Radon in public water supplies usually decays to low concentrations before the water is delivered to users, especially if the water has been treated. Because a private well system delivers only to one or two households, there is a greater chance the radon will not dissipate before the water is consumed. Since radon will decay, storage alone can allow radioactivity to diminish. However, a household using 150 gallons of water per day would require 1,800 gallons of storage to achieve 90 percent reduction and 3,700 gallons to achieve 99 percent reduction. The U.S. EPA has not established a Maximum Contaminant Level for radon in drinking water supplied by public systems.
Radon likely is released to outdoor air before reaching home faucets.
Some public water systems mix their water supplies that have elevated radon or radium levels with other sources to lower the percentage in the drinking water. Such mixing does not eliminate the radon or radium, but can bring it within acceptable EPA levels. When water is supplied by a municipal system, radon is generally released while the water is being treated in the system. In addition, when water is held in a storage area, the possibility of high levels of radon is minimized because radon quickly escapes into the open atmosphere. Radon in outdoor air is diluted to insignificantly low concentrations.