Radon is a naturally-occurring radioactive gas.  It is a product of uranium and is ubiquitous in the earth's crust.  Hot spots for radon in water include Maine, New Hampshire, Connecticut, North Carolina and Arizona.

Large municipal water systems usually aerate water outdoors, dispersing radon gas to the atmosphere.  Open water sources such as rivers, lakes or reservoirs allow radon to escape naturally.  Radon is most likely to be present in unaerated water from private wells or community wells serving fewer than 500 people.  Showering, dish-washing and laundering agitate radon laden water and release the gas to indoor air.

Before testing your water for radon, test the air.  If the air level is low, don't worry about the water.  If your indoor radon level is high and you use groundwater, you should test your water also.  Some states have programs that will test water for radon at a modest cost,  Commercial laboratories, including mail-order companies, charge between $20 and $35 per sample. 

Test results are expressed in picocuries of radon per liter of water.  According to the EPA, one should definitely take action if the level is 10,000 picocuries per liter or higher. 

Granular activated carbon units and home aerators can reduce radon in water. An activated carbon unit for radon removal resembles a water-softener tank.  Properly designed and installed, it should reduce waterborne radon levels by 90%.

Home aerators haven't been as extensively tested as carbon devices.  Aeration tanks are often placed in the basement.  Pumped-in air agitates the water and causes radon to bubble off, a pipe vents the gas to the outdoors.