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Hot springs in non-volcanic areas are attributed to the interaction of water with hot rocks deep in the earth's crust:

In non-volcanic areas, the temperature of rocks within the Earth also increases with depth—this temperature increase is known as the Geothermal Gradient. If water percolates deeply enough into the crust, it comes into contact with hot rocks and can circulate to the surface to form hot springs.
Hot Springs/Geothermal Features, National Park Service

If I understand correctly, this heat comes from the decay of radioactive isotopes:

... the vast majority of the heat in Earth's interior—up to 90 percent—is fueled by the decaying of radioactive isotopes like Potassium 40, Uranium 238, 235, and Thorium 232 contained within the mantle.
Probing Question: What heats the earth's core?

Hence...

Question: Are hot springs in non-volcanic areas radioactive?

I'm guessing the answer is either "no" or "yes, but not very", because people swim in them. But it seems like radioactive isotopes should be seeping into the water.

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  • $\begingroup$ most things are radioactive and this includes spring water,location is more important than temparature. en.wikipedia.org/wiki/Background_radiation $\endgroup$
    – trond hansen
    Commented Feb 29, 2020 at 17:05
  • $\begingroup$ Note that the mantle and the crust are physically very different: While radioactive elements can build-up in the mantle, they will be cycled through the crust and occur there in lower cocentrations. $\endgroup$
    – AtmosphericPrisonEscape
    Commented Feb 29, 2020 at 22:31
  • $\begingroup$ @AtmosphericPrisonEscape - That's backwards. The concentrations of uranium, thorium, and potassium are enhanced in the Earth's crust (compared to carbonaceous chondrites), and slightly depleted in the mantle. Those three elements are strongly lithophile. $\endgroup$
    – David Hammen
    Commented Mar 1, 2020 at 11:36
  • $\begingroup$ keep in mind those radioactive source for the earths heat are thousands of miles away, why do you think any would add anything to the water? $\endgroup$
    – John
    Commented Mar 2, 2020 at 3:00

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The answer is contained within your own question. Virtually everything is radioactive, including your own body, the air you breathe, and everything you eat. The radioactivity is usually beta radiation (emission of energetic electrons), which is the least harmful form of radioactivity.

Radon gas, which is emitted from granite rocks containing uranium, sometimes builds up to dangerous levels in our homes and is an alpha emitter. Alpha radiation is very ionising and dangerous, but needs to be ingested into the body to cause serious harm.

The answer to your question, therefore, is yes, hot springs are radioactive, but so are cold springs. As hot springs are more likely to have passed through granite rocks, they are probably on average more radioactive than cold springs, but neither are radioactive enough to cause concern.

The radioactivity which is dangerous enough to cause concern is radon gas, which varies from place to place according to the underlying rocks. Maps of radon hotspots can be obtained from the internet. If you happen to live in a hotspot, you need to keep your house well ventilated, otherwise the radon will build up to dangerous levels, sometimes levels which would not be tolerated in a nuclear power station or research establishment.

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    $\begingroup$ This answer starts off as self-evident facts without need references and citations, but then it says "The radioactivity which is dangerous enough to cause concern is radon gas..." which 1) I am not sure is true (what spring water has dangerous amounts of radon gas in it? I'd like to see a source) and 2) ignores radium, which certainly can be a problem! livescience.com/61397-tap-water-radium.html This is why answers that don't cite sources are not trusted no matter how good they sound. $\endgroup$
    – uhoh
    Commented Mar 1, 2020 at 5:43
  • $\begingroup$ Any radon gas which gets into spring water is in tiny quantities and is soon lost. It doesn't have the chance to build up like it does in some houses. We also don't spend half our lives wallowing in hot springs, like the ones at Bath in England, therefore whatever nasties they may contain don't have enough time to do us serious harm. Houses are another matter. Radon levels vary tremendously from place to place.. $\endgroup$
    – Michael Walsby
    Commented Mar 1, 2020 at 8:27
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    $\begingroup$ This is Earth Science, not The Great Outdoors. The question is not if the levels are of any concern. You're essentially saying "I don't know if hot springs are more radioactive than cold springs, but [something else] is more radioactive", which is true but not useful. They key phrase, "probably on average more radioactive than cold springs", appears to be just a guess. This line in particular needs to be explore in more depth, with sources. $\endgroup$
    – gerrit ♦
    Commented Mar 1, 2020 at 13:10
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    $\begingroup$ @MichaelWalsby If your comment gets deleted, please do not repost it. Please review what comments are for. I deleted your comment twice because it was disrespectful of people who die of cancer. $\endgroup$
    – gerrit ♦
    Commented Mar 2, 2020 at 14:48
  • $\begingroup$ Why delete it? There was nothing wrong with it. No, it is you who are disrespectful of people wo die of cancer by deleting a comment which might have saved one or two. My own brother died of cancer because he would not listen to my advice. You delete comments because you don't want anyone to see any view which contradicts yours, or for them to get the impression that you are devious. I could have put myself in a position to delete comments and carry out all the devious tricks the in-crowd get up to, but I'm not interested in censoring their views or in needlessly editing their posts. $\endgroup$
    – Michael Walsby
    Commented Mar 2, 2020 at 19:02

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