Radon in water: wells, health risk, testing and treatment

Radon comes from the decay of uranium in the earth's crust. When it is released from underground rocks such as granites or shales, radon can move easily toward the surface in air or water, and groundwater sources such as springs, wells and boreholes normally carry higher radon concentrations than surface water from reservoirs, rivers or lakes. Surface water in contact with the air lets radon escape into the atmosphere.

Even neighbors differ. Two adjacent wells may show very different radon concentrations, and levels in well water can vary through the year. A neighbor's result therefore says little about your well.

The main danger is the air it creates. Radon in a water supply poses an inhalation risk and a smaller ingestion risk, and most of the risk from radon in water comes from radon released into the air when water is used for showering and other household purposes. Research has shown that the lung cancer risk from breathing radon in air is much larger than the stomach cancer risk from swallowing water with radon in it.

Put plainly, the vast majority of radon-related health risk is from breathing radon. How radon harms the lungs is covered in the radon and lung cancer guide.

That ratio is the bridge between the two numbers. A worked example: 4,000 pCi/L in the water contributes about 0.4 pCi/L to the air, one tenth of the 4 pCi/L action level and about the level of outdoor air. The radon escapes when water is agitated: showers, faucets, dishwashers and washing machines.

The arithmetic is an average estimate, not a measurement of your home. Air and water radon are separate: soil gas radon does not predict well water radon, nor the reverse. EPA recommends fixing a home when the radon level is 4 pCi/L or higher.

Test the air first. Although radon gets into some homes through water, it is important to first test the air, and if you find a radon problem in the air and your water comes from a well, have the water tested. Well owners who are concerned can go further and test for radon in both air and water, so they can weigh the mitigation options more completely.

Air tests and water tests use different devices and procedures. How water is sampled is on water testing, and the air test on radon testing.

There is no federal number for private wells. At present no action level is set at the federal level for radon in drinking water, and as of 2015 EPA had set no maximum contaminant level for it. Some states set their own, and those that have set limits put them between 4,000 and 10,000 pCi/L.

You need your air result too: if the air has not been tested, test it before deciding how to reduce radon, and test the air whatever the water result.

Radon is treated where water enters the house. Point-of-entry treatment removes radon before the water reaches the home's pipes and usually uses granular activated carbon (GAC) filters or aeration systems; both are straightforward and effective techniques. Point-of-use devices treat water at one tap, such as drinking water, and do not reduce the risk from radon released into the air from all the water used in the home.

Vermont discourages GAC for radon because the radon collected on the filter could create a radiation hazard. Details, certification and upkeep are on radon water treatment.

If your water comes from a public system and you are concerned, contact your water supplier; some public water systems treat their water to reduce radon before delivering it. The risk from a public system sourced from groundwater is considered lower than from a private well, so ask your provider whether it tests for radon.

If you have tested the air, found elevated levels, and the water comes from a private well, have the water tested before a sale. Some states regulate who may test: in Maine, a Maine registered radon tester must do the test unless the owner or occupant does it, it is illegal for a realtor or home buyer to do a radon in air or radon in water test, and a home for sale needs an independent registered tester.