Radon is a naturally occurring, colorless, odorless, and tasteless radioactive gas that forms from the breakdown of uranium and thorium found in nearly all soil and rock. Because this gas is fairly soluble, it can dissolve into water sources, particularly those drawn from underground. Radon gas breaks down quickly, giving off radioactive particles that can be harmful when inhaled or ingested. The presence of radon in water is a concern, but it is often manageable with the right understanding and mitigation steps.
How Radon Enters Water Supplies
Radon enters water supplies primarily through contact with bedrock and soil that contain naturally occurring uranium and radium deposits. As uranium decays into radium, and radium further decays into radon, the gas migrates through the ground and dissolves into the surrounding groundwater. Water drawn from underground sources, such as private wells, is therefore at the highest risk for elevated radon levels.
Private well systems are more vulnerable because they draw water directly from the aquifer without the extensive treatment and storage common in public systems. Since the water is used quickly, dissolved radon has little time to escape. Public water systems often source water from surface bodies like lakes and reservoirs, where radon is readily released into the atmosphere due to aeration and exposure. Even when public systems use groundwater, the water is usually subjected to aeration, stored in large reservoirs, and diluted, causing much of the radon to dissipate before reaching the tap.
The Primary Health Concern: Inhalation vs. Ingestion
The most significant health risk from waterborne radon is not from drinking the water, but from inhaling the gas released into the indoor air. When water containing dissolved radon is agitated or heated, such as during showering or washing dishes, the gas escapes from the water and mixes with the air inside the home. This process, called volatilization, results in airborne radon that is then inhaled.
Once inhaled, the radioactive decay products of radon lodge in the lungs, where they emit radiation that can damage tissue and increase the risk of lung cancer. This inhalation pathway accounts for the vast majority of cancer risk associated with radon in water supplies. While ingesting contaminated water carries a small, secondary risk of stomach or other internal organ cancers, this risk is extremely low compared to the danger posed by inhaling the released gas. For every 10,000 picocuries per liter (pCi/L) of radon in water, the indoor air level increases by approximately 1 pCi/L, illustrating how waterborne radon contributes to the overall indoor air concentration.
Testing Your Water for Radon
Testing is the only way to determine if radon is present in your water supply, especially for homeowners who rely on a private well drawing from groundwater. The Environmental Protection Agency (EPA) recommends that anyone with a private well test their water for radon, particularly if the indoor air has already tested high for the gas. Public water customers can check their annual Consumer Confidence Report or contact their water utility to see if testing has been performed.
The testing process involves collecting a water sample directly from the tap in a specialized vial, which is then sealed and sent to a certified laboratory for analysis. Radon concentration in water is measured in picocuries per liter (pCi/L). While a national legally binding standard does not currently exist, guidance suggests taking action if the water level exceeds 4,000 pCi/L to 10,000 pCi/L. Testing should ideally be done twice to get an accurate average reading and confirm the need for mitigation.
Strategies for Removing Waterborne Radon
When water testing reveals high levels of radon, homeowners have two primary, effective strategies for removal that treat the water at the point-of-entry (POE) into the home. These whole-house systems ensure that all water used for household activities is treated before it reaches the taps. Choosing the correct system depends on the measured concentration of radon and the homeowner’s budget.
The first method is aeration, which involves spraying the water or mixing it with air inside a tank to strip the radon gas out of the liquid. The released radon-rich air is then safely vented to the outdoors, often achieving a removal efficiency of up to 99%. Aeration systems are highly effective, making them the preferred technology for very high radon concentrations, often exceeding 10,000 pCi/L.
The second strategy uses a Granular Activated Carbon (GAC) filtration system, where the water flows through a large tank filled with activated carbon material. The radon gas adheres to the surface of the carbon through adsorption, effectively trapping and removing it from the water. GAC systems are generally less costly than aeration units and are suitable for moderate radon levels, typically below 10,000 pCi/L. A consideration is that the trapped radon continues to decay, causing the carbon tank to become radioactive over time, which necessitates careful handling and disposal of the spent filter media.