Radon in well water: how it differs from soil-gas radon

Most homeowners worried about radon in well water do not need to be: the EPA's published rule of thumb is that roughly 10,000 picocuries per litre (pCi/L) of radon in water contribute about 1 pCi/L to the air inside the house, so water-borne radon is almost always a much smaller contributor than the soil gas seeping up through the slab. The pathway only matters if you draw from a private well sitting on granitic bedrock — and even then, the fix is a separate, well-understood piece of equipment.

Two different pathways, often confused

When the EPA published its Map of Radon Zones, it was mapping one specific thing: the geological likelihood that radon gas from uranium decay in soil and bedrock would migrate up through the ground and accumulate inside a house. That is the dominant pathway in essentially every part of the country. Soil-gas radon is what the 4 pCi/L EPA action level was set against, what radon test kits measure, and what mitigation systems like sub-slab depressurisation are designed to fix.

Radon in well water is a separate, secondary pathway. The mechanism: groundwater moving through fractured granitic rock dissolves radon released by uranium-bearing minerals in the aquifer. The radon stays in solution under pressure and arrives at the tap still dissolved. Then, every time the household runs hot water — showers, dishwashers, washing machines, kitchen taps — a fraction of the dissolved radon comes out of solution and enters indoor air. So the water pathway ends, eventually, in the same place: a contribution to the radon concentration you breathe.

The difference is the size of the contribution and the universe of homes affected. Soil gas affects every house. Water radon only affects homes that use untreated private wells, and even then it typically delivers a small fraction of the indoor-air dose.

Why only private wells

Municipal water customers do not need to think about radon in water in any practical sense, for two compounding reasons.

First, surface-water systems — reservoirs, rivers, lakes — start with water that has had repeated contact with the atmosphere. Radon-222 has a half-life of 3.8 days, so almost all of it has decayed away by the time water reaches the treatment plant, and any residual radon off-gasses during open-air storage and treatment. Surface water is effectively radon-free at the tap.

Second, even most municipal groundwater systems aerate, settle, and store the water before delivery. Aeration in particular is highly effective at stripping dissolved radon. By the time the water reaches a customer's house, the concentration is generally a small fraction of what it was in the source aquifer. Some smaller community groundwater systems — particularly in granitic regions — can have measurable residual radon, but they fall under (or were proposed to fall under) federal drinking-water rules that municipal-only customers are not in a position to fix at the household level anyway.

Private wells are different. A private well pulls water directly from the aquifer, pressurises it into a closed system, and delivers it to the home with no aeration step. Whatever was dissolved in the bedrock is what comes out of the tap. That is the only household configuration where water-borne radon is plausibly a meaningful contributor to indoor air.

The 10,000:1 rule of thumb

This is the single most useful number in the water-radon conversation, and the one that should anchor any decision. The math works in the homeowner's favour almost every time:

• A well-water radon reading of 1,000 pCi/L — already in the higher band typical of New England private wells — implies roughly 0.1 pCi/L added to indoor air. That is well below the 4 pCi/L EPA action level and barely detectable above background variation.
• A reading of 5,000 pCi/L implies about 0.5 pCi/L in air — still small relative to the action level, but no longer negligible if the soil-gas contribution is already near the limit.
• A reading of 20,000 pCi/L — uncommon but seen in some Maine and New Hampshire wells — implies about 2 pCi/L from water alone. In a house with a soil-gas baseline of 3 pCi/L, that is enough to push the total above 4 pCi/L on its own.

The corollary: if your home tested low for indoor air radon, the water concentration would have to be extraordinary to change that. The water pathway is a meaningful problem in a narrow band of cases, not a general worry.

The proposed federal rule that was never finalised

In 1999 the EPA published a proposed rule for radon in drinking water (EPA-815-Z-99-006) under the Safe Drinking Water Act. The proposed rule had a distinctive two-track structure that is still cited in the literature:

• A maximum contaminant level (MCL) of 300 pCi/L in water for public water systems whose state did not adopt an indoor-air mitigation programme.
• An alternative MCL of 4,000 pCi/L in water for systems in states that did adopt a Multimedia Mitigation (MMM) programme — that is, a programme targeting the much larger indoor-air radon problem from soil gas.

The logic was unusual but rational: dollars spent stripping the last bit of radon out of drinking water buy far less risk reduction than dollars spent mitigating soil-gas radon in homes. The MMM track was the EPA's attempt to push states toward the higher-leverage intervention rather than locking them into the lower-leverage one.

The proposed rule was never finalised. The combination of cost-benefit arguments (it would have cost public water systems hundreds of millions of dollars per year for a relatively small share of total radon risk reduction), the political difficulty of the multimedia framing, and competing priorities meant it sat in proposed status. As of 2026 there is no enforceable federal MCL for radon in drinking water. The 300 / 4,000 pCi/L numbers are widely used as informal action levels by state programmes and laboratories, but they are not federal regulations.

Health context: stomach vs. lung

Radon in drinking water reaches the body through two routes. Drinking the water exposes the lining of the stomach to alpha radiation as the dissolved radon and its decay products move through the GI tract. Breathing air in which dissolved radon has off-gassed exposes the lungs to alpha radiation in the same way that soil-gas radon does.

The 1999 National Research Council report on radon in drinking water tried to quantify both. The headline conclusion was that the inhalation pathway — radon released from water into household air — accounted for the large majority of the cancer risk attributed to radon in water, and that the ingestion pathway contributed a small additional stomach-cancer risk. Neither risk is large compared with the indoor-air lung-cancer risk from soil-gas radon in high-radon areas. For framing on that larger pathway, see our explainer on radon and lung-cancer numbers.

The practical implication: water radon does not introduce a fundamentally new disease pathway. It is mostly the same lung-cancer mechanism as soil gas, just delivered through a different route, plus a small ingestion contribution. That is why the EPA's policy argument has consistently been that fixing the indoor-air pathway is the better-leverage intervention.

Where in the country water radon actually matters

The geography of high water-radon concentrations tracks fractured granitic bedrock aquifers. The USGS has published nationwide maps and underlying data; the high-concentration regions are recognisable to anyone who has worked with the county-level soil-gas data, because they often overlap.

• New England. Maine, New Hampshire, Massachusetts, Rhode Island, Connecticut. Coastal Maine and the New Hampshire seacoast in particular have seen private-well radon readings in the tens of thousands of pCi/L.
• The Reading Prong — eastern Pennsylvania, northern New Jersey, southeastern New York. The same geologic feature that drives the famous indoor-air problem also drives elevated well-water radon.
• The Appalachian belt, including parts of western North Carolina, northern Georgia, and into the southern Blue Ridge.
• The Rocky Mountain region in patches, particularly in Colorado and parts of Wyoming and Montana where granitic batholiths are widely used as private-well aquifers.
• Scattered local pockets elsewhere, anywhere a private well draws from a uranium-bearing granitic or pegmatitic unit.

Outside these regions, water radon is almost always low enough to ignore even for private-well households.

How to test well-water radon

Well-water radon is not detected by the air-radon test kits used for indoor-air screening. It requires a separate test, with a water sample drawn into a special sealed vial and shipped to a certified lab. Key practicalities:

• Collect the sample from a cold-water tap that has not been treated by a water softener, filter, or aerator. Run the tap for a few minutes to clear stagnant water in the lines, then fill the vial fully with no headspace and cap it under the running stream.
• Ship to a lab certified by the National Radon Proficiency Program (NRPP), the National Radon Safety Board (NRSB), or your state radon programme.
• Typical cost is in the $25 to $50 range for a single sample, including shipping. Some state radon programmes subsidise testing for private wells in high-radon counties.
• Results come back as picocuries per litre, the same unit as air-radon, which is convenient but also a common source of confusion when comparing to the 4 pCi/L indoor-air action level — these are different media and the numbers should not be compared directly.

If you are buying a house with a private well in a high-radon region, a water-radon test alongside the indoor-air test is reasonable due diligence; if you have lived in the same house for years and your indoor-air tests have always come back well under the action level, the water test is far lower priority.

Treatment: GAC vs. aeration

If a well-water test comes back high enough to warrant treatment, two well-established technologies are used at the point of entry to the house.

Granular activated carbon (GAC)

A tank packed with activated carbon adsorbs the dissolved radon as the water passes through. GAC is mechanically simple, has no moving parts, and is generally appropriate for lower concentrations — typically up to a few thousand pCi/L. The trade-off is that the carbon bed accumulates radon and its solid decay products (lead-210, polonium-210), which can become a low-level radioactive material requiring careful disposal when the carbon is changed out. Installed cost is typically $1,000 to $2,500, with periodic media replacement.

Aeration

An aeration unit — packed-tower, spray-tower, or shallow-tray — bubbles air through the water to strip dissolved radon, which is then vented outside. Aeration is recommended for higher concentrations because it does not accumulate radioactive residue on a media bed. The standard for residential systems is ANSI/AARST SGM-SF-2017 ("Soil Gas Mitigation Standards for Existing Homes: Specifications for Water Aeration System Components"), which covers component specs, venting, and removal efficiency. Installed cost is typically $3,000 to $5,000+, depending on flow rate and configuration.

Decision framing: water-band table

The following table maps measured well-water radon concentration to the implied indoor-air contribution and the treatment approach typically discussed in the literature. Concentrations are in pCi/L. The air contribution uses the EPA 10,000:1 rule of thumb.

These bands are illustrative, not regulatory. There is no federal MCL for radon in drinking water; the 300 / 4,000 pCi/L numbers come from a proposed rule that was never finalised, and individual states set their own action levels.

When to actually test the water

A pragmatic three-condition rule that tracks how most state radon programmes and home inspectors approach it:

1. The home is served by a private well, not a municipal system.
2. The home tested high for indoor-air radon on a proper short-term or long-term test, or sits in a documented high-radon county.
3. Either soil-gas mitigation has been installed and the post-mitigation air number is still close to the action level, or no soil-gas pathway is plausibly explaining the elevated reading.

When all three conditions hold, a $25 well-water test is straightforward due diligence. If only the first condition holds — private well in a low-radon area, no elevated indoor reading — the water test is low priority.

Bottom line

Radon in well water is real, well-characterised, and almost always a second-order problem. The 10,000:1 transfer ratio means a private well would have to be in a high concentration band before the water contribution rivals what comes up through the slab from soil gas. The EPA tried to regulate it in 1999, with a proposed 300 pCi/L MCL and an alternative 4,000 pCi/L MCL conditional on a state indoor-air programme; the rule was never finalised, and those numbers remain proposed, not enforced. If you have a private well in a granitic-bedrock region, test the air first, the water second, and let the numbers — not the worry — drive the spending.

Sources

• US EPA — Proposed Rule: National Primary Drinking Water Regulations; Radon-222 (EPA-815-Z-99-006, 1999). Source of the 300 / 4,000 pCi/L proposed MCLs and the 10,000:1 air-to-water transfer ratio of thumb.
• National Research Council — Risk Assessment of Radon in Drinking Water (National Academies Press, 1999). Authoritative review of the inhalation and ingestion risk from radon in drinking water.
• US Geological Survey — Radon in Groundwater of the United States. Mapping and well-water concentration data, water.usgs.gov.
• US EPA — Consumer's Guide to Radon Reduction (EPA-402-K-10-005). Plain-language overview of radon, including the water pathway and treatment overview.
• ANSI/AARST SGM-SF-2017 — Soil Gas Mitigation Standards for Existing Homes: Specifications for Water Aeration System Components. Industry standard for residential water aeration systems.
• American Lung Association — Radon. Patient-facing summary of the lung-cancer pathway from indoor radon exposure.

Related guides

• EPA Map of Radon Zones, explained — how the soil-gas pathway is mapped and what the three zones actually mean.
• DIY radon testing guide — choosing and running a short-term or long-term indoor-air test.
• Sub-slab depressurisation, explained — the standard fix for the larger soil-gas pathway.
• Highest-radon US counties — where the soil-gas pathway is most aggressive, often overlapping the granitic-bedrock well regions.