Radon in Well Water: Aeration vs. Carbon Treatment

By SubcontractorHub Editorial Team·Published October 2026

Dealers meet radon when a buyer's inspector, a state radon office, or a granite-country health department adds it to a well panel. It does not stain a fixture and it does not smell. A hardness strip will not see it. This page is the field sequence: what the law does and does not say, how the sample has to be taken, and how to choose between aeration and carbon without selling a point-of-use filter as a whole-house fix.

Start the rest of the well panel from the well water testing guide. Radon is an add-on, not a substitute for coliform and nitrate.

Why the Shower Matters More Than the Glass

Radon is a radioactive gas that dissolves into groundwater as water moves through rock, especially granite and similar bedrock. At the kitchen tap it is still dissolved. When that water is sprayed, heated, or agitated, the gas leaves the water and enters the room. Showers, dishwashers, and washing machines are the releases. Drinking is a smaller pathway, associated with the stomach rather than the lung. The lung pathway is why an indoor-air test still belongs on the job even after the water is treated.

EPA has long used a planning transfer: a large concentration in water contributes a much smaller concentration to household air, on the order of one unit in air for every ten thousand units in water. That ratio is a planning sketch for a typical house, not a measurement of the house you are standing in. Ventilation, how long people shower, and how much water the home uses all move the air number. Do not quote the ratio to a homeowner as their result.

Soil gas under the slab is still the usual source of indoor radon. A well can add to it. A water system that ignores the air test, or an air system that ignores a high water test, leaves a pathway open. Say that before you quote equipment.

No Single Federal Limit for a Private Well

Public water systems are covered by the Safe Drinking Water Act. A private well that serves one home is not that kind of system. EPA does not send a sampler to the wellhead, and there is no finalized federal maximum contaminant level for radon in drinking water that you can print on a proposal as national law.

In 1999 EPA proposed a community-water regulation for radon. It was never finalized. The proposal is sometimes repeated online as if it were the current private-well rule. It is not. It would not have made a homeowner's well into a regulated public system. States and some counties publish their own guidance for private wells, and those sheets disagree with each other. The number that matters is the one your state radon program or health department is using for that address, plus the indoor-air result.

If a rep needs a sentence for the kitchen table, use this one: there is no national private-well limit in force, so we follow the state program and we test the air as well as the water. Do not fill the silence with a picocurie figure you remember from another state.

How to Collect the Water Test

Order the kit from a laboratory the state program accepts. Hardware-store air canisters do not answer the water question. Take the sample from a cold, unsoftened, untreated tap if you can reach one. A sample after a carbon tank or an aerator you forgot to remove is not the well.

• Remove the faucet aerator so the water does not splash and lose gas.
• Run the cold water long enough that you are sampling the well, not the house piping.
• Fill the vial slowly, with no bubbles, and cap it so no air remains.
• Ship it the day you draw it. Radon's half-life is about 3.8 days. A kit that sits in a truck over a weekend is a low result, not a safe well.

Write the tap, the time, and whether treatment was bypassed on the work order. The next tech should not have to guess which water was tested. The annual list this sample sits beside is the well water testing guide.

Aeration versus Granular Activated Carbon

Two whole-house methods are in real use. They are not substitutes for each other at every concentration, and neither one is a drinking-water pitcher.

Aeration

An aeration system sprays or bubbles air through the water in a tank and blows the released radon outdoors, above the roofline, the way a soil-gas fan vents a basement. The gas leaves the property instead of accumulating in a media bed. Aeration is the method dealers reach for when the water result is high, because the vent is the point. The unit needs a place for the blower, a drain for the tank, and a vent route that does not discharge under a soffit or next to a bedroom window. Iron and manganese will plate the trays or the packing if you skip pretreatment. Ask the manufacturer what the unit is rated to remove at the flow you measured, and retest the water after startup. Do not print a removal percentage from memory.

Granular activated carbon

A point-of-entry carbon tank adsorbs radon as water passes through. It costs less to set on a slab than a blower and a vent, which is why it gets proposed too often. The bed does not destroy the radioactivity. Decay products build up in the carbon, and the tank can emit gamma radiation. That is why carbon is generally discussed for lower water results, with shielding, setbacks from living space, and a plan for media that may need to be handled as radioactive waste. A state program may restrict or discourage carbon for radon. If the program says no, the proposal says no.

Size a carbon shell for contact time with the carbon filter sizing calculator only after you know carbon is the allowed method. That tool is built around chlorine and chloramine capacity, not a radon loading rate. Use it for tank and flow planning, then confirm the radon specification with the media supplier. It will not tell you whether carbon is legal or wise for this well.

Pretreatment, and What Not to Sell

Radon equipment fails in ordinary ways before it fails as radon equipment. Sediment packs a bed. Iron and manganese foul aeration media and carbon. A well that cannot hold a backwash will compact whatever tank you put in front. Sequence the train: sediment, then iron and manganese if the test shows them, then the radon stage, with the vent or the carbon tank where the state wants it. The well water treatment system selector is the order-of-stages check. The iron filter sizing calculator and the backwash rate calculator are how you find out whether the well can wash the tanks you are about to specify.

Do not sell an under-sink filter, a refrigerator cartridge, or a pitcher as the radon fix. Those devices, even when they touch the drinking tap, do nothing for the shower and the laundry, which are the release points. If the homeowner wants drinking water polished for something else, that is a separate stage at the sink.

After installation, retest the water with the same vial method, and retest indoor air if the air was the reason the job started. A startup sheet without a retest is a story. Put the retest on the proposal so it is part of the price, not a favor.

What the Dealer Writes on the Proposal

Record the lab, the picocuries per liter, the sample date, and the tap. Record the indoor-air result if you have it. Name the method, aeration or carbon, and the pretreatment in front of it. Name the vent termination if you are aerating. State that the state program's guidance was the decision rule, and attach it. Leave installed price to the quote you actually build. This page does not carry a national equipment price.

A water treatment dealer who keeps that packet on the customer record can explain the retest without reconstructing the job from memory. The platform side of that visit is the same proposal workflow as the rest of the well train: result, stages, and a follow-up already scheduled.

Frequently Asked Questions