Radon in well water

Radon can dissolve in groundwater, especially where wells draw from granite and other uranium-bearing rock. Homes on private wells can get some of their indoor radon through the tap.

How water adds radon to indoor air

When water comes out of a faucet or showerhead, or runs through a dishwasher or washing machine, some of the dissolved radon escapes into the air. EPA lists the water supply as one of the ways radon enters homes. For most households the main risk from radon in water is breathing it after it is released, with a smaller risk from drinking it. In most homes, radon from soil gas contributes far more to indoor levels than radon from water, but in areas with very high radon in groundwater the water can matter.

Public water vs private wells

Radon escapes from water that is stored and treated, so homes on large public systems that use surface water (rivers and reservoirs) rarely get much radon from their water. Private wells deliver groundwater directly from the rock to the house. The USGS notes that private well owners are responsible for testing their own water, since federal drinking water rules do not cover private wells.

Is there a federal limit?

No enforceable federal limit for radon in drinking water has been finalized. EPA proposed a radon rule in 1999 that paired a standard for water with programs to reduce radon in indoor air, but it has not been finalized. Some states publish their own guideline levels for radon in well water. Your state radon or drinking water program can tell you what applies.

How to test well water

• Test your indoor air first. If air results are elevated and you have a private well, consider testing the water too.
• Water tests require a special sampling procedure because radon escapes quickly. Get a kit from a certified lab or your state program and follow its instructions exactly, often filling a vial underwater from a tap with no aerator.
• Results are reported in pCi/L of water, which is not the same scale as pCi/L of air.

Treatment options

Two approaches are common for whole-house treatment:

• Aeration systems bubble air through the water and vent the radon outdoors. They are usually the choice for higher levels.
• Granular activated carbon (GAC) filters adsorb radon. They can work for lower levels, but the carbon can accumulate radioactivity, so sizing and disposal need care.

Point-of-use filters at a single tap do not address radon released by showers and appliances. A water treatment professional can recommend a system based on your results.

Related

If you have a well, you may also want our sister topic of general well water testing, including bacteria, nitrate, and arsenic. For radon in air, start with how to test your home.

Testing the rest of your well water

A radon test is not a full water test. For bacteria, nitrate, and the other basics, see well water testing on our sister site Septic & Well Guide.

How much does water radon add to indoor air?

When water containing radon is sprayed, heated, or agitated, during showering, washing dishes, or doing laundry, some of the dissolved radon escapes into the air. A commonly cited rule of thumb in radon guidance is that about 10,000 pCi/L of radon in water adds roughly 1 pCi/L to indoor air, on average. Actual contribution depends on how much water the household uses, the home's size and ventilation, and where water is used.

That means:

• For most homes, radon from soil gas is a much larger source than radon from water.
• But in areas with granite or other uranium-bearing bedrock, well water can carry tens of thousands of pCi/L, adding a meaningful amount to indoor air, along with a smaller risk from drinking it.

Interpreting a water test result

No federal drinking water standard for radon applies to private wells. EPA proposed a regulation for public water systems in 1999, with a maximum contaminant level of 300 pCi/L and an alternative higher level of 4,000 pCi/L for systems in states with programs to reduce radon in indoor air. That proposal hasn't been finalized. Some states publish their own guidance levels for private well owners. Check with your state radon program or health department. Many advise treatment at several thousand pCi/L or higher, and they always recommend testing indoor air too.

Testing well water correctly

Water radon tests are sensitive to how the sample is taken, because radon escapes quickly when water is agitated:

1. Use a kit from a certified lab that analyzes radon in water, often a vial filled underwater or with minimal aeration.
2. Run the water for several minutes first, so the sample comes from the well, not from the pressure tank or pipes.
3. Collect the sample from a tap without an aerator, or remove the aerator.
4. Fill the vial with no air bubbles, cap it immediately, and send it to the lab quickly. Radon decays with a half-life of about 3.8 days.

Treatment options compared

Method	How it works	Notes
Aeration	Bubbles or sprays water to release radon, which is vented outdoors	Often very effective, even at high levels. Larger, more costly equipment that needs venting and maintenance
Granular activated carbon (GAC)	Radon adsorbs onto carbon in a tank	Lower cost, but less suited to very high levels. The tank can accumulate radioactivity and may need shielding or special disposal

Point-of-entry systems treat all water entering the house, which matters because the main exposure from water radon is breathing it during water use, not drinking it. Point-of-use filters at a single tap don't address shower and laundry release.

Testing for other contaminants

While you're testing for radon, consider testing for uranium, gross alpha, and radium, which can occur in the same geology, along with routine well tests for bacteria, nitrates, and arsenic. A state-certified lab can recommend a panel based on your area.

Indoor air comes first

Because soil gas usually contributes more, EPA's general advice is to test indoor air first. If indoor air is elevated, fix the soil-gas pathway with a mitigation system. If indoor air remains high after that, or if water tests show very high radon, consider water treatment.