Guide

Radon in Well Water

Air first, water second

In almost every house with a radon problem, the source is soil gas entering through the foundation. Water is a secondary path, and it is worth investigating only in a specific situation: you are on a private well, and indoor air levels are elevated in a way the foundation does not explain, often after a mitigation system has already been installed and the numbers still have not come down enough.

Doing it the other way round, testing water first, is a common way to spend money on the wrong problem. Start with an air test.

Why private wells and not public water

Groundwater passing through uranium-bearing rock picks up dissolved radon. Whether that reaches your tap depends on what happens to the water in between.

Public water systems typically store, treat and move water through processes that agitate and aerate it, and much of the dissolved radon escapes before delivery. Water also spends time in storage, and radon's short half-life means a lot of it simply decays. A private well draws directly from the aquifer to a pressure tank to the tap with none of that, so whatever was dissolved in the groundwater is still there.

York County has a substantial number of homes on private wells, which is why this is worth a page rather than a footnote.

Where it actually gets into the air

Radon leaves water when the water is agitated, warmed or sprayed. The high-release fixtures are predictable:

• Showers, by a wide margin, because hot water is broken into droplets in an enclosed room
• Washing machines, especially on a hot cycle
• Dishwashers
• Any tap running hard into a sink

A distinguishing pattern: a bathroom on an upper floor reading higher than the basement is unusual for soil gas, which concentrates at the lowest level, and is consistent with a water source. It is a hint rather than a diagnosis.

The rough conversion, and why it is rough

Waterborne radon is measured in pCi/L of water, and a commonly cited rule of thumb is that roughly 10,000 pCi/L in water contributes about 1 pCi/L to indoor air. Treat that as an order-of-magnitude guide, not a calculation: the actual contribution depends on how much water the household uses, the size and ventilation of the house, and how that water is used.

The practical implication is the useful part. Water radon concentrations have to be very high before they contribute meaningfully to indoor air. If your water tests in the hundreds of pCi/L, it is not what is driving a 12 pCi/L air reading, and the foundation is.

The two fixes

Aeration

Water is sprayed or bubbled in a tank at the point of entry, the radon comes out of solution, and it is vented outside. Aeration is highly effective across a wide range of concentrations and it does not accumulate radioactivity in a filter medium. It costs more to install, needs power, and requires a vent to the outdoors and some space to put the tank.

Granular activated carbon

Water passes through a carbon bed at the point of entry and radon is adsorbed. Cheaper, simpler, no power, no vent. The trade-off is that the carbon accumulates radon decay products over time, so the unit becomes a low-level radiation source that has to be sited sensibly, away from occupied space, and eventually replaced and disposed of appropriately. It also suits lower concentrations better than very high ones.

Either way this is point-of-entry equipment, treating all the water entering the house, not a tap filter.

The order to do things in

1. Test indoor air with a short-term kit. See short-term vs long-term
2. If air is above 4 pCi/L, deal with the foundation first, because it is almost always the dominant source
3. Retest air after mitigation and confirm the result
4. If air is still elevated after a correctly performing system, and you are on a private well, test the water
5. Treat the water at the point of entry if the concentration warrants it, then retest the air again

If you are testing the well anyway, it is worth testing for the ordinary things at the same time, since the sampling visit is the expensive part. Related: what to do with a high result and how the soil-gas system works.

Common questions

Do I need to test my water for radon?

Only if you are on a private well and your indoor air test came back elevated. Soil gas is the dominant source in almost every house, so air testing comes first. If a mitigation system has been installed and air levels are still above the action level, water becomes a suspect.

Is drinking the water the risk?

The larger risk is inhalation, not ingestion. Radon leaves the water when it is agitated and heated, so a shower releases it into the air you breathe. There is an ingestion risk too, but it is the smaller part.

Does a normal water filter remove it?

No. A standard under-sink or pitcher filter does nothing for dissolved radon. The two working approaches are whole-house aeration and a properly sized granular activated carbon unit, both installed at the point of entry.