Radon

Radioactive gas from decaying uranium — enters water through granite and uranium-bearing formations

EPA MCL

Proposed: 300 pCi/L (or 4,000 pCi/L with state air programs)

Health concern

Lung cancer (second-leading cause in US, after smoking)

Testing method

Certified lab alpha-track or liquid scintillation; $30-50 mail-in

Radon is a colorless, odorless, radioactive gas produced by the natural decay of uranium in rock and soil. If you live in a granite- or uranium-bearing geology, you almost certainly have some radon in your air and probably in your water. It's the second-leading cause of lung cancer in the United States, responsible for an estimated 21,000 deaths per year.

How it gets into well water

Uranium-bearing minerals in granite, granitic gneiss, and certain sedimentary formations decay through a chain that produces radium and then radon. Groundwater moving through fractured crystalline bedrock picks up the dissolved gas. When that water comes out of your tap — especially in showers, dishwashers, and washing machines where there's agitation — the radon off-gasses into your indoor air.

Most radon exposure at home comes from soil gas seeping up through foundations. But water-borne radon adds to that burden, and in areas with high-radon geology it can be the bigger source in well-water households.

Where it's a problem

Radon in well water is most severe in regions with uranium-rich granitic or metamorphic bedrock:

• Colorado Front Range and mountain communities — Pikes Peak granite has unusually high uranium content. Colorado mountain wells commonly test at 1,000-3,000 pCi/L, with levels above 5,000 pCi/L common in the Woodland Park / Divide area.
• New England — Maine, New Hampshire, Vermont, and Massachusetts all have granite-bedrock regions with elevated well-water radon. Maine in particular has been extensively mapped.
• Appalachian crystalline regions — North Carolina, Virginia, and Georgia foothills.
• Pacific Northwest — parts of Washington and Oregon.

What the numbers mean

Radon is measured in picocuries per liter (pCi/L). The EPA's proposed drinking water standard is 300 pCi/L, which applies in states that don't have their own indoor-air radon programs. Where states do have air programs, an alternate standard of 4,000 pCi/L applies because the calculation assumes air radon is being addressed separately.

Either way, typical high-radon-area mountain wells that test 1,000-3,000 pCi/L are well above the strict standard and approaching the alternate one. Levels above 5,000 pCi/L are not unusual. If you're in a known radon geology and haven't tested, you probably should.

Testing

Radon in water requires a specific test — you can't infer it from an indoor air test. Look for:

• Certified lab with radon-in-water accreditation
• Liquid scintillation or alpha-track method
• Typical cost: $30-50 per sample, mail-in kit

Sample collection matters: you need to draw water into a sealed vial without agitation, because any disturbance lets the radon off-gas before it reaches the lab. Follow the kit instructions exactly.

Treatment

Two treatment approaches actually work:

• Aeration systems (the right answer) — bubble air through the water to drive off dissolved radon, which is vented safely outside. Removes 95%+. Cost: $3,500-6,000 installed.
• Granular activated carbon (GAC) — adsorbs radon onto carbon media. Works, but the carbon bed becomes radioactive over time and needs careful disposal. Less favored by regulators.

Reverse osmosis removes radon from the treated water but only at the single tap it's installed at, and it doesn't address the whole-house exposure from showers and appliances. RO is not a solution for radon.

Aquifers where this is a concern