Radon in Water: When Your Well Is Part of the Problem

Most radon that ends up in a home's air comes from soil gas moving up through the foundation. But if your home uses well water instead of a municipal water supply, there's a second, less obvious pathway worth understanding: radon dissolved in groundwater can be released into the air every time that water is used indoors.

How radon gets into groundwater

The same uranium-bearing rock and soil that produces soil-gas radon can also produce radon dissolved directly into groundwater as it passes through those formations. Private wells, which draw water straight from underground aquifers, are far more likely to carry meaningful radon levels than municipal water systems, which typically draw from surface water sources (lakes and rivers) where radon has already dispersed into the air before treatment, or which hold water in open reservoirs long enough for radon to dissipate naturally.

Why it matters indoors

Radon dissolved in water doesn't pose much risk just from drinking it occasionally, the bigger concern is inhalation. Every time water runs, whether it's a shower, a dishwasher cycle, or a washing machine, some of the dissolved radon gas releases into the air, known as off-gassing. Showers are typically the biggest contributor since they involve heated water, spray dispersion, and an enclosed space, all of which increase how much radon escapes into the air you're breathing.

For most homes, radon from soil gas is a far bigger contributor to indoor air levels than radon from water. But in homes with a private well and notably high water-radon concentrations, water can be a meaningful secondary source, sometimes enough to keep indoor air levels elevated even after a standard sub-slab depressurization system is installed and working correctly.

How to know if water is a factor

If your home is on a private well and:

• Indoor air radon tests come back elevated even after a soil-gas mitigation system is installed and confirmed working, or
• You simply want a complete picture before deciding on a mitigation approach,

a separate water radon test is worth doing. This is a distinct test from a standard indoor air radon kit, it requires a water sample collected in a specific way (usually directly from an indoor tap, avoiding aeration) and sent to a lab that tests water specifically. Many of the same companies that sell air test kits also sell water test kits, but they are not interchangeable.

Treatment options for radon in water

If a water test confirms elevated radon, two main treatment approaches are used:

• Granular activated carbon (GAC) filtration. Water passes through a tank of activated carbon that adsorbs radon and other contaminants. This is often less expensive upfront but can allow radioactivity to accumulate in the filter media over time, which occasionally requires special handling or disposal, and doesn't work as well on very high radon concentrations.
• Point-of-entry aeration systems. These treat all water entering the home by mechanically stripping dissolved radon gas out before it ever reaches a faucet, typically venting the removed gas safely outside. Aeration is generally considered the more effective and more commonly recommended approach for higher water-radon levels, though it costs more to install than carbon filtration and requires ongoing maintenance of pumps and vents.

How this fits with air mitigation

Water treatment and soil-gas mitigation address two different entry pathways and are usually evaluated separately. A home can need one, the other, or both. The general order of operations most contractors recommend:

1. Test indoor air first, since soil gas is the more common and often larger contributor.
2. If air levels are elevated, install standard sub-slab (or sub-membrane) mitigation and retest.
3. If air levels remain elevated after a properly functioning air system, and the home is on a private well, test the water specifically.
4. If water radon is confirmed elevated, evaluate aeration or carbon filtration with a water treatment specialist.

Who to call

Not every radon mitigation contractor works on water treatment, it's a somewhat different specialty that overlaps more with well and water treatment professionals. When researching a contractor, it's worth asking directly whether they handle both air and water mitigation, or whether they can refer you to a water treatment specialist if testing shows it's needed. You can browse licensed and certified contractors by state in our directory, and many listings include contact information to ask this question directly before scheduling any work.