both. The air test measures the radon concentration in the home’s living space, which is what the EPA action level of 4 picocuries per liter directly addresses. The water test measures radon dissolved in the supply, which contributes to that indoor air level whenever water is used.
The sequence usually starts with an air test, since indoor air is where the health risk is most directly measured and where the action level applies. If the air test comes back elevated, a water test can help determine how much of the airborne radon originates from the water supply versus the soil beneath the home. That distinction matters because the remedy differs: soil-sourced radon calls for an air mitigation system, while water-sourced radon calls for water treatment.
In practice, many well-served homes in radon-prone areas benefit from testing both at once. A combined approach avoids treating one pathway while leaving the other unaddressed. Homeowners interested in coordinating radon testing alongside other indoor-air concerns can review the related guide on mold and radon testing, which discusses bundling these assessments efficiently.
When should you test your well water for radon?
Certain situations make a radon water test particularly worthwhile for well owners. A real estate transaction involving a well-served home is one clear case, since a buyer is committing to that water supply for years and can address any problem before closing. Drilling a new well or experiencing a significant change in water source, flow, or quality is another reasonable trigger for testing.
Households that have already found elevated radon in their indoor air have a strong reason to test the water, because the water may be a contributing source. If air mitigation is installed but indoor levels remain higher than expected, waterborne radon released during showering and laundry could be the explanation. In that case, a water test clarifies whether water treatment is also needed to bring the home fully under control. Without testing the water, a homeowner in that situation might wrongly conclude the air system has failed when the real source is the supply itself.
Routine periodic testing also has a place for long-term well users who want to stay ahead of changes. Groundwater conditions can shift over time as the aquifer and surrounding geology change, and a supply that once tested low may change. Testing every few years, and after any major change to the well or water system, keeps the picture current. Because radon in water is invisible and odorless, testing is the only reliable way to know, and the EPA encourages well owners in radon-prone regions to include it in their water-quality monitoring.
Two main treatment methods reduce radon in water, and both are point-of-entry systems that treat all water as it enters the home. Aeration systems bubble air through the water to strip out the radon and vent it safely outdoors; they are highly effective and do not add anything to the water. Granular activated carbon filtration adsorbs radon onto carbon media and works well for lower concentrations, though the carbon can accumulate radioactivity over time and requires proper maintenance and disposal.
Choosing between them depends on the radon concentration and household water use, a decision best made with a qualified water-treatment professional. Treating water at the point of entry, before it reaches taps and showers, addresses both the inhalation and ingestion pathways at once.
Comparing aeration and carbon filtration
Aeration is generally favored for higher radon concentrations because it removes the gas rather than accumulating it. The system agitates the water to release radon, which is then vented outdoors, leaving the treated water radon-free without adding any media to maintain. Its main drawbacks are a higher upfront cost and the need for a vent and some space for the equipment.
Granular activated carbon filtration is simpler and often less expensive upfront, making it attractive for lower radon levels. Water passes through a carbon bed that adsorbs the radon. The trade-off is that the carbon can accumulate radioactivity over time, which requires monitoring and eventual proper disposal of the spent media. For homes with high water radon, the accumulated radioactivity can become a handling concern, which is part of why aeration is preferred at higher concentrations.
What does this mean for Colorado well owners?
Colorado’s geology produces elevated radon in both air and, in some areas, well water, so private-well households across the Front Range and rural areas have a genuine reason to consider both tests. While municipal customers in the region rarely face a waterborne radon concern, the many homes on private wells in foothill and rural Colorado are a different case. For them, the water pathway is worth understanding alongside the more familiar air pathway.
The practical message is to treat air and water as related but separate questions. A clean air test does not rule out a water concern, and a clean water test does not rule out soil-sourced radon entering through the foundation. Testing both, where a well is involved, gives the full picture and prevents addressing one pathway while ignoring the other. This is especially relevant for foothill and rural households whose water comes straight from the ground without any treatment that might strip radon out first.
For households with diagnosed radon in either pathway, the path forward is well established. Soil-sourced radon is handled with an air mitigation system that vents soil gas above the roof, while waterborne radon is handled with point-of-entry treatment such as aeration or carbon filtration. Working with qualified professionals for testing and treatment, and retesting after any system is installed, ensures the household ends up genuinely protected rather than merely reassured.
References
- Radon in Drinking Water — U.S. Environmental Protection Agency
- Radon Testing Guidance for Colorado — Colorado Department of Public Health and Environment
- Radon Exposure and Health — Centers for Disease Control and Prevention
Private-well owners across the Front Range can reach out through our contact page to connect with a vetted professional for combined radon air and water testing.