Source #3 – Radon in Water Sources

Although soil gas is the primary source of radon in homes, radon is water-soluble and can enter the water supply from surrounding bedrock and contribute to indoor radon levels. Homeowners concerned that radon may be entering the home through the water supply should first conduct an air sampling and treat air problems according to standard recommendations. If indoor radon levels cannot be successfully mitigated subsequent water sampling may confirm a radon-in-water problem.

Radon in Water Studies

To date, epidemiological studies have not found an association between radon in drinking water and cancer of the digestive and other systems. From 1983 to 1988, four national studies were conducted on radon in private & public water supplies in the United States.  In fact, one particular study examined 9,000 measurements of radon in water from national and state surveys from all but 10 states. These studies concluded that the highest radon-in-water concentrations generally occur in portions of the Appalachian Mountains, Rocky Mountains, and Basin and Range.  The studies also concluded:

• Private well sources and small public water supplies tend to be higher in radon than large public water supplies.
• Radon concentration in the water tends to be high in areas where aquifers are comprised of uranium bearing granite, metamorphic rocks, or fault zones (as found in the mountain states).
• Large public water supplies tend to be lower in radon because they use high-capacity sand and gravel aquifers, which generally comprise low-uranium rocks and sediments.
• Private water supplies were higher in radon than public water supplies by factors of 3 to 20. 
• Rhode Island, Florida, Maine, South Dakota, Montana, and Georgia were the states with the highest radon in private well water.
• The New England states overall had the highest radon concentrations in water from all sources.

Radon Concentration in Air versus Water

EPA guidelines suggest that it takes approximately 10,000 pCi/L of radon in water to give an airborne concentration of 1pCi/L. 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 risk of lung cancer from breathing radon in air is much larger than the risk of stomach cancer from swallowing water with radon on it.  However, recent experimental work suggests the number of fatal cancers from radon ingestion in drinking water may be more significant than once thought.  Although the uncertainty of this work is large, it has raised the possibility that radon ingestion may represent a major pathway for risk when stomach cancer is included in the risk analysis.

Water to Air Radon Transfer

The ability for radon to transfer from water to air relies on 4 principles:

• The waterborne radon level.
• The amount of water used.
• The type of water activity (i.e. shower vs. faucet)
• The air and water temperatures.

The primary source of concern with regards to radon entering the air via water is the off-gassing of radon from hot water use in the home.  The Radon molecules dissolve in water then escape into the air as steam is created during hot water activities such as showering, hot water laundering, dishwashing, etc. Although dish washing and clothes washing can contribute to this source, the main source is a running shower or bath that emits steam and humidity into the air.  Radon and radon decay products can readily cling to these particles and remain in the air longer than typically expected.

Figure 3-5 illustrates that the primary source of concern with regards to radon entering the air via water is the off gassing of radon from hot water use in the home. 

Levels of radon in household well water supplies can range from 0 to over 1 million pCi/L.  Using the air to water rule of 1:10,000 pCi/L assumption, it would take a waterborne radon level of 40,000 pCi/L to result in the EPA guideline level of 4 pCi/L in the air.  However, the following considerations should be noted:

• The 1:10,000 ratio is not valid in all cases.
• Water is rarely the only contributor to the airborne radon level.
• Conditions at a specific household, such as size of house or amount of water used, may validate levels significantly lower than 40,000 pCi/L of radon in its water supply.
• Levels far greater than the whole house average can be found in areas of hot water use, such as showers, laundry rooms, etc.

The EPA does not currently have a maximum containment level (MCL) for radon in drinking water. However, in July 1992, in its proposed (revised) regulations on radionuclides for community drinking water supplies, the EPA proposed an MCL of 300 pCi/L.  This level was proposed because radon removal treatment is considered affordable and technologically feasible for public water supplies.  Using the air-to-water rule of 1:10,000, radon at this level contributes to about 0.03 pCi/L of radon in the air.  This corresponds to an estimated lifetime cancer risk of 2 x 10^-4 (2 in 10,000) and is consistent with the EPA’s risk guidelines for regulating other drinking water contaminants.

Note that the MCL is only directly applicable to public water supplies.  The current guidelines used by individual states for radon in private household water supplies typically range between 10,000 and 20,000 pCi/L.  This is expected to change with the proposed MCL of 300 pCi/L.  Subsequently, radon mitigators may find an increasing emphasis placed on water radon in the future even though the contribution is smaller than the EPA air guidelines of 4 pCi/L in most cases.