# Uranium or radon in well water: the limits, and what removes each > EPA's uranium MCL is 30 ug/L, removed by anion exchange or RO. Radon in water has no final MCL, only a 1999 proposal; aeration vents it. | | | |---|---| | Source | https://ownerspec.com/water/guides/uranium-radon-in-well-water/ | | Publisher | OwnerSpec (https://ownerspec.com/) | | Published | 2026-09-14 | | Updated | 2026-09-14 | | Facts verified | 2026-09-14 | --- ## Short answer Uranium in well water has an enforceable federal limit and radon does not. EPA's radionuclides rule at 40 CFR 141.66 sets the uranium Maximum Contaminant Level at 30 ug/L, combined radium-226 and radium-228 at 5 pCi/L, and gross alpha particle activity at 15 pCi/L, a gross alpha figure that by definition excludes radon and uranium. Radon in water has only a 1999 EPA proposal: 300 pCi/L as the MCL, or an alternative MCL of 4,000 pCi/L for states with an indoor-air program, and the proposal was never finalized, so states such as Maine set their own guideline, also 4,000 pCi/L. Treatment splits by contaminant. Uranium is removed by anion exchange or reverse osmosis; radium by ion exchange, lime softening or reverse osmosis. Radon is a gas, so aeration vents it outdoors, and EPA's archived Q&A warns that granular activated carbon collects radioactivity on the filter itself. Get a certified lab result for each before buying anything. ## Fact sheet - **Answer:** 40 CFR 141.66 sets the uranium MCL at 30 ug/L, combined radium-226 and radium-228 at 5 pCi/L and gross alpha at 15 pCi/L. Radon in water has no final federal MCL; EPA's 1999 proposal named 300 pCi/L, or 4,000 pCi/L with a state indoor-air program. Anion exchange or reverse osmosis removes uranium; aeration removes radon, and EPA says GAC collects radioactivity on the filter. - **Basis:** EPA National Primary Drinking Water Regulations for radionuclides, 40 CFR 141.66: uranium 30 ug/L, combined radium 5 pCi/L, gross alpha 15 pCi/L; radon limits are EPA's 1999 proposal and Maine CDC's guideline, not a final federal rule. - **Assumptions:** Results come from a certified laboratory, not a home strip; no strip reads uranium, radium or radon; Treatment classes are the technologies EPA names for each contaminant; no percent-removal figure is stated because the primary EPA PDF was not readable this session; The 1999 radon proposal is cited as a proposal; no final federal radon-in-water MCL exists as of 2026-09-14 - **Method:** Limits read from 40 CFR 141.66 on Cornell LII, radon figures and the GAC caution from EPA's archived radon-in-drinking-water Q&A, Maine's guideline from the Maine CDC MEG analysis paper, all read on 2026-09-14; nothing was tested. - **Primary source:** [Cornell LII, 40 CFR 141.66, Maximum contaminant levels for radionuclides](https://www.law.cornell.edu/cfr/text/40/141.66) (university or extension service) - **Facts verified:** 2026-09-14 - **Last updated:** 2026-09-14 - **Canonical URL (cite this):** https://ownerspec.com/water/guides/uranium-radon-in-well-water/ If your lab report has no line for uranium, gross alpha or radon, leave this page for now: none of the three shows in a glass or on a strip, and the [well water test panel guide](/water/guides/well-water-test-panel/) explains how to add them to the annual order. ## What does the lab result tell you? A Maximum Contaminant Level (MCL) is the enforceable federal limit for a public water system, and for a private well it is the yardstick a lab result is read against. Uranium is reported in micrograms per liter (ug/L); radium, gross alpha and radon are reported in picocuries per liter (pCi/L), a unit of radioactivity. | Lab result | Likely cause | Confirm with | |---|---|---| | Uranium at or above 30 ug/L | Naturally occurring uranium in the bedrock the well draws from | A certified lab test for uranium; no home strip reads it | | Gross alpha above 15 pCi/L | Combined natural alpha emitters, which may include radium and uranium | Follow-up lab tests for radium-226/228 and uranium, because 40 CFR 141.66 defines gross alpha as excluding radon and uranium | | Combined radium-226/228 above 5 pCi/L | Radium from the same geology as uranium | A certified lab radium test | | Radon in water reported in pCi/L, far above any indoor-air reading | Radon gas dissolved from bedrock, released into the air when water runs | A certified lab radon-in-water test, read against the 4,000 pCi/L guideline where a state publishes one | Gross alpha is a screen, not a diagnosis. A high gross alpha result tells you to order the specific tests; it does not tell you which treatment class to buy. ## What do the numbers mean? 40 CFR 141.66 sets three limits that matter here: 30 ug/L for uranium, 5 pCi/L for combined radium-226 and radium-228, and 15 pCi/L for gross alpha particle activity. Community water systems have had to meet them since December 8, 2003, per the same section. Radon is the gap in that rule. EPA's archived radon-in-drinking-water Q&A describes a 1999 proposal of 300 pCi/L for community systems in states without an enhanced indoor-air program, and an alternative MCL of 4,000 pCi/L where the state runs one; EPA never finalized it. Because no federal radon-in-water MCL exists, states fill the space. The Maine CDC adopted 4,000 pCi/L as its Maximum Exposure Guideline, and the same paper says it does not recommend treating water for radon without first testing indoor air, naming a home above 20,000 pCi/L in water and 4 pCi/L in air as a candidate for both. Your state's figure may differ from Maine's. The number on your report is compared against your state health department's guideline first, and against the 1999 proposal only as a reference point, not a rule. ## Which treatment class fits? | Result | Treatment class | Why | |---|---|---| | Uranium above 30 ug/L | Anion exchange or reverse osmosis | 30 ug/L is the enforceable MCL in 40 CFR 141.66(e); both technologies are the ones the EPA radionuclides rule names for uranium | | Combined radium-226/228 above 5 pCi/L | Ion exchange, lime softening or reverse osmosis | 5 pCi/L is the MCL in 40 CFR 141.66(b); these are the technologies named for radium | | Radon above 4,000 pCi/L, or above a stricter state guideline | Aeration | EPA's archived Q&A says aeration bubbles air through the water and carries the radon out through an exhaust fan, and warns that GAC collects radioactivity on the filter | | Radon between 300 pCi/L and the 4,000 pCi/L guideline | Aeration preferred; GAC only with the hazard understood and the state radiation program consulted | The 1999 proposal set 300 pCi/L as the MCL absent a state program; the lower the result, the smaller the GAC concern, but the EPA caution does not go away | Anion exchange is a resin bed that swaps negatively charged ions such as the uranyl carbonate form of uranium for chloride. Reverse osmosis is a membrane that rejects dissolved ions at the tap it is plumbed to, which is why an under-sink RO fixes the kitchen tap and nothing else; the [under-sink RO review](/water/reviews/best-under-sink-reverse-osmosis-system/) covers that class. No percent-removal figure appears on this page. The EPA document that carries those figures was not readable this session, so the treatment classes above are named without a removal percentage rather than with a number this page cannot show. **Before you buy anything** Do not order an aeration unit, an anion exchange tank or a whole-house RO on a gross alpha result alone. Gross alpha above 15 pCi/L only says "test further"; the uranium and radium results decide the treatment class, and a radon-in-water result decides whether aeration is needed at all. A certified mail-in lab panel that adds uranium, radium and radon is the purchase that settles it; the [water test kit review](/water/reviews/best-water-test-kit-well-and-city/) separates the strips that cannot read any of these from the lab kits that can. ## Who should not pick each treatment? Every class above has a reader it does not fit. - **Anion exchange for uranium:** not the first move when the same well is high in sulfate or nitrate, because those ions compete for the resin sites and shorten its life. The [nitrate guide](/water/guides/nitrate-in-well-water/) covers the 10 mg/L nitrate limit that would show on the same panel. - **GAC for radon:** not a default choice because it is cheaper. EPA's archived Q&A says the filter may become a handling hazard and require special disposal; a household considering it confirms with its state radiation program first. - **Reverse osmosis for uranium:** not a whole-house fix on its own, and no fix for radon. RO works only at the tap it is plumbed to, and radon leaves the water at every unfiltered fixture upstream, a shower included. Uranium and radium travel with the same geology, so a well high in one is tested for the other. The [arsenic guide](/water/guides/arsenic-in-well-water/) covers the other bedrock contaminant that often sits on the same report. ## What should you do first? 1. Order a certified lab test that names uranium, combined radium-226/228, gross alpha and radon in water; a strip reads none of them. 2. Read uranium against 30 ug/L, radium against 5 pCi/L and gross alpha against 15 pCi/L, the three 40 CFR 141.66 limits. 3. Read radon against your state health department's guideline, then against Maine's 4,000 pCi/L and the 1999 proposal's 300 pCi/L as reference points. 4. Match the class to the result: anion exchange or RO for uranium, ion exchange, lime softening or RO for radium, aeration for radon. 5. Retest the treated water at the same lab after installation; a published spec is not a result from your well. ## Frequently asked questions ### What is the federal limit for uranium in drinking water? 40 CFR 141.66(e) sets the Maximum Contaminant Level for uranium at 30 ug/L, and the same section sets combined radium-226 and radium-228 at 5 pCi/L and gross alpha particle activity at 15 pCi/L. Community water systems have had to comply since December 8, 2003. A private well is not regulated, so the 30 ug/L figure is the yardstick, not a requirement. ### Does the EPA have a final limit for radon in drinking water? No. EPA's archived radon-in-drinking-water Q&A describes a 1999 proposal: community water systems in states without an enhanced indoor-air program would meet 300 pCi/L, and systems in states with such a program would meet an alternative MCL of 4,000 pCi/L. That proposal was never finalized, so no enforceable federal radon-in-water MCL exists and states publish their own guidelines. ### Why not use a carbon filter to remove radon from well water? EPA's archived Q&A says GAC filters tend to cost less than aeration devices, but that radioactivity collects on the filter, which may cause a handling hazard and require special disposal methods. Aeration devices bubble air through the water and carry the radon gas out through an exhaust fan instead of trapping it, which is why EPA describes aeration as the alternative. ### What removes radium from well water? The EPA radionuclides rule at 40 CFR 141.66 sets combined radium-226 and radium-228 at 5 pCi/L, and the treatment classes named for radium are ion exchange, lime softening and reverse osmosis. The primary EPA PDF with percent-removal figures was not readable this session, so this page names the technologies without a removal percentage. Confirm the radium result with a certified lab before sizing anything. ### What is Maine's guideline for radon in well water? The Maine CDC adopted 4,000 pCi/L as its Maximum Exposure Guideline for radon in drinking water, the same figure as EPA's proposed alternative MCL. The same paper says Maine CDC does not recommend treating water for radon without first testing indoor air, and calls a home above 20,000 pCi/L in water and 4 pCi/L in air a candidate for both. Other states publish their own figures. ## Sources - [Cornell LII, 40 CFR 141.66, Maximum contaminant levels for radionuclides](https://www.law.cornell.edu/cfr/text/40/141.66) (university or extension service) - [EPA archive, Radon in Drinking Water: Questions and Answers](https://archive.epa.gov/water/archive/web/html/qa1.html) (government or regulatory source) - [Maine CDC, Maximum Exposure Guideline for Radon in Drinking Water (MEG analysis paper)](https://www.maine.gov/dhhs/mecdc/sites/maine.gov.dhhs.mecdc/files/MEG%20analysis%20paper.pdf) (government or regulatory source) --- Citation: OwnerSpec, "Uranium or radon in well water: the limits, and what removes each", https://ownerspec.com/water/guides/uranium-radon-in-well-water/, facts verified 14 September 2026. Canonical citation target: the URL above (the public HTML page), not this Markdown representation. Reuse: quoting with attribution and a link is permitted. See https://ownerspec.com/legal/terms/.