Radon in Well Water in Pennsylvania: Testing, Health Risks, and Treatment Options

How Radon Gets Into Well Water — and Then Into Your Air

Radon-222 is a radioactive gas produced by the radioactive decay of radium-226, which is itself the decay product of uranium-238. In areas where the underlying bedrock contains uranium-bearing minerals — granite, gneiss, schist, and related formations that dominate Pennsylvania's geological landscape — radon is continuously generated in the rock matrix and in the soil above it. Most of this radon migrates upward through soil and enters buildings through foundation cracks, floor-wall joints, and utility penetrations, which is the soil gas pathway that drives indoor air radon levels in the vast majority of Pennsylvania homes. But a second pathway exists: radon dissolves into groundwater moving through fractured bedrock, and when that groundwater is pumped to the surface through a private well, it delivers dissolved radon directly into the household water supply.

Surface water sources — rivers, lakes, and reservoirs that feed most Pennsylvania municipal water systems — lose their radon content to the atmosphere before the water reaches any household. Radon degasses rapidly at the air-water interface, and the long residence times and large surface areas of surface water systems result in negligible radon concentrations by the time water reaches a treatment plant. Pennsylvania homeowners served by municipal systems that draw from the Susquehanna, Delaware, Allegheny, or their tributaries have no meaningful radon-in-water exposure from their tap. The concern is specific to private wells and to small community water systems that draw from groundwater — both of which are common in Pennsylvania's rural counties, particularly those overlying the Reading Prong and the Appalachian Plateau.

The risk pathway from well water to indoor air is direct and well-characterized. When water containing dissolved radon enters the household and is agitated — during showering, running the dishwasher, operating the washing machine, or even running the kitchen tap at high flow — the dissolved radon gas is released from solution and enters the indoor air. Showering is typically the highest radon-releasing activity, because the combination of hot water, fine droplets, and forced air movement maximizes gas transfer from water to air in an enclosed space. A ten-minute hot shower in a poorly ventilated bathroom can release a measurable pulse of radon into the interior atmosphere of a home.

Pennsylvania DEP and the EPA distinguish carefully between the two risk pathways from radon in water: inhalation and ingestion. Drinking water that contains dissolved radon does carry a small radiation dose to the stomach lining and gastrointestinal tract, and that risk is real. But it is small compared to the lung cancer risk from breathing radon that has been released from water into indoor air. EPA's analysis of radon-in-water risk estimates that for every 10,000 pCi/L of radon in household water, approximately 0.1 lung cancer deaths per million people per year are attributable to the inhalation pathway, compared to approximately 0.006 deaths per million per year from the ingestion pathway. The inhalation risk is roughly 17 times larger than the ingestion risk. Pennsylvania DEP's public guidance reflects this: the primary concern with radon in well water is its contribution to indoor air radon, not the direct health effect of drinking the water.

Pennsylvania homes at highest risk for radon in water are those served by drilled wells into fractured bedrock in the state's high-uranium geology areas. Reading Prong counties — Berks, Northampton, Lehigh, Bucks, Montgomery, Chester, Lancaster, and Lebanon — are the most widely recognized high-risk zone, but elevated radon in groundwater has been documented across much of the Piedmont and the Appalachian Plateau in western Pennsylvania as well. Bedrock well depths matter: wells drilled deeply into fractured granite or gneiss that intersect multiple fracture zones have more contact with radon-bearing rock and typically show higher dissolved radon than shallow wells in the same area. Age of the well is less relevant than geology and well depth. A 30-year-old drilled well and a newly installed well in the same geological unit will typically show similar radon levels, because the source — uranium decay in the surrounding bedrock — is essentially constant on human timescales.

Small community water systems that draw from groundwater wells — common in rural Pennsylvania boroughs and townships that are too small or too dispersed for municipal surface water treatment — can also have elevated radon in their distribution water. These systems are regulated as public water supplies under the Safe Drinking Water Act, but because no federal MCL for radon has been finalized, they are not required to test or treat for radon. Homeowners served by small community water systems with groundwater sources should contact their water authority to ask whether radon testing has been conducted and what results were found.

The 10,000:1 Conversion Rule

Pennsylvania DEP uses a straightforward rule of thumb for estimating how much radon in well water contributes to the indoor air radon level above what is already present from soil gas: for every 10,000 picocuries per liter (pCi/L) of radon measured in the well water, approximately 1 pCi/L is added to the home's indoor air. This conversion factor is an estimate based on average household water use patterns — the number of showers taken per day, the volume of water used per household member, the typical ventilation characteristics of residential buildings — and was derived from EPA and state program research on actual radon transfer from water to air in residential settings.

The math is worth working through because it illustrates when radon in water becomes a significant problem and when it does not. A private well testing at 10,000 pCi/L in water contributes approximately 1 pCi/L to indoor air. At a soil-gas baseline of 2 pCi/L — a common result in moderate-risk Pennsylvania counties — the combined indoor air level would be approximately 3 pCi/L, still below the EPA action level of 4 pCi/L. The same well in a home where soil gas alone produces 4 pCi/L would push the combined indoor level to approximately 5 pCi/L — above the action level. Context matters, and the additive nature of the two sources is why testing both air and water is important when indoor air is elevated and the home uses a private well.

A well testing at 20,000 pCi/L in water contributes approximately 2 pCi/L to indoor air. A well testing at 40,000 pCi/L contributes approximately 4 pCi/L — by itself equal to the EPA action level. A well testing at 100,000 pCi/L — a high but not unusual result in Reading Prong geology — contributes approximately 10 pCi/L to indoor air, which combined with even a modest soil-gas baseline would produce indoor air levels associated with significant long-term health risk. Pennsylvania DEP's Radon in Water fact sheet notes that radon concentrations in Pennsylvania private wells have been measured ranging from below detectable levels to over 600,000 pCi/L in water, with the highest values concentrated in the Reading Prong and related geology.

The practical implication for Pennsylvania homeowners in Zone 1 counties who use private wells is that the two radon sources interact and must both be evaluated when indoor air is elevated. A home where sub-slab depressurization has successfully reduced soil-gas radon from 12 pCi/L to 1.8 pCi/L post-mitigation may still test above 4 pCi/L in indoor air if the well supplies water at 50,000 pCi/L — contributing another 5 pCi/L. A homeowner who installed an ASD system and then retested indoor air expecting a sub-2 pCi/L result, only to find 6 pCi/L, may be experiencing exactly this scenario. Before concluding that the ASD system is underperforming, test the well water.

The 10,000:1 rule is an estimate, not a precise physical calculation. Actual radon transfer from water to air in a specific home depends on water usage volume and patterns, the ventilation rate of the space where water is used (a well-ventilated kitchen releases radon outdoors; a poorly ventilated bathroom concentrates it), water temperature (higher temperature water releases dissolved gas more readily), and the specific fixtures and appliances through which the water passes. A household with high-flow showers, a dishwasher, and a washing machine on the same well will transfer more water-sourced radon to indoor air than a household with the same well water concentration but lower overall water consumption. The rule provides a useful first estimate that informs whether further investigation is warranted — it does not replace actual measurement of indoor air radon.

Testing Protocol: Air First, Then Water

The decision sequence for addressing potential radon-in-water exposure follows a logical priority order established in Pennsylvania DEP guidance: test indoor air first, because soil gas is almost always the dominant radon pathway in Pennsylvania homes, and because the appropriate response to air radon — sub-slab depressurization — is independent of whether water is also a contributing source. Testing water before addressing air radon simply because the home has a well reverses the priority order and may result in treating a secondary source while leaving a larger primary source unaddressed.

The decision tree is straightforward. Step 1 is to test indoor air using a certified radon test — either a short-term charcoal canister test (2–7 days) or a long-term alpha track detector (90 days to one year), conducted by a DEP-certified radon tester or using a mail-in kit from a DEP-certified laboratory. If indoor air tests below 4 pCi/L, the radon situation is below the EPA action level and no further action is required. Even if the home has a private well with some dissolved radon, the combined air level is already below the action threshold and water testing is unlikely to change the management decision.

Step 2 is triggered when indoor air tests at or above 4 pCi/L and the home is served by a private well or a small community groundwater system. In this situation, testing well water for radon is warranted to determine what fraction of the indoor air radon is attributable to water versus soil gas. This matters because the treatment approaches are completely different — ASD for soil gas, aeration or GAC for water — and knowing the relative contribution of each source allows for appropriately targeted treatment. A home where 80% of the indoor air radon comes from soil gas and 20% from water needs ASD as the primary intervention and may or may not need water treatment depending on the post-mitigation indoor air result. A home where significant contributions come from both sources needs both addressed.

Testing radon in water requires a water sample collected from the well following a specific protocol that prevents radon from degassing before the sample reaches the laboratory. The sample cannot be collected from an ordinary kitchen tap that has been running — the turbulence of flow through standard faucet aerators releases dissolved radon before it can be captured in the sample. A DEP-certified laboratory or the DEP Radon Division can provide specific collection instructions and a proper sample container. The general protocol involves allowing the well pump to run briefly to flush the pressure tank, then collecting a water sample slowly through a tap without an aerator, filling the sample container completely without splashing or turbulence to minimize degassing, and sealing and refrigerating the sample immediately for shipment to the laboratory. Radon in water is highly time-sensitive — samples must reach the laboratory within a short window after collection, typically 24 to 48 hours, because radon's 3.8-day half-life means that a delayed sample will underreport the true concentration.

The cost of radon in water testing is modest — approximately $25 to $50 per test — and is entirely separate from indoor air radon testing. Pennsylvania DEP's Radon Division at 800-237-2366 can assist homeowners in identifying certified laboratories for water testing and in obtaining test kits. The EPA's Safe Drinking Water Hotline at 800-426-4791 provides guidance on testing for regulated and unregulated contaminants including radon in water. Results are reported in pCi/L of water, and the 10,000:1 conversion rule is applied to estimate indoor air contribution.

Treatment Options: Aeration vs. GAC Filtration

When radon in well water is confirmed to be a significant contributor to indoor air levels, two treatment approaches are proven effective. Both must be installed as point-of-entry systems — treating the entire household water supply before it enters the distribution plumbing — because only a whole-house approach reduces radon release in all the locations where water is used throughout the home.

Aeration Systems

Aeration treatment works by exposing the radon-bearing water to large volumes of air, causing dissolved radon to transfer from the water to the air phase, and then venting that radon-laden air to the outdoors. The two most common aeration system designs for residential radon removal are spray aeration — where water is pumped through a spray nozzle into a sealed chamber through which air flows — and packed tower aeration, where water flows downward over a packed media while air flows upward countercurrent. Both designs maximize the air-water contact surface area, which is the primary determinant of radon transfer efficiency.

Aeration systems achieve 95 to 99 percent radon removal from water — the highest removal efficiency of any available treatment technology. A well testing at 100,000 pCi/L in water treated by an effective aeration system would deliver water at 1,000 to 5,000 pCi/L to the household, reducing its air contribution from approximately 10 pCi/L to 0.1 to 0.5 pCi/L. At higher incoming radon concentrations — wells above 50,000 pCi/L — aeration is generally the preferred treatment because GAC systems may not achieve adequate removal at those levels without very large carbon beds that create maintenance and cost challenges.

Installed cost for residential aeration systems in Pennsylvania typically ranges from $3,000 to $4,000, depending on system design, flow rate requirements, and installation complexity. Aeration systems require a venting system that exhausts radon-containing air from the aeration chamber to the outdoors — the discharge cannot be into the home, into an attached garage, or into any enclosed space. This venting requirement means that the installation location of the aeration system must provide a pathway for an outdoor exhaust run, which affects where the system can be sited within the home. Annual maintenance involves cleaning the aeration chamber to remove mineral deposits and inspecting the air pump and blower components.

Granular Activated Carbon (GAC) Filtration

GAC filtration passes water through a tank containing a bed of activated carbon granules. Radon is adsorbed onto the surface of the carbon — it binds to the carbon rather than passing through with the water — while the water itself flows through the bed and exits the tank with significantly reduced radon content. The adsorption process is not as efficient as aeration's volatilization mechanism, but it is effective at moderate radon concentrations and requires no external air venting system, making it easier to site in a mechanical room or utility space.

GAC systems achieve 85 to 95 percent radon removal from water. At incoming concentrations up to approximately 50,000 pCi/L, a properly sized and maintained GAC system provides adequate treatment for most residential applications. Installed cost is typically $1,000 to $1,500 — substantially less than aeration. The lower upfront cost, however, must be weighed against the maintenance implications of radioactive radon accumulation on the carbon bed.

The critical maintenance requirement for GAC systems treating radon in water is carbon bed replacement. Radon adsorbed onto the activated carbon undergoes radioactive decay, producing short-lived decay products — polonium-214, lead-214, bismuth-214, polonium-218 — that accumulate in the carbon bed over time. A GAC tank that has been treating radon-bearing water for one to two years develops significant radioactivity within the carbon bed and may emit gamma radiation detectable outside the tank housing. Pennsylvania homeowners with GAC systems treating radon should: replace the carbon bed on the schedule recommended by the manufacturer or a qualified water treatment professional (typically every one to two years depending on water flow volume and incoming radon concentration), keep the tank housing intact and in good condition to provide shielding, consider a radiation shield around the tank housing, and consult with the tank manufacturer or a radon professional about proper handling and disposal of the spent carbon, which may qualify as low-level radioactive waste requiring disposal through a licensed facility rather than in standard household waste.

Why Point-of-Use Filters Do Not Solve the Problem

Under-sink filters, refrigerator water dispensers, pitcher filters, and other point-of-use devices treat only the water that passes through that specific fixture. They have no effect on the radon released into indoor air from showers, dishwashers, washing machines, and other household water uses that constitute the vast majority of household water consumption. A homeowner who installs a point-of-use filter in the kitchen believing it addresses the radon-in-water problem has addressed only the ingestion pathway — the smaller of the two risk pathways — while leaving the inhalation contribution from whole-house water use completely unresolved. Point-of-use devices have legitimate uses for reducing certain regulated contaminants at the drinking tap, but they are not a solution for radon in water that contributes to indoor air exposure.

Pennsylvania-Specific Considerations

Pennsylvania's radon landscape combines two risk factors that amplify each other in specific communities: unusually high-radon geology and widespread private well use. These factors overlap significantly in the same rural and exurban counties, creating a population of homeowners who face potential radon exposure from both soil gas and water simultaneously and may not be aware that the two sources need to be evaluated and addressed separately.

The Reading Prong — Pennsylvania's most intensively studied high-radon geological feature — runs through Berks, Northampton, Lehigh, Bucks, Montgomery, Chester, Lancaster, and Lebanon counties. These counties also contain large rural and semi-rural populations that rely on private wells drilled into the same Reading Prong bedrock that produces elevated soil gas radon. Uranium concentrations in Reading Prong granite and gneiss are among the highest in the eastern United States, and the same uranium that produces soil gas radon also produces radon in the groundwater moving through fractured Reading Prong bedrock. A home in rural Montgomery County on a drilled well into Reading Prong geology may face elevated radon from both soil gas and water — a dual-source problem that requires addressing both pathways. Homeowners in Harrisburg and surrounding Dauphin County, and in Erie and northwestern Pennsylvania where private wells serve rural communities, face this combined exposure profile.

Beyond the Reading Prong, elevated radon in groundwater has been documented across the Appalachian Plateau in western Pennsylvania, portions of the Great Valley, and areas of the Ridge and Valley physiographic province where carbonate bedrock is interbedded with uranium-bearing formations. Pennsylvania does not maintain a comprehensive statewide radon-in-water database analogous to its radon-in-air testing program, but DEP's Radon Division can provide regional guidance on which areas have documented elevated water radon based on testing data it has collected.

The regulatory framework for radon in private well water in Pennsylvania is straightforward: there is none, beyond voluntary guidance. Private wells are not subject to the federal Safe Drinking Water Act, which applies only to public water systems. There is no finalized federal MCL for radon in drinking water — the EPA proposed an MCL of 300 pCi/L in 1999 but withdrew it and issued an alternative maximum contaminant level (AMCL) of 4,000 pCi/L as a component of a multimedia mitigation program; neither has been adopted as binding regulation. Pennsylvania has not promulgated a separate state MCL for radon in private wells. The practical implication: testing and treatment of radon in private well water is entirely voluntary for Pennsylvania homeowners, driven by their own initiative rather than by any regulatory requirement. DEP's guidance is that when indoor air radon is elevated and the home uses a private well, water testing and treatment are appropriate next steps — but no agency will require it.

Real estate transactions involving Pennsylvania homes with private wells add a specific dimension to the radon-in-water question. A seller who has installed ASD to address indoor air radon but has not tested or treated their well water may be offering what appears to be a comprehensively addressed radon situation — documented mitigation, a certified post-mitigation test result — when the water source continues to contribute to indoor air. Buyers in EPA Zone 1 counties purchasing homes with private wells should request both an indoor air radon test and a radon-in-water test during the inspection contingency period. If the indoor air result is below 4 pCi/L but a seller-provided post-mitigation test is only a few months old, it may be worth requesting a water test anyway in high-geology-risk areas, because a seasonal change or a change in water use patterns could alter the indoor air contribution from water enough to matter.

Water treatment contractors who install aeration or GAC systems for radon in water do not require PA DEP radon mitigation certification — that certification covers air mitigation systems under PA Code § 240, and its scope does not extend to water treatment. However, not all water treatment contractors have experience with radon-specific treatment. Aeration systems for radon require venting design that is analogous in some respects to radon vent pipe design for ASD systems — the discharge cannot be into an enclosed or conditioned space, must not create a re-entry pathway for radon into the building, and must be positioned to avoid short-circuiting radon back into the air intake. GAC systems for radon require carbon bed sizing and replacement scheduling specific to radon treatment, which differs from carbon filtration for taste, odor, or other contaminants. Homeowners seeking radon-in-water treatment should ask specifically about the contractor's experience with radon removal, request references for prior radon-in-water installations, and confirm that the system design includes appropriate venting for aeration systems and a documented maintenance schedule for GAC systems.

Frequently Asked Questions

Does radon in well water increase indoor radon levels in Pennsylvania homes?

Yes. When water containing radon is used for showering, dishwashing, or other household purposes, radon gas is released into the indoor air. Pennsylvania DEP uses the rule of thumb that for every 10,000 picocuries per liter (pCi/L) of radon in well water, approximately 1 pCi/L is added to indoor air. A private well testing at 40,000 pCi/L in water would contribute approximately 4 pCi/L to indoor air — equal to the EPA action level.

Should I test my well water or indoor air for radon first in Pennsylvania?

Test indoor air first. Pennsylvania DEP guidance states that radon entering through soil is almost always a larger risk than radon entering through water. If indoor air tests below 4 pCi/L, radon in water is unlikely to be a significant contributor. If indoor air tests at or above 4 pCi/L and the home is served by a private well or groundwater community system, also test the water. Surface water supplies (rivers, reservoirs) rarely have elevated radon.

What are the treatment options for radon in well water in Pennsylvania?

Two point-of-entry treatment methods are effective: aeration and granular activated carbon (GAC) filtration. Aeration systems spray water or mix it with air and vent the radon outdoors — they achieve 95–99% removal efficiency and are preferred for higher radon levels. GAC systems filter water through a carbon bed that retains radon — they achieve 85–95% removal and cost less upfront but require periodic replacement of the carbon bed (typically every 1–2 years) due to radioactive buildup. Point-of-use filters at individual taps are not effective for reducing airborne radon risk from water use throughout the home.

How much does radon in water testing cost in Pennsylvania?

Radon in water testing costs approximately $25 to $50 per test. Testing requires a water sample collected from the well and analyzed by a laboratory. Pennsylvania DEP can assist homeowners in obtaining radon-in-water test kits. The Safe Drinking Water Hotline at 800-426-4791 also provides guidance on testing and certified laboratories.

Is there a Pennsylvania or federal standard for radon in drinking water?

There is currently no adopted federal maximum contaminant level (MCL) for radon in public drinking water. The EPA proposed an MCL of 300 pCi/L but it has not been finalized. Private wells are not regulated under the federal Safe Drinking Water Act. Pennsylvania DEP recommends that homeowners with private wells treat radon in water if testing reveals elevated levels, particularly when indoor air radon is also elevated.

Which Pennsylvania homes are most at risk for radon in well water?

Homes served by private wells or small community groundwater systems in Pennsylvania's high-radon geology areas are at greatest risk — particularly in Reading Prong counties (Berks, Northampton, Lehigh, Bucks, Montgomery, Chester) and other areas with uranium-bearing bedrock. Surface water supplies are rarely a concern. Homes in rural Pennsylvania that rely on drilled wells into fractured bedrock have the highest probability of elevated radon in water.