Radon in water mitigation: aeration or carbon by test result, cost and retest

A radon-in-water mitigation system is a whole-house unit that takes radon out of well water before the water reaches any faucet, shower or appliance. There are two kinds: aeration, which strips the gas out with air and vents it outdoors, and granular activated carbon (GAC), a large carbon tank that traps it. Your water result decides which one fits. The U.S. Environmental Protection Agency's Consumer's Guide to Radon Reduction (EPA 402/K-10/005, 2025 edition) suggests carbon for "moderate levels, around 5,000 pCi/L or less in water." Maine, Connecticut and Virginia draw their carbon line just under 10,000 pCi/L and New Hampshire at 10,000, and all four send higher results to aeration. Vermont discourages carbon for radon.

This guide covers the fix: which system at which level, upkeep, why a carbon tank becomes a source of radiation, dated official costs, and the retest. For the health risk and every state's guidance level in full, see our radon water test explainer. If radon is one of several problems in your water, the treatment train guide shows how the units are ordered.

Before you buy: test the air and confirm the water number

Radon from a well reaches your lungs through the air. The National Research Council's 1999 report Risk Assessment of Radon in Drinking Water (the National Academy of Sciences study EPA cites) found the contribution to indoor air from household water use "is very low—only about one ten-thousandth the water concentration," because "only about half of the radon in the household water supply escapes into the air and then it is diluted into the large volume of air inside the home." That ratio is the familiar rule of thumb, which New Hampshire, Virginia and Pennsylvania state as 10,000 pCi/L in water adding about 1 pCi/L to indoor air, on average.

The same arithmetic explains why water is usually the smaller source. EPA's archived radon in drinking water page (last updated June 30, 2014) says "Only about 1-2 percent of radon in the air comes from drinking water." The NAS report put U.S. lung-cancer deaths from indoor radon at about 19,000 a year (its 1998 estimate) and found that "only 160 are estimated to result from inhaling radon that was emitted from water used in the home." Its conclusion is the one to keep in mind when you shop: "Except in situations where concentrations of radon in water are very high, reducing the radon in water will generally not make a large difference in the total radon-related health risks to occupants of dwellings."

Those are averages. Maine CDC says "many Maine homes have high air radon levels due only the radon which escapes from the water." So the order matters:

1. Test the indoor air. Maine CDC's treatment tip sheet: "If you have tested just your well water, test your air too before making treatment decisions." Vermont asks you to test the air "before considering how to reduce radon levels."
2. Confirm the water result. New Hampshire's environmental agency (NHDES) recommends "at least two radon tests (at least one month apart when possible), prior to making any treatment decisions." Connecticut's action level is based on "the average radon level of two water samples (drawn simultaneously from the same location)." Radon samples go in small glass vials with no air bubble; our guide to collecting a well water sample walks through the vial method.
3. Test for uranium and gross alpha too if you have a bedrock well. NHDES: "A well that has high levels of radon is more likely to have high levels of uranium and/or gross alpha." Aeration won't fix those: Connecticut lists heavy metals among what aeration is "Not Effective Against," and NHDES's treatment table sends uranium to anion exchange or reverse osmosis.

Divide the water result by 10,000 to see roughly how much of your air reading the well explains. If the air is high and the well explains little, the fix is a soil-gas system, not a water system.

Which system at which level: what each agency says

Agencies agree on the two technologies and disagree on where carbon stops being acceptable. Each row comes from that agency's current page or fact sheet; check the date column. The NHDES sheets were read through Internet Archive captures of May 26, 2025, because des.nh.gov returned 403.

Nobody calls carbon the better choice at high levels; the disagreement is only about how high it can go. The Virginia Department of Health shows how often each band comes up: "Of 1,500 known radon in water test results in Virginia, about 13% exceeded 4,000 pCi/L, but only about 4% exceeded 10,000 pCi/L."

Decision table by water result

Read your confirmed water result (the average of your samples) in the first column. The state cells quote each agency separately; your own state's program has the final word.

The "adds to indoor air" column divides by 10,000, the NAS average. Your own house can differ, which is why the air test comes first.

How aeration works, and what it needs to keep working

Aeration treats radon as what it is: a dissolved gas that wants to leave the water. Connecticut's Department of Public Health describes the method in its publication on aeration treatment of private drinking water systems (March 2009, still posted; link is the Internet Archive copy of July 1, 2026, as portal.ct.gov timed out): it "consists of passing large amounts of air through water and then venting the air to the atmosphere." Maine CDC says the system "mixes your water with air inside a plastic tank and then vents the air and radon outdoors, away from the house," and calls it the most common type, removing "95-99%" of radon (its tip sheet says "between 85% and 99%"). Virginia notes these systems "can be installed in either a holding tank located in the home or some new systems may be installed directly in the well itself," and that "They can effectively reduce even extremely high radon levels."

The big advantage is what aeration leaves behind: nothing to dispose of. Connecticut: "there is no disposal or regeneration of treatment media necessary. This is especially important when the contaminant being treated would constitute a hazardous waste disposal problem, such as radon." The trade-off is a machine with air lines and a vent. Connecticut's installation points: take intake air from a clean spot (indoors, "at least a foot above the floor, but preferably near the ceiling"); in an "air-tight" house an interior intake may create a vacuum that stops heating-system combustion gases from venting unless a fresh air source is provided. The waste air "has a higher concentration of the contaminant removed from the water," so its discharge "should be above the eave of the roof," away from windows and downwind, and its connections "need to be air tight within the home to prevent leakage of hazardous gas." Both lines should slope so condensation drains.

Upkeep is the part buyers underestimate. EPA's table says aeration "requires annual cleaning to maintain effectiveness and to prevent contamination." Connecticut warns that "Bacterial slime will grow in aerators and storage tanks, requiring continuous or periodic chlorination," and recommends a drain line that makes flushing the storage tank "at least once or twice each year" easier. Connecticut also suggests asking "How loud is the air blower?"

Iron and manganese decide much of the design. Connecticut's 2021 radon sheet says they "need to be taken into account when considering installation of an aeration system," and Wisconsin DNR says aeration systems "must be properly designed to prevent fouling by iron, iron bacteria or water hardness." Air turns dissolved iron and manganese into particles that stain fixtures and clothing "unless a post-treatment filter is installed" (Connecticut). If your report shows either, read our pages on iron in well water and manganese before you choose equipment.

How carbon (GAC) works, and why the tank becomes radioactive

A GAC system for radon is a big filter tank, not a faucet cartridge. Connecticut's DPH fact sheet Radon in Your Well Water (January 2021; link is the Internet Archive copy of March 19, 2025, the file posted since 2021) describes "a fiberglass tank filled with granular activated carbon, a fine material that traps radon as the water passes through it." EPA says "GAC filters usually cost less than aeration systems," and puts typical reduction at 85 to 95 percent, "Less efficient for higher levels than aeration."

The carbon wears out. Connecticut: "The carbon also captures other contaminants, which is beneficial, but it shortens the life of the carbon. The carbon eventually becomes saturated and can no longer trap radon." Service periods "vary based on the amount of carbon, household water usage, and radon level." Wisconsin DNR adds that GAC devices "often require a pretreatment system."

Then there is the radiation. Virginia's health department: "the trapped radon atoms will continue to undergo radioactive decay and will emit gamma radiation." That is why agencies tie carbon to lower levels (Wisconsin warns of hazardous levels at 4,000 pCi/L and higher) and to regular carbon changes. Each in its own words:

New Hampshire's own sheet gives two intervals: "every 1 to 2 years" in the text and "annual carbon changeout" in its table. None of these agencies publishes a dose figure for a home tank or one disposal route for spent carbon, so we don't either. Ask your state radon program how spent carbon is handled where you live, and keep the tank well away from living space, as Virginia advises.

What a radon water system costs: official figures, with dates

Four of the agency pages we read publish prices. We show the three that carry a date, from 2020 to 2025 (Wisconsin DNR's undated page gives about $3,500 for aeration). These are printed ranges, not quotes.

A note on the Pennsylvania DEP fact sheet: the file at DEP's "Radon in Water" address is the Spanish-language edition (2940-FS-DEP4492, Rev. 11/2020). We read the Internet Archive capture of September 6, 2025, because files.dep.state.pa.us refused connections. Its prices: aeration "de unos $3,000 a $4,000," activated carbon "de unos $1,000 a $1,500."

All three dated sources put carbon below aeration. Its running cost is carbon replacement; aeration's is the blower's electricity, annual cleaning and possible chlorination. Testing is the small line item: Vermont's health department lab sells a radon in water kit for $25, and Pennsylvania DEP gives $25 to $50 for a radon in water test.

Who should install it

Rules differ by state:

• Connecticut: DPH "recommends hiring a nationally certified radon mitigation professional to install a system to reduce radon in your water," and the same for replacing GAC carbon.
• Maine: under Maine law, only the owner or occupant, or someone registered with the state, may "install and/or advise on the installation of a radon reduction (mitigation) system," and for a home that is for sale the work "must be done by a registered radon service provider."
• New Hampshire: "Certification is not required for radon-in-air testing or for radon-in-water treatment."
• Virginia: the national radon certification bodies "do not yet have certification programs for radon professionals to test or mitigate radon" in water, according to VDH (September 2025).
• Wisconsin: "Any aeration or GAC treatment system must be approved by the Department of Safety and Professional Services plumbing program."
• Vermont: contact "a certified water treatment specialist."

Whoever you hire, Connecticut suggests asking whether the system has been "tested and certified by a third party," what maintenance it needs, and whether plumbing changes add cost. For carbon, Vermont names NSF/ANSI Standard 53.

Retest: prove the system works

A radon system is only as good as the test after it. Vermont: "Re-test your drinking water for radon after any treatment system is installed to make sure levels are below the health advisory level." Connecticut's aeration publication goes further: "After installation, retest both the raw water (prior to treatment) and the treated water at a state certified laboratory," then test both "annually or more frequently (quarterly or semi-annually) if high levels of contaminants are present in the raw water." Maine CDC recommends re-testing "every 2 years after a mitigation system is installed to ensure levels are acceptable and that the system is working properly."

Use the vial method for both: one set from a tap before the system, one after it. EPA's typical reductions give a yardstick: aeration (95 to 99 percent) should leave 1 to 5 percent of the raw radon, carbon (85 to 95 percent) 5 to 15 percent. A treated result well above that is a reason to call the installer. Retest the indoor air as well. Maine says "It is important to know the air and water levels in your home to understand the total health risk of radon," and a water system does nothing for radon coming up through the foundation.

Find radon labs in the finder, or go straight to the labs certified for radon in your state in Connecticut or Pennsylvania. You can enter raw and treated results in the results checker.

Worked examples

Illustrative numbers, not real results. Each lands in a row of the decision table.

House A, Virginia, drilled well. Two water samples average 7,000 pCi/L. Winter air test: 2.0 pCi/L.

• Row: 5,000 to under 10,000. By the NAS average, the water adds about 0.7 pCi/L to the air, about a third of the 2.0 reading.
• Virginia: carbon "may be used" at this level, kept well away from living space, with the bed changed every year. Connecticut would act (above 5,000) and allows carbon below 10,000. But 7,000 is above EPA's "around 5,000" for carbon, and New Hampshire's guidance would not call for treatment with air below 4.
• Reasonable plan: if you treat, compare a carbon quote that includes yearly carbon changes with an aeration quote, which avoids the radiation buildup.

House B, Maine, drilled well. Water averages 25,000 pCi/L. Air: 5.5 pCi/L.

• Row: 10,000 or more. The water adds about 2.5 pCi/L to the air on average, nearly half of the 5.5 reading. Maine's tip sheet says a treatment system is recommended at 10,000 or higher, and Maine typically recommends carbon only under 10,000: this is an aeration case.
• Even with the water's share gone, about 3 pCi/L would probably remain from the soil, in the 2 to 4 range where Maine says to consider reducing levels.
• After installation, EPA's 95 to 99 percent implies treated water of about 250 to 1,250 pCi/L. Retest every 2 years, as Maine CDC recommends.

House C, Vermont, drilled well. Water: 4,500 pCi/L. Air: 1.2 pCi/L.

• Row: 4,000 to under 5,000. The water adds about 0.45 pCi/L, more than a third of the air reading.
• Vermont says to take action at or above 4,000, and discourages carbon, so the fit there is aeration. The same numbers would not trigger action under Connecticut's 5,000 level or New Hampshire's guidance (air below 4). Your state changes the answer.

For health background, testing schedules and full state guidance, see the radon water test page.

Questions people ask