Radon in Well Water: When Does It Matter?

How Much Radon in Well Water Affects Indoor Air

The transfer from water to air is inefficient, which is the useful thing to know first. The widely used rule of thumb is that it takes something on the order of ten thousand picocuries per litre in water to add roughly one picocurie per litre to indoor air. Water concentrations have to be very high before they register meaningfully against what the soil is already supplying.

That ratio is why water is rarely the answer to a high reading, and why chasing it before addressing soil gas usually wastes both money and time.

Why Municipal Water Is Rarely a Radon Source

Public water systems draw from sources that are aerated, stored and treated, and radon is volatile enough that most of it leaves during that handling. By the time water reaches a tap on a municipal line, the contribution is generally negligible.

Private wells are different because the water goes from the ground into a pressure tank and into the house with none of that intervening exposure. If the aquifer sits in rock producing radon, the water arrives carrying it.

Where it shows up, and what to do

The signature is location. Soil gas produces its highest concentrations on the lowest level, closest to the ground. Water-borne radon is released wherever water is agitated, so it appears in bathrooms during showers and in the laundry, and a house where upstairs readings behave oddly relative to the basement is worth a second look.

The sequence that makes sense is soil gas first, then water only if a system that is demonstrably working has not brought the house where it should be. Water testing is a separate laboratory test rather than an air kit, and treatment is its own equipment: aeration at the point of entry, or granular activated carbon for lower concentrations.

Still deciding what to do about a reading? The testing page covers how a measurement is taken, and what a system costs breaks the ranges down by method. If you got here from a map or a ZIP-code lookup, what Ohio's radon maps show explains what those averages are actually measuring.