Radon in Well Water

What Radon Is and How It Gets Into Well Water

Radon in a private well is not industrial pollution - it is the slow radioactive decay of the rock your water sits in.

Radon-222 is an invisible, odorless, radioactive noble gas. It is born from the decay chain of Uranium-238, which breaks down into Radium-226 and then into radon. Because uranium and radium occur naturally in nearly all soil and rock, radon is everywhere in the environment - but how it reaches your tap depends entirely on where your water comes from.

Surface water (lakes, rivers, reservoirs) carries almost no radon: the gas is highly volatile, so it off-gasses harmlessly into the open air before the water ever reaches an intake. Groundwater is the opposite case. Deep private wells draw from bedrock aquifers - fractures inside solid rock where water creeps slowly under pressure. When uranium-rich rock decays, the radon it releases has nowhere to escape, so it dissolves into the pressurized groundwater. The moment your pump activates, it pulls that radon-saturated water up the casing and into your home's plumbing.

Human activity does not create radon in groundwater. But human engineering does open the door: drilling a deep bedrock well is what connects that confined, radon-charged rock directly to the inside of a house. The deeper and more granitic your well, the more likely radon is part of the picture.

The Water-to-Air Connection: Why You Breathe Radon More Than You Drink It

This is the single most important - and most misunderstood - fact about radon in well water. The danger is mostly airborne.

Radon in your water creates two exposure paths: ingestion (drinking it) and inhalation (breathing the gas after it escapes the water). The vast majority of the health risk is inhalation. Every time water is heated, depressurized, or agitated - a shower, a running washing machine, a dishwasher cycle - the dissolved radon violently off-gasses into the indoor air. Once airborne, it decays into heavy radioactive particles that lodge in lung tissue and release bursts of alpha radiation that sever DNA. According to the EPA, lung cancer from inhaled, off-gassed radon accounts for roughly 89 percent of all waterborne-radon deaths.

Drinking the water exposes the stomach to radon and carries a small stomach-cancer risk, but it is comparatively minuscule. In its landmark 1999 report, the National Academy of Sciences estimated that radon escaping from household water causes about 160 lung-cancer deaths a year, versus roughly 20 stomach-cancer deaths from ingestion - about 11 percent of the waterborne total.

The 10,000-to-1 transfer rule

Health physicists quantify the link with the 10,000-to-1 transfer rule. Radon is measured in picocuries per liter (pCi/L) - a unit of radioactive decay rate. The rule states that for every 10,000 pCi/L of radon dissolved in your water, indoor air radon rises by about 1 pCi/L.

That math is why a high-radon well matters even to people who never drink the tap water. A well testing at 40,000 pCi/L contributes about 4.0 pCi/L to the home's air - all on its own enough to reach the EPA's 4.0 pCi/L indoor-air action level, entirely separate from any radon seeping up through the foundation. It also means water and air radon must be considered together: see the if-your-test-is-high section for how to tell which one to fix first.

Where Radon Occurs: The US Hotspots

Radon turns up in every state, but it concentrates where the bedrock is granitic and crystalline. The rock under your property dictates your risk.

Groundwater radon is intensely local. Aquifers in granitic, metamorphic, and crystalline bedrock carry the highest probability of severe contamination, and USGS mapping shows distinct hotspots. If your well sits in one of the regions below, radon belongs on your first test - and even then, your neighbor's number is only a hint.

In New England, a 2022-2023 USGS study found that 55 percent of New Hampshire is underlain by groundwater with a 50 percent or higher probability of exceeding 2,000 pCi/L, with dense clusters in Rockingham, Carroll, and Coos counties. In southeastern Pennsylvania, the worst crystalline schist lithology shows median groundwater radon near 4,300 pCi/L. Up and down the Appalachian Piedmont in North Carolina and Georgia, granite fractures above the Fall Line routinely exceed 8,000 pCi/L, with historical Georgia outliers measured up to 140,000 pCi/L. In the Mountain West, Montana wells in Quaternary and Tertiary sediments average 300 to 3,000 pCi/L with peaks near 14,000, and California's Sierra Nevada granitic aquifers carry significant localized radioactivity.

Health Effects and the Action Levels

Radon is the second-leading cause of lung cancer overall and the leading cause among people who never smoked.

The harm from radon is cumulative, not acute. Inhaled radon decay products are alpha emitters - heavy, charged particles that cannot penetrate skin but do devastating damage when they lodge in lung tissue. Over years of exposure, that repeated DNA damage drives the mutations that cause lung cancer. The EPA ranks radon as the second leading cause of lung cancer in the US, behind only smoking.

Because there is no enforceable federal limit for private wells, the practical benchmarks are the EPA's 4.0 pCi/L indoor-air action level and a patchwork of state water action levels that generally run from 4,000 to 10,000 pCi/L. The American Association of Radon Scientists and Technologists (AARST) recommends mitigating water once it reaches 4,000 pCi/L. The state thresholds are covered in the regulatory section below.

The EPA Limit That Never Was

One of the most confusing things about radon in water is that there is no legally enforceable federal standard - and there almost was, twice.

Private wells are entirely exempt from the Safe Drinking Water Act (SDWA), so the legal and financial burden of testing and treatment falls on you alone. But the EPA's long, stalled attempt to regulate radon in public water systems is the framework states now use to advise well owners.

In 1991, under the 1986 SDWA amendments, the EPA proposed a Maximum Contaminant Level (MCL) of 300 pCi/L. Water utilities and the American Water Works Association estimated national compliance would cost about $2.5 billion a year for relatively small health benefit compared with soil-gas radon (the EPA's own estimate was $272 million a year). Congress halted the rule with the 1996 SDWA amendments and ordered a National Academy of Sciences cost-benefit study, which concluded that fixing waterborne radon would prevent roughly 160 deaths a year, against the ~21,000 deaths caused by total radon exposure.

In 1999 the EPA returned with a novel dual-option proposed rule: a strict 300 pCi/L MCL, or an Alternative Maximum Contaminant Level (AMCL) of 4,000 pCi/L for utilities and states that funded a Multimedia Mitigation program to reduce the far larger indoor-air radon threat. That compromise stalled after the 2000 election, and as of 2026 the EPA has never finalized any federal MCL for radon in drinking water.

The AARST recommends mitigation at 4,000 pCi/L regardless of where your state sets its line. Because these limits vary so widely, the number on your lab report only tells half the story - what you do with it depends on your state and on your indoor-air reading.

How to Test for Radon

Radon is colorless, tasteless, and odorless, so testing is the only way to detect it - and a standard well panel will not catch it.

Routine well-water checks for bacteria, lead, or nitrates do not include radon. You have to request it specifically, and the sampling is unusually fussy. Radon testing uses special zero-headspace vials: if any air bubble is trapped inside the sample, the gas escapes into it and the lab reads a falsely low number. The sample also begins decaying immediately, so it must ship the same day.

How to draw the sample

1. Remove the aerator screen from an indoor faucet.
2. Run cold water for 10-15 minutes to purge the pressure tank and draw fresh well water.
3. Reduce the flow to a slow, pencil-thin trickle so you do not agitate the water.
4. Fill the vial to the absolute brim, forming a rounded (convex) top.
5. Cap it underwater or carefully so that zero air bubbles are trapped inside.
6. Invert and tap the vial - if you see any bubble, empty it and start over.
7. Ship the same day; radon decays immediately.

Reading Your Results

Results come back in picocuries per liter (pCi/L). There is no single federal pass/fail line, so the table below blends the AARST recommendation with common state action levels. Run your number through the transfer rule too - divide by 10,000 to see roughly how much your water alone adds to your indoor air.

So a 12,000 pCi/L result is not just "above a state line" - it is adding about 1.2 pCi/L to your air before a single bit of soil gas is counted. If your number lands in the optional-treatment band, the deciding factor is your indoor-air test, covered next.

If Your Test Comes Back High: Do This Now

The intuitive first move - boiling the water - is exactly the wrong one. Here is the right sequence.

While you wait for a permanent system to be quoted and installed, take these steps to cut the airborne hazard:

1. Maximize ventilation. Run bathroom exhaust fans during and for 30 minutes after every shower, and run an outdoor-venting stove hood when using the sink or dishwasher.
2. Minimize aerosolization. Take shorter, cooler showers to reduce steam and off-gassing, and wash laundry on cold cycles instead of hot.
3. Test the indoor air. Deploy a long-term alpha-track test or a continuous digital radon monitor so you know the combined air hazard from water plus soil gas.
4. Decide water vs. air using the transfer rule. If your water is, say, 3,500 pCi/L (adding only ~0.35 pCi/L) but your air reads 6.0 pCi/L, the air problem is mostly soil gas - fix that first with Active Soil Depressurization, and treat the water only if the water itself is pushing the air over the limit.

Treatment Options Compared

Radon is a dissolved gas, so softeners, sediment filters, and reverse osmosis do essentially nothing for it. Only two methods work whole-house - and they are very different.

Treatment has to be Point of Entry (POE) - it must treat the water as it enters the house, before the water heater and showers. A Point-of-Use under-sink filter is a dangerous half-measure for radon: it might make the kitchen tap safer to drink, but it does nothing to stop radon from off-gassing in the shower, which is where the inhalation risk lives. The two whole-house methods that actually work are aeration and granular activated carbon, and they handle the radioactive gas in opposite ways.

Aeration: the gold standard

Aeration systems force water into a sealed chamber and inject filtered air, agitating the dissolved radon out of the water; a blower then vents the captured gas through a dedicated PVC pipe above the roofline. They remove 99.0 to 99.9 percent of radon and handle even extreme concentrations (100,000+ pCi/L) easily, with minimal hazard because the gas goes outdoors. The trade-off: aeration breaks your water pressure (the water is sprayed into an atmospheric tank), so the system needs a secondary submersible repressurization pump (usually 0.5 hp) and a second pressure tank to push clean water back to the house at 40-60 psi. Many state health departments require aeration above 10,000 pCi/L.

Granular activated carbon (GAC): cheaper, but it hoards the radiation

A GAC system is a tall fiberglass tank of coconut-shell charcoal that adsorbs radon into its pores. It removes 80 to 95 percent and needs no electricity, but it does not destroy radon - it concentrates it. As the bed loads up, the tank itself becomes a source of gamma radiation (unlike alpha particles, gamma rays penetrate the tank walls and reach nearby occupants), and Lead-210 buildup can make proximity comparable to ambient X-ray exposure. The carbon must be replaced every 1 to 2 years, and heavily saturated carbon often has to be handled as radiological hazardous waste through your county health department - not the trash. GAC is only appropriate for lower-tier contamination.

Prevention and Well Construction

Geology decides whether radon is present, but smart drilling and construction can blunt the risk before a home is occupied.

The National Ground Water Association advises drillers on best practices to limit radon intrusion. Shallow dug or sand-point wells rarely have a radon problem because they never reach the deep, confined bedrock fractures where radon accumulates - though they trade that for far higher surface and bacterial contamination risk. When a deep bedrock well is necessary, a driller can sometimes case off high-uranium zones if cleaner, high-yield fractures are available higher in the borehole.

For new homes in known radon hotspots, builders should install passive Radon-Resistant New Construction: a 4-inch gravel layer under the slab, a heavy vapor retarder over it, and a 3-inch PVC pipe routed from the gravel up through the roof. If the finished home later tests high - from soil gas or water transfer - a fan splices into that attic pipe to convert it instantly to Active Soil Depressurization.

Aerator installs, casing work, and well construction all call for a licensed water-well contractor. You can find a licensed driller near you and cross-check their license with your state agency. For routine care, see our well maintenance guide and well water upkeep basics.

Financial Assistance Programs

A $4,000-$7,000 aeration system is a real burden for a rural family. Several federal, state, and nonprofit programs can help.

The federal USDA Section 504 Home Repair Program (Rural Development) offers very-low-income rural homeowners - those below 50 percent of the Area Median Income - 20-year loans up to $40,000 at a fixed 1 percent interest rate to remove health and safety hazards, including radon systems. Homeowners aged 62 and older who cannot repay a loan may qualify for direct grants up to $10,000.

State and nonprofit programs add more help:

• State Indoor Radon Grants (SIRG) - the EPA funds states to subsidize free testing. By law the money cannot go directly to individual homeowners for mitigation, but states use it for free kits: Tennessee distributes roughly 6,000 free test kits a year, and North Carolina hands out 2,000 to 3,000 free DIY kits each National Radon Action Month.
• Maine - the Normand Dubreuil Radon Air Abatement System Program (Free ME from Lung Cancer) provides direct grants to low-income single-family homeowners to fully fund installation of a radon mitigation system.
• USDA Emergency Community Water Assistance Grants (ECWAG) - aimed at communities rather than individuals, providing up to $150,000 to repair systems or up to $1,000,000 for a new water source after an emergency degrades water quality.

Keep reading

• Well Water Testing: get the right test
• Uranium in Well Water
• Arsenic in Well Water
• All Well Water Contaminants
• State Well Guides