Where is Waterborne Radon Found? Radon is produced by the radioactive decay of radium in the ground. Granites and other rocks may contain high concentrations of radium and produce significant amounts of radon. Aquifers in these rocks accumulate the radon, increasing the concentration in the water.
The highest radon concentrations in water are found in ground water supplies. Radon is more common in smaller ground water supplies . . . those serving 25 or fewer people.
Radon in rocks of surrounding aquifers diffuses into the water, and radon remains trapped in the water until exposed to air. Once the water is exposed to air, the radon can move rapidly to the air, where it becomes a concern because it may be inhaled. Agitating and heating water increases radon movement to the air. Clothes washing machines, dishwashers and water heaters can become sources of airborne radon.
Drinking water contaminated with radon may provide a risk of cancer, but the risk is higher when the radon moves from the water into indoor air. Radon can accumulate inside buildings to levels which may be harmful.
It is clear that:
- Radon moves easily from water to the air, so waterborne radon can contribute to airborne radon.
- Health risks from radon in water, even if they are small compared to those of radon in air, are greater than almost anything else the EPA regulates in water.
- Some homes in the United States have very high concentrations of radon in water and this may be the primary source of airborne radon in those homes.
Radon Water Treatment
Four methods can be used to treat water contaminated with radon. Each of these has been used to treat water in homes or in small water supplies and each can be cost-effective depending on the circumstances.
Blend ground water with surface water
Here we simply dilute the water containing high radon concentrations with water containing no radon or a very low concentration. The amount of dilution needed is directly proportional to the radon concentration in the ground water. This approach is effective only for ground water concentrations at or below 1,200 pCi/L. Beyond that level, the amount of water needed for dilution would too great.
Storing the water before use
Radon has a half-life of 3.8 days (a half-life is the length of time required to reduce the radioactivity by one-half.) When we store the water for one half-life (3.83 days) the radon concentration is cut in half. When we store it for two half-lives (7.6 days) we anticipate a reduction in the concentration by a factor of four. And each additional 3.83 days of storage reduces the concentration by an additional factor of two.
Obviously, the usefulness of this approach depends on the length of time over which it is possible and reasonable to store the water.
Granular Activated Carbon Filtration
Activated carbon may be used to remove radon from water as well as many other pollutants and odors. As water is passed through the activated carbon filter, the radon is adsorbed onto the carbon. Filter effectiveness depends greatly on the contact time with a longer contact time assuring increased removal.
All carbon cartridge filters should be replaced every few months due to the buildup of wastes, bacteria and other contaminants. It is advisable to pre-filter sediment (using a sediment filter) to increase the life of an activated carbon filter. Note: Because radon trapped in a carbon filter eventually decays and becomes other radioactive materials that can pose a health risk, regular filter changes are required. Handling and disposal of these filters as radioactive waste may be required. Check with local authorities before disposing of the used filter.
Because radon is highly volatile, it transfers easily to the air when water is aerated. Aeration can remove as much as 95 percent of the radon. The dollar investment in an aeration system for a home water supply is dependent on the volume of water treated. The initial investment usually ranges from a few hundred dollars to $2,000.00 (2011 estimate), with annual operating costs around $100.00.
Remember, when the radon is removed from the water by aeration, it is transferred the air. Concentrations in the air can be significant and hazardous. Aeration process air cannot be vented in a manner that allows it to enter a home or building. And it should NOT be vented to where people or animals may concentrate. In some cases air filters may be required.
Important Note: It may be that completely removing waterborne radon will reduce the risk from radon by only few percent. The challenge here is most radon enters the home from soil and rocks and not from water. However, there are places where waterborne radon contributes more than half of the total radon. Still, for most homes, we should be concentrating on lowering the radon in the air. The most cost-effective measure may be to allow the radon to move from the water to the home air and then ventilate or filter the air. The most important points to treat the air are in your basement, laundry room, bathrooms, kitchen, and any room where water is heated and/or stirred or agitated.
For more information on all aspects of radon visit the United States Environmental Protection Agency at: www.epa.gov/radon