Radon in the ground can dissolve into ground water. When this ground water is brought into a building directly (without the benefit of being held in an atmospheric holding tank) the dissolved gas is released from the water.

As a rule of thumb for residential structures on private wells, if a well contained 10,000 pCi/l of radon in the water, the amount of radon entering the building from normal water usage-after dilution by normal residential ventilation-would add 1 pCi/l of radon into the air in the building above and beyond that which comes from the soil.

There are no current U.S. standards regarding the amount of radon allowed in public drinking water supplies, although standards have been proposed.

Many EPA publications recommend testing for radon in water for all homes with private wells that have found elevated radon concentrations in air (levels greater than 4 pCi/l). However, elevated indoor radon concentrations are most often caused by radon in soil gas, not radon in household water. In fact, only in a few areas of the United States would radon from well water be expected to make a significant contribution to radon in air concentrations, and Michigan is not among those areas. (It should be noted that the majority of the health risk from radon is associated with inhalation, not ingestion, and drinking or cooking with water that contains some radon is not known to pose a significant health risk.)

MDEQ recommends that elevated indoor radon levels be treated first as a soil gas problem, using conventional radon reduction techniques in an attempt to lower those levels. Radon in water testing need not be routinely conducted, and would be recommended only if the conventional techniques are not successful. Then the homeowner may wish to test the water to determine whether it is contributing to the radon in air levels. However, it should be noted that, as of this time, there are no lists of laboratories certified to measure radon in water.

For more information on radon in water and/or the proposed regulation of radon in public water supplies, contact the EPA Drinking Water Hotline (1-800-426-4791) or visit any of these websites:

• National Academy Press, Risk Assessment of Radon in Drinking Water (1999)
• EPA Office of Water, Drinking Water Standards Program - Radon (includes link to the proposed drinking water rule)
• EPA Office of Water, Office of Groundwater and Drinking Water, NAS Report on Radon in Drinking Water