Radionuclides in Water

Dr. Brian Redmond, Licensed Professional Geologist, Professor
Featured Water Professional

An element is determined by the number of protons the atomic nucleus has. An isotope is determined by how many neutrons the nucleus of that element has. Some combinations (isotopes) of protons and neutrons are completely unstable – they don’t exist. Other combinations are completely stable. Then there are the isotopes which, although unstable, don’t immediately decay. Such isotopes (radionuclides) have a radiologic half-life (t1/2) which is the time in which half of the original material emits radiation and becomes something else; the shorter the half-life, the faster they fall apart and the more radiation is generated.

Isotopes can be designated in several ways. One example is Carbon-14 which can also be written as C-14, 14C, or 6C14. The letter C indicates the element, in this case, carbon. The 14 refers to the atomic mass of the isotope where protons and neutrons have an approximate atomic mass of 1 atomic mass unit (amu) each. Since carbon by definition has 6 protons and the atomic mass is the sum of protons + neutrons, C-14 must have 14 – 6 = 8 neutrons.

The atomic mass is usually written as a superscript to the left of the symbol (letter/s) of the element.  The atomic number (how many protons the element has) is a subscript to the left of the symbol of the element.  Unfortunately, the word processing program used for this article will not allow both a superscript and a subscript in the same place so, if the atomic number is included as a subscript on the left, the superscript atomic mass is shifted to the right of the elemental symbol.  

The emitted radiation is usually Alpha (α), Beta), or gamma (γ) radiation although fission can spit out neutrons and most of cosmic radiation is actually protons (p+) moving at near the speed of light.  The penetration power of each type of radiation differs greatly: alpha has the least and gamma the most.  

Alpha radiation (α) is actually a very fast-moving (lots of kinetic energy) bundle (particle) of two protons and two neutrons.  This ‘bundle’ is the nucleus of a 2He4 atom but it is not an atom because it lacks the two electrons a helium atom would have.  Alpha particles are emitted by nuclei that have too many protons (or too few neutrons) to be stable.  Once the alpha particle is gone, what remains is an element two numbers less and with an atomic mass of four amu less.  For example:  92U238 ––> 90Th234 + α.  You could think of the α as being 2α4.

As an approximation, Beta decay can be thought of as when a nucleus, unstable because it has too few protons for the number of neutrons it has, takes a neutron, splits it into a proton + an electron, and throws that electron out of the nucleus at great speed (the beta particle).  The nucleus that is left after the beta particle is ejected now has one proton in place of a neutron: its atomic number has gone up by one.  Since the sum total of protons + neutrons is the same (the mass of the electron is negligible compared to that of protons and neutrons), the atomic mass of the new isotope remains the same. 

The temperature of something can be thought of as a measure of how quickly (how much kinetic energy) the atoms and molecules that make something up are moving.  The alpha and beta particles are moving much faster than anything around them but as they interact with surrounding atoms and molecules, they transfer some of their momentum to whatever they hit, slowing themselves down and speeding up whatever they hit.  

Eventually, the alpha and beta particles are moving about the same speed as everyone else.  When that happens, the alpha particle acquires two electrons and becomes an ordinary helium atom.  The beta particle becomes an ordinary electron indistinguishable from any other electron. 

The Earth is not massive enough to hold helium gas which, if released into the atmosphere, escapes into space.  Every wonder where companies get the helium gas to blow up those party balloons?  Alpha decay produces helium gas within the Earth where it can remain trapped within the rock.  Natural gas (mostly methane) is also generated within the Earth.  Sometimes there are structural traps within the Earth in which such gasses can accumulate.  Drill a natural gas well into such a trap and a small fraction of that natural gas may be helium.  Squeeze and cool the natural gas to the point where the methane becomes liquified and it is easy to separate out the helium gas.  

Meanwhile, everyone else is moving ever so slightly faster because of the transferred momentum from the alpha and beta particles which is a fancy way of saying that the temperature of the surrounding material goes up a little bit.  This is how radioactive decay, in this case, alpha and beta decay, can produce heat.  The Radioactive Decay of the alpha and beta-emitting isotopes in the Earth slowly heats up the Earth, driving processes like plate tectonics, volcanism, geothermal heat sources, and earthquakes.  

Gamma radiation is not a particle like alpha and beta radiation but is a very highly-energetic (very short wavelength) part of the electromagnetic spectrum which includes radio waves, infrared radiation, visible light, ultraviolet, and x-rays.  Ultraviolet radiation and x-rays, like their even more energetic brother, gamma radiation, have enough energy to ionize atoms and break chemical bonds; they all are ionizing radiation.  

A whimsical description of each might be that being hit by an alpha particle is like being hit by a brick; it doesn’t usually penetrate the body much but can be quite damaging.  Being hit by a beta particle is like being shot with a 22-caliber bullet.  It does penetrate into the body some distance and can damage tissue deeper in the body but does not usually pass completely through the body.  Gamma radiation is like being shot with a powerful Flash Gordon ray gun (Jedi sword?), something that can go completely through the body.  

The problem arises when Ionizing Radiation ionizes atoms or breaks molecular bonds in living tissue.  The broken bonds will re-establish themselves but will not necessarily be bonded in the same way they were before.  The changes in the bonding in what could be a complicated organic molecule could produce a change in the molecule which could promote cancer.  What kind of cancer can develop depends on what living body tissue the radiation can reach.  For this reason, various national and international agencies have set limits on how much Dissolved Radionuclides can be in drinking water. 


Uranium in the Earth’s Crust 

While there is some argument about how much uranium is in the core of the Earth (some think a lot, some  very little), uranium is surprisingly common in the crust of the Earth, more so than in the mantle.  The crustal average of uranium is a few ppm (parts per million) which is 40 times that of Silver and 500 times that of gold.  

Granite, the most common rock in continental crust, has a uranium concentration of from a few ppm to as much as 100 ppm.  Black shales may have up to 400 ppm.  Uranium has an affinity for phosphate and can be found in phosphorite deposits (in the mineral, apatite) in concentrations of more than 800 ppm.  Phosphorite is mined and converted into phosphate fertilizer which could still contain the uranium and some of its derivatives.  

Unlike most of the Rare Earths, which are not all that rare in amount but are rarely concentrated, uranium is sometimes concentrated in amounts worth mining, typically on the order of 0.1% or 1000 ppm.  Uranium ores contain minerals such as carnotite, K2(UO2)2(VO4)2•3H2O, Uraninite (aka pitchblende) which is mostly UO2, and autunite, Ca(UO2)2PO4)2•10-12H2O.  

The higher concentrations of uranium, produced in a variety of processes, can be found within certain types of igneous, metamorphic, and sedimentary rocks.  An example of one such concentrating process is the precipitation/crystallization of uranium minerals from water under certain conditions, leading to a concentration of uranium in a sediment which eventually becomes a sedimentary rock.  Depositional environments which favor the formation of black, organic-rich mud or phosphates would also be favorable to the deposition of uranium, hence, elevated uranium in black shales and phosphorites.  

However, uranium, as a resource, is as nonrenewable as it can get.  Since there is no process on or in the Earth that can create new uranium and because uranium is unstable (radioactive), the amount of Earth’s uranium has been slowly diminishing ever since the Earth first formed some 4.5 billion years ago.  

Uranium is basically made up of two uranium isotopes (there are trace amounts of others):  U-238 (92 protons + 146 neutrons) and U-235 (92 protons + 143 neutrons); uranium by definition has 92 protons.  Neither of those two combinations of protons and neutrons is completely stable; they will eventually fall apart, emit radiation, and change into another (radioactive) element.  Unstable, though, is a relative term.  

How unstable an isotope is can be described by its half-life which is the time over which half of the original unstable isotope decays, emitting radiation.  In the case of the two uranium isotopes mentioned above, the half-lives (t1/2) are very long:  t1/2 for U-238 = 4.468 billion years, (t1/2) for U-235 = 703 million years.  You might note that the t1/2 of U-238 is about the age of the Earth which means that the Earth today has about half of the U-238 it started out with; the U-235 is disappearing faster (shorter half-life) but some is still around.  

Uranium is an energy resource because U-235 is fissionable.  Under the right conditions, such as in a nuclear power plant, throwing neutrons at a U-235 nucleus can induce it to break apart (fission) into several large, very radioactive pieces while releasing lots of energy and more neutrons which can be used to induce fission in other U-235 nuclei.  

Unfortunately (or maybe fortunately) the U-238 is not fissionable and, because of the different decay rates, the uranium of today is about 99.3% U-238 compared to the 0.7% of U-235.  If you want to use uranium to power a nuclear power plant, the amount of U-235 in the uranium should be around 5% of the uranium which means you have to enrich (increase the U-235 ratio) in the uranium which is very difficult and expensive since there is only a very small mass difference between the two isotopes.  And if you want to make an atomic bomb, the amount of U-235 in the uranium needs to be close to 95%.  This is why Iran, which is trying to make atomic bombs, has so many centrifuges to separate the two isotopes in multiple stages of centrifuges.  

As an interesting aside, uranium is created in supernova explosions in something called the r-process in which the U-235 to U-238 ratio is about 1.65 compared to the current Earth ratio of 0.007.  

Another interesting aside is that, unlike the uranium isotopes, few people care what the isotopes of Iron are.  As it happens, iron has four stable isotopes of which Iron-56 (Fe-56) makes up 91.7% of all iron in the Earth followed by Fe-54 @ 5.8%, Fe-57 @ 2.2%, and Fe-58 @ 0.3%.  There is no economic difference in which or how much of a given iron isotope you might have.  

The point of all this is that because uranium is a resource, mined, and processed, there are uranium ore waste piles called tailings which, when leached, can be a source of uranium in water.  The separation of the isotopes can create more uranium waste.  Uranium from which as much U-235 has been extracted is known as depleted uranium.  Depleted uranium, because of its high density, is sometimes used to make heavy bullets and shells with greater penetrating power.  Such ammunition was used in the Iraq war which means there must be depleted uranium bullets/shells in the Iraqi desert landscape, a potential source of uranium contamination for what little water Iraq has.  

It should be noted, however, that the uranium isotope ratio is not a factor in how much uranium gets into water.  What matters is that human activity can further concentrate uranium and then distribute it in such a fashion that it becomes more likely to enter the water cycle. 

Getting the Uranium into and Out of Water

Most uranium compounds are essentially insoluble in water, certainly uranium oxides which include all of the uranium minerals mentioned above.  How, then, might what appears to be a very insoluble element or uranium compound get into water?  Consider sodium (Na).  The sodium ion (Na+) is one of the most common ions dissolved in seawater.  Paired with chloride (Cl), sodium can precipitate as ordinary salt, NaCl.  But where does all that sodium come from?  

Granite is the most common rock in continental crust and the most abundant mineral in granite is feldspar.  There are three variations of feldspar, one of which is a sodium feldspar, NaAlSi3O8.  But this sodium compound, unlike salt (NaCl), is very insoluble in water.  But granite can be weathered, a chemical process which can convert the feldspar into a clay, releasing some of the sodium into a water solution.  Once in solution, the sodium ion tends to remain in solution, gradually building up the concentration of sodium ions in the oceans over many hundreds of millions of years.  

Now consider that same granite with a few ppm of uranium.  The uranium can substitute for calcium in calcium feldspar and when the granite weathers, some of the uranium can be released into solution.  How soluble the uranium is in water depends on a number of factors including: water temperature, pH, redox potential, ionic strength, nature of other dissolved ions and complexes, and the oxidation state of the uranium.  

Without getting carried away with this, in general, the higher the water temperature and the lower the oxygen content of the water (redox potential), the more dissolved uranium the water can hold.  Low and very high pHs promote uranium solution as do carbonate and phosphate complexes.  Change any of those factors and you can increase or decrease the amount of dissolved uranium in the water.  

How soluble the uranium ion is depends on the oxidation state of the uranium ion.  A more familiar example of this is dissolved iron which can be ferrous iron (Fe++) or ferric iron (Fe+++).   Ferrous iron is much more soluble in water than ferric iron.  Groundwater or mine water, depleted of oxygen, can hold in solution considerable iron as ferrous iron.  Bring that water to the surface and let oxygen get into the water and the ferrous iron will oxidize into the less soluble ferric iron which immediately begins to precipitate as ferric hydroxide [Fe(OH)3], leaving bright red/orange stains on sinks, toilets, and laundry.  

It is a similar story for dissolved uranium in water which usually is in one of two oxidation states:  the uranous ion (U++++) or the uranic ion (U++++++).  That’s a lot of plusses so the oxidation states are usually designated by Roman numbers such that the uranous ion becomes U(IV) or UIV and the uranic ion becomes U(VI) or UVI.  In oxygenated water, the uranic ion combines with oxygen to form the uranyl ion (UO2++).  

With the iron ions, iron in the higher oxidation state (Fe+++) is less soluble than iron in the lower oxidation state (Fe++).  And, like uranium, the oxidation states of iron could also be designated using Roman numerals: Fe+++ = Fe(III) and Fe++ = Fe(II).  

For uranium, it is the exact opposite.  The uranyl ion is more soluble than the uranous ion.  This means that should well-oxygenated water loaded with uranyl ions lose its dissolved oxygen, it could trigger the deposition of uranium oxide which is why uranium can be concentrated in the low-oxygen sediments that become black shales.  

Sea water has a lot more dissolved ions in it (higher ionic strength) than fresh water does.  Should fresh water loaded with uranyl meet sea water, it could trigger the deposition of uranium oxide (uraninite).  This can happen along coasts where sea water meets fresh water: the sea water/fresh water interface.  As sea level rises and falls over geologic time, moving the interface back and forth, it can concentrate uranium at the current interface in a sediment or permeable rock in a process known as a roll front. 

The Toxicity of Uranium in Water

When the U-238 and U-235 isotopes decay, they produce alpha radiation (α).  Having lost two protons, what was once a uranium isotope is no longer uranium (element 92) but is now an isotope of thorium (element 90).  The isotopes have also lost two neutrons so 92U238 becomes 90Th234 and 92U235 becomes 90Th231.  

In the U.S., the Environmental Protection Agency (EPA) limit for Uranium in drinking water is 0.03 mg/L which, in terms of the amount of radiation (actually the number of radioactive decays, known as the activity) produced by that amount of uranium, would be about 27 pCi/L.  The World Health Organization (WHO) prefers a dissolved uranium limit of no more than 0.002 mg/L.  

To put this into perspective, a Curie (Ci) is the number of decays (the activity) that one gram (a tiny pinch) of radium would produce (37 billion disintegrations/second).  If you were exposed to that much radioactivity, don’t bother to run; it’s too late.  A pico Curie (pCi) is one millionth of one millionth of a Curie but even that tiny amount is a significant exposure to radiation (2.2 decays/minute).  L is a liter; it can be a liter of water or a liter of air.  Europe prefers to use Bequerels/cubic meter (Bq/m3); 1 pCi/L = 37 Bq/m3.  

Another producer of alpha radiation is Radon gas which can be found in both air and water.  The U.S. Environmental Protection Agency (EPA) limit for radon in air is 4 pCi/L.  There is no national consensus on what the radon limit in water should be yet.  The EPA has proposed a limit of 3000 pCi/L while Massachusetts has set a limit of 10,000 pCi/L.  

You might notice that while the concentration of uranium in water can be given either as something like mg/L or, based on its (radio)activity, as pCi/L, radon concentrations, either in air or water or only given in terms of pCi/L.  Suppose that both the uranium and radon in water had an equal activity, say, 27 pCi/L.  Because radon has such a much, much shorter half-life than uranium does, in order to have the same activity there must be much, much more dissolved uranium in the water than radon.  27 pCi/L of uranium is roughly equivalent to 0.03 mg/L of dissolved uranium, an amount that can be reasonably measured.  

Far, far less radon is required to produce the same activity (because it has such a short half-life), so small an amount that it would be infeasible to try to measure the concentration of radon in terms of mg/L.  But its (radio)activity is easily measured.  This is also true of other isotopes with short half-lives.  

There then remain two interesting questions:  Why is the limit for radon in water so much higher than radon in air?  Why are the limits in water for uranium and radon not the same? 

The answer lies in part in how a health agency determines at what exposure level is something unacceptable.  The something could be an exposure to radiation, poison, or other toxins.  It could include occupational exposures which might have higher acceptable limits than for the general public.  It could depend on different kinds of exposure (hence, different limits for air, drinking water, or food).  

One approach, recognizing that there usually is no such thing as zero risk to exposure to anything, is to ask, at what exposure level would exposure to something result in one extra death in a population of one million people?  Exposure to alpha and gamma radiation from something like uranium and radon is associated with cancer which can be fatal.  But there is a difference between exposure to radon in air and radon in water (you wouldn’t normally expect to be exposed to uranium in air unless you were in a dusty uranium mine).  

Unlike gamma radiation, alpha radiation has a very limited penetration power.  If the alpha emitter is outside the body, even if the alpha particle reaches the body surface, it will be stopped by the thin layer of dead skin cells at the top of the skin.  Dead skin cells cannot become cancerous; it requires living tissue to do that.  Radon, however, is a gas and can be inhaled.  Should a radon atom enter the lungs and decay, the emitted alpha particle would hit living lung tissue.  Consequently, radon in air is linked specifically with lung cancer, not other kinds of cancer.  It is the lungs that are exposed to alpha radiation from the radon, not other body organs.  

If the radon is in water, the entire digestive system might be exposed to radiation from the radon.  However, speculating a little here, there are two factors which could significantly lower the exposure.  Water is much denser than air and with its low penetration power, the alpha particles may never make it out of the water to living tissue.  The other factor is that you breathe in much more air than you drink water.  

But, then, why are the limits for uranium and radon in water not the same?  They both emit alpha radiation but the limit for uranium is much lower than that for radon.  [Remember that we are talking about uranium and radon activities (pCi/L), not mass concentrations (mg/L)]

Elements and compounds that enter the body have what is known as a biological half-life, the length of time in which half of the element or compound has been broken down (in the case of a compound) or expelled from the body; like radiologic half-life, this is an example of exponential decay.  Nicotine, for example, has a biologic half-life of a few hours; that of caffeine is about five hours.  For water, that biologic half-life is one to two weeks.  The biologic half-life should not be confused with the radiologic half-life.  Being radioactive, uranium and radon have both types of half-lives.  

If the uranium and radon remain in the ingested water, you would expect half of each to leave with the water with half gone in a week or two.  However, the radiologic half-life of uranium and radon is vastly different.  The half-life of U-238, the most stable uranium isotope, is about the same as the age of the Earth but the half-life of Rn-219, the most stable isotope of radon, is less than four days.  This means that most of the radon that enters the body in water decays and emits alpha radiation before it can leave the body.  This would not be the case with the relatively much more stable uranium.  

But there is a complication.  About two thirds of the uranium that enters with the water doesn’t leave with the water but is incorporated into bone (like lead, it substitutes for calcium).  In bone, the uranium has a biologic half-life of many months.  Worse yet, while in water, much of the alpha radiation may be blocked but in bone, alpha radiation from the uranium can easily reach living tissue although the greater danger may not be the radiation but the chemical toxicity of the uranium on calcium and phosphate metabolism.  There has not been much study of this aspect but it has been suggested that sustained exposure to levels of dissolved uranium of more than about 0.3 mg/L (0.3 ppm) can lead to kidney failure. 

Getting Uranium Out of Water

Reverse Osmosis is usually the method used to remove uranium from water – it has an efficiency of > 90% although other methods can approach this such as ion exchange and Various Filtration Systems. However, as is the case for the removal of all radioactive species in water, if the concentration of radioactive isotopes in the water is high, the wastewater from the treatment system or the filtrate on a filter or the used activated charcoal can concentrate radioactive isotopes to such an extent that they could be classified as low-level radiation waste which could create an expensive proper-disposal problem.

Plutonium and Thorium

Plutonium in the Earth’s Crust

This subtitle heading is a little misleading as, up to about 1945, there was no plutonium whatsoever anywhere on or in the Earth.  When the Earth first formed it assuredly had some plutonium but the longest-lived isotope of plutonium, Pu-239, has a half-life of 24,400 years.  After more than 4.5 billion years, any of that original plutonium is long gone and there is no natural process on Earth that would create more (spontaneous fission of U-238 can produce the neutrons necessary to produce an insignificant amount of plutonium).  

Natural process?  By 1945 we Americans became the first to produce plutonium.  Any plutonium anywhere on the Earth today is manmade.  Recent human activity has become so globally significant and pervasive that there is a proposal to create a new geologic epoch, the Anthropocene, beginning in 1950, based on detectable amounts of plutonium in the newly laid sediments in a supposedly pristine Canadian Arctic lake.  

Nuclear power plants usually use U-235 as their fissionable fuel.  The uranium needs to be enriched to at least 5% U-235 for the generation of nuclear power but this means that some 95% of the uranium fuel is non-fissionable U-238, useless for generating power.  The power-generating U-235 fission is induced by thermal (not moving too fast) neutrons which split (fission) the U-235 nucleus.

Thorium in the Earth’s Crust

Unlike plutonium, thorium is naturally present in the Earth’ crust, averaging 12 ppm and typically on the order of 6 ppm in soils.  Concentrated amounts of thorium can be found in a phosphate mineral, monazite, containing some 6% or more thorium phosphate.  Thorium is about four times as abundant as uranium which is understandable since, although uranium-238 has a half-life (4.47 billion years) of about the age of the Earth (4.54 billion years), the half-life of thorium-232 (13.9 billion years) is on the order of the age of the universe (13.8 billion years).  

Economic deposits of thorium have not been nearly as well mapped as those of uranium but there are an estimated 6 million tonnes of thorium reserves globally, with the greatest amount in central Asia.  Curiously, the estimated global uranium reserves are also about 6 million tonnes with the greatest amount in Australia.  Should the value of thorium increase, what is considered to be a thorium reserve would also increase. 

Making Plutonium

Uranium-238 normally decays by the emission of an alpha particle but there is another, very unlikely kind of decay.  5 times out of 10 million decays, instead of emitting an alpha particle, the U-238 nucleus will spontaneously fission, breaking up into several large, highly radioactive chunks, and emitting 1 to 3 fast neutrons; alpha decay does not produce free neutrons.  Should one of those fast neutrons be slowed by a moderator, such as heavy water, it would become a thermal (slow) neutron which would greatly enhance its chance of splitting (fissioning) a U-235 nucleus, producing more highly radioactive chunks, more fast neutrons, and lots of energy.  

U-235 does not spontaneously fission but if it is concentrated enough, a rare neutron from the spontaneous fission of U-238 can induce fission in a U-235 nucleus, resulting in a spray of neutrons which can induce fission in other U-235 nuclei, perhaps enough for a sustained chain-reaction or even a nuclear explosion.  

In a nuclear reactor with abundant U-238, some of the neutrons generated by the induced fission of U-235 will hit a U-238 nucleus which, instead of fissioning, will absorb the neutron to become U-239.  92U239 is not fissionable but it has a very short half-life (23.5 minutes), spitting out a beta particle (β) and becoming 93Np239.   The 93Np239. also has a very short half-life (2.1 days) so it spits out another beta particle and becomes 94Pu239 which happens to be fissionable; it can be used in another type of nuclear reactor or made into a plutonium atomic bomb like the one dropped on Nagasaki.  

Note that atomic bomb explosions both have and create more plutonium which is then sprayed all over the neighborhood and, eventually the world (nuclear fallout) including that pristine Canadian Arctic lake.  Worse yet, the hydrogen bomb requires an atomic bomb to produce the millions of degrees temperature for the fraction-of-a-second hydrogen (actually tritium + deuterium) fusion.  Tiny amounts of plutonium are now everywhere, perhaps marking the beginning of the Anthropocene, and all because of us.  

Except in the near vicinity of a recent atomic bomb blast, the amount of plutonium in a given area, although measurable, is probably insignificant.  However, highly radioactive used fuel rods from nuclear power plants will contain plutonium and if not properly stored and eventually disposed of (probably deep burial; Yucca Mountain, Nevada?), could be a significant source to the local environment including groundwater.  

Being a source of plutonium, those used fuel rods could be vulnerable to terrorists who might want to make their own atomic bombs without having to go through the very difficult and expensive process of separating uranium isotopes.  

Failing that, a terrorist could use the highly radioactive isotopes, including some plutonium, to make a dirty bomb, a simple chemical bomb that would spread the radioactive isotopes over an area, possibly rendering it uninhabitable for many thousands of years.  Even a crop duster could spread the stuff although its pilot may well be exposed to a lethal amount of radiation (drones anyone?).  

If you want to maximize plutonium production, usually for atomic bombs, nuclear power plants can be modified, becoming breeder reactors for plutonium.  That plutonium then has to be stored somewhere like Hanford, Washington which has already been shown to have leaked plutonium into the local groundwater. 

Characteristics of Plutonium

Elemental plutonium, which is what weapons-grade plutonium is, is highly reactive with water and oxygen; it will oxidize and form tiny plutonium oxide (PuO2) particles both in air and water. In air these particulates can be inhaled with a major impact on the lungs, possibly leading to lung cancer.  Some plutonium could pass from the lungs into the blood and then the kidneys.  

Plutonium is not readily absorbed by the digestive system, mostly being excreted, especially in urine, but, if it gets into the blood, it will likely end up in the bones and liver where it will stay for a very long time.  Wherever plutonium is in the body, it will irradiate living tissue with alpha radiation, possibly leading to some form of cancer.  No-one has yet been definitively shown to have died specifically from exposure to plutonium (except in Nagasaki).  

Although both uranium and plutonium are alpha emitters, plutonium, with its much shorter half-life compared to uranium, can be a problem in much lower amounts than is the case for uranium.  Like radon, plutonium concentrations are usually described in terms of pCi/L rather than mg/L. 

Characteristics of Thorium

All of the naturally-occurring thorium is of one isotope, thorium-232, which is not a fissionable isotope.  However, if it were bombarded with fast neutrons from a fast neutron source like uranium-235, the thorium would absorb the neutron, forming thorium-233, quickly undergo two beta decays, and become uranium-233 which is fissionable.  

Although unstable (radioactive), U-233 has a relatively long half-life of 162,000 years and could be, and has been, used as the nuclear fuel in a thorium nuclear reactor.  Such reactors are not yet common and could become more popular although there are various pros and cons.  One downside is that such reactors can be prolific sources of tritium.  Fortunately, thorium or its derivatives are not well-suited to make a thorium nuclear bomb.  Nevertheless, should thorium become more popular in nuclear reactors, its mining, processing, storage, use, and waste could provide more opportunities for thorium to enter the hydrologic cycle.  

Most of the thorium that is ingested from food or water is excreted within several days.  The small amount that makes it into the blood ends up in the bones where it can linger for many years.  

Plutonium and Thorium in Water

The EPA has a maximum contaminant level of 15 pCi/L for all alpha emitters (gross alpha particle activity) in water except for uranium, radium, and radon which are separately regulated.  Plutonium and thorium are included in that 15 pCi/L limit although some have advocated a much lower limit for plutonium.  

Plutonium in drinking water is a potential major threat but, although some plutonium has appeared in groundwater near plutonium storage areas such as in Hanford, Washington, there is, as yet, no known significant impact on public water supplies.  

However, should significant plutonium get into a water supply, water treatment could complicate the problem.  How soluble plutonium is in water depends, among other things, on its oxidation state.  Two common oxidation states are Pu(IV) and Pu(VI).  Like uranium, the higher oxidation state is more soluble.  Chlorination of water supplies would oxidize the more common Pu(IV) to the more soluble Pu(VI).  

Nevertheless, barring a nuclear war, the plutonium dispersed in the general environment, while detectable, is insignificant and unlikely to be a problem in drinking water.  Concentrated sources of plutonium are all under government control and highly monitored.  Depending on your opinion of the reliability of governments, this may not be too reassuring but national agencies like the EPA require the periodic testing, including of alpha emitters like plutonium, of all public water supplies.  

If you have your own private water supply, and if you have some reason to suspect that you might have some plutonium or other alpha emitter in your water, you can test for it.  They could be present as suspended solids (PuO2 for plutonium, ThO2 for thorium) or in tiny amounts of dissolved solids.  Either way, testing for gross alpha particle activity should pick them up.  

Should the gross alpha particle activity exceed the EPA limit of 15 pCi/L, the same water treatment methods used to remove uranium will work with plutonium and thorium with the same caveat that, with high levels of such contaminants, water treatment can result in concentrated amounts of radioisotopes in the treatment waste that could require special, expensive proper disposal. 

Fission Products from Atomic Bombs and Nuclear Reactors

Earlier it was mentioned that the fissioning of plutonium and certain uranium isotopes produced several highly radioactive ‘chunks.’  These chunks, found in used nuclear fuel rods and making up nuclear fallout from atomic bomb explosions, notably include:  Carbon-14, Cobalt-60, Strontium-90, Iodine-131, and Cesium-137, along with some Plutonium-239 (discussed above) and tritium (already discussed in another article).

Isotope Half-life Emitted Radiation
H-3 12.3 years β–
C-14 5720 years β–
Co-60 5.26 years β–, γ
Sr-90 29 years β–
I-131 8.06 days β–, γ
Cs-137 30 years β–, γ
Pu-239 24,400 years α


C-14 (14C) is produced naturally in small amounts by neutrons (a form of cosmic radiation) interacting with the nitrogen gas in the stratosphere:  7N14 + n  ––>  6C14 + p+.  It does decay back into nitrogen: 6C147N14 + β, γ + a neutrino, but with a half-life of about 5730 years it has time to spread out and reach equilibrium (the amount lost to radioactive decay equals the amount being generated in the stratosphere) with nonradioactive carbon isotopes on and near the surface of the Earth including as part of the carbon in living things.  That equilibrium is about 14 disintegrations/minute per gram of carbon (a measure of radioactivity).  It is part of the natural background radiation.   

Should something living die, carbon is no longer being exchanged with the carbon in the surface environment so 14C lost to decay is not replaced with new 14C; the (radio)activity of the carbon decreases.  Should calcium carbonate precipitate out of water to form limestone (CaCO3), the 14C content of the limestone would begin to decrease.  This is the basis of carbon dating.  

Fission in nuclear reactors and atomic bombs produces neutrons which can then interact with the nitrogen in the air, producing 14C.  This recent human source of 14C has the potential to screw up carbon dating since the extra human-generated 14C could make something seem to be younger than it really is.  

Carbon is present in water; it includes dissolved organic matter and, one of the more common anions, bicarbonate (HCO3).  There could be some detectable beta and gamma radiation from 14C but, barring a nuclear war or a very serious nuclear reactor problem (and if that happened, there would be far more serious things to worry about), the amount of radiation from 14C should be insignificant.  

It is also the case that groundwater, isolated from surface replenishment of 14C, would have an even lower 14C activity.14C is the second-most important source of radiation in the typical human body (40K is #1), but almost all of it comes from what you eat, not what you drink.


Cobalt is often used in steel alloys common in nuclear reactors and atomic bombs but there is only one stable isotope of cobalt, 59Co.  Atomic bombs and nuclear reactors are great sources of neutrons and if a Co-59 atom absorbs a neutron, it becomes 60Co.  With its short half-life, 60Co is a prolific source of beta and gamma radiation; it then becomes stable 60Ni.  There is no significant natural source of 60Co.  

Cobalt is more soluble in sea water than freshwater and at a lower pH.  Like iron, cobalt has two oxidation states: Co(II) or Co++ and Co(III) or Co+++.  Also like iron, the higher oxidation state (Co+++) is much less soluble in water than the lower oxidation state.  But, unlike iron, the dissolved cobalt is not present as a simple Co++ ion but rather forms complexes with the water, such as [Co(H2O)6]++.  The cobalt could also be present as a suspended solid in the water.  

Cobalt is a micronutrient necessary for the production of vitamin B12 (C63H88CoN14O14P).  Most of the excess cobalt ends up in the liver, some in the kidneys.  Once in the body, then, it is the liver that would have the highest exposure to 60Co.  The biologic half-life of cobalt is a little complicated.  In the liver, 60% of the cobalt has a biologic half-life of 6 days, 20% has a biologic half-life of 60 days, and the last 20% lingers with a half-life of 800 days.  

Many water treatment systems will remove cobalt from water, systems including: activated charcoal filtration, reverse osmosis, and ion exchange. However, if this includes the highly radioactive 60Co, it could easily lead to high concentrations of radioactive cobalt in the treatment waste with consequent disposal problems.


Non-radioactive strontium, 82.6% 88Sr, 9.9% 86Sr, and 7.0% 87Sr, is relatively common in the Earth’s crust (15th most abundant crustal element), especially in igneous rocks.  Chemically, it is similar to calcium and will end up in the bones with a biologic half-life of somewhere between 14 days and 49 years.  The EPA recommends that there be no more than 4 mg/L of nonradioactive strontium in drinking water.  Groundwater can have as much as 50 mg/L of dissolved strontium but it is usually < 1 mg/L.  Many water treatment systems will remove strontium from water.

90Sr is one of the large radioactive ‘chunks’ produced by the fission of uranium or plutonium; there is no significant natural source of 90Sr. The beta and gamma-emitting 90Sr promotes the development of bone and bone marrow cancer. The EPA drinking water limit for 90Sr is 8 pCi/L which corresponds to an exposure of 4 mrem/yr.


131I is another large ‘chunk’ produced by the fission of uranium.  Non-radioactive iodine, 100% 127I, is mostly found in marine sediments with an average crustal abundance of some 300 ppb (0.3 mg/L); sea food, especially sea weed, is an important source of dietary iodine.  Thyroid hormones require iodine so, once in the body, most iodine is concentrated in the thyroid gland; it has a biologic half-life of around two months in normal thyroids.  A certain small amount of iodine is necessary for human health (~ 0.15 mg/day for an adult); an iodine deficiency could lead to goiter which is why commercial food-grade salt is iodized.  

Older readers may remember when 2% tincture of iodine (elemental iodine dissolved in a mixture of ethanol and water) in small bottles could be found in pharmacies.  It was used as a surface wound antiseptic.  Hikers could buy iodine tablets (potassium iodide), used to disinfect river and spring water for emergency drinking purposes.  Alas, somebody discovered that iodine could be used in the illicit manufacture of methamphetamine and that was the end of easy public access to iodine.  

The beta and gamma-emitting 131I has a short radiologic half-life of about 8 days (there is no significant natural source of 131I ) and is linked with the development of carcinoma of the thyroid.  Ironically, such carcinomas and hyperactive thyroids are treated with the clinical ingestion of 131I whose purpose is to destroy the carcinoma or reduce the amount of active thyroid tissue in a hyperactive thyroid.  This treatment leads to an interesting story.  

In the 1970s the Susquehanna Nuclear Power Plant in Berwick, Pennsylvania, was under construction.  As was prudent, the area around the budding power plant was tested for the presence of radionuclides before any radioactive material came on-site.  This included water from the adjacent Susquehanna River, water which was to be used in the power plant cooling towers.  Lo and behold, detectable amounts of 131I were found to be in what was to become the intake water for the cooling towers.  It was very important to establish where this 131I was coming from, otherwise, someone might conclude that it was the result of improper operation of the power plant.  

The plant owner and operator, Pennsylvania Power & Light (PP&L), persuaded two Wilkes College professors (I, the author of this article, was one of them) to undertake a study to track down the origin of the 131I in the river.  The amount of 131I in the water was very low but river diatoms facilitated the 131I  measurement by concentrating the iodine.  We set out diatom traps upriver and sampled them for 131I.  

Some thirty miles upriver of what was to be the power plant water intake, was the discharge of the treated effluent from a major sewage treatment plant.  Upriver of the discharge, no 131I.  We then tested the water in the sewage treatment system, from raw intake, through primary and secondary treatment.  We noticed pulses of 131I throughout the sewage treatment all the way to the effluent discharge; the sewage treatment did not remove iodine from the treated sewage which was then discharged into the river.  

We then asked all of the regional hospitals serviced by this sewage treatment plant to notify us when they had outpatient treatments with 131I for carcinoma of the thyroid and hyperthyroidism.  Within a day of such treatments, we were seeing 131I at the sewage treatment plant.  Incidentally, it is important to note that the amounts of 131I we saw, although detectable (a few µg/L), were way below any health limits nor did the excretion of 131I by the treated patients violate any State laws.  Mystery solved.  

Iodine is slightly soluble in water at about 300 mg/L.  Most people can detect an iodine taste in water at a concentration of 150 to 200 mg/L.  The World Health Organization recommends that the daily intake of iodine from all sources not be more than 1.1 mg/day.  There don’t appear to be official limits for iodine in drinking water probably because the level of iodine in drinking water is usually quite low and higher concentrations are unlikely to last.  Iodine can be removed from drinking water by activated carbon filtration.  

The EPA standard for 131I in drinking water is 3 pCi/L but it is based on a long-term average, thereby evening out any intermittent sources. Nuclear power plants keep supplies of nonradioactive potassium iodide to distribute to surrounding populations should there be a major accident which releases 131I into the environment. The idea is to load up everyone’s thyroids with non-radioactive iodine so that the thyroid will not take in 131I, allowing it to be quickly flushed from the body.


137Cs is yet another large radioactive ‘chunk’ produced by the fission of uranium or plutonium. Non-radioactive cesium, 100% Cs-133, is very rare and very unlikely to be present in significant amounts in water.137Cs is also very rare, completely manmade, but because of its radioactivity, can be significant in very small amounts. The EPA has an annual average drinking water limit of 200 pCi/L for 137Cs (it is a beta and gamma emitter) which corresponds to an exposure of no more than 4 mrem/yr. Once in the body, most of the 137Cs concentrates in the bone and muscles with a biologic half-life of about 70 days; it is removed by excretion. Note that the biologic half-life is considerably shorter than the radiologic half-life (~ 30 yrs). Many water treatment systems will remove cesium from water.


Argon-40 gas, which is not radioactive, makes up almost 1% of the Earth’s atmosphere, making it the third most abundant gas in the atmosphere; carbon dioxide is a distant fourth at ~ 0.042 % (420 ppm).  It is generated by the beta decay of an unstable isotope of potassium, K-40 (‘K’ comes from kalium, which means potassium in Latin).  K-40 has a half-life of 1.27 billion years and, along with thorium and the uranium isotopes, has been a major source of geothermal heat in the Earth, powering plate tectonics, volcanism, and earthquakes.  

K-40 decays in two distinct ways.  89% of the time it spits out a beta particle and becomes stable calcium-40.  11% of the time, it grabs one of its orbital electrons in a process called electron capture, pulls it into the nucleus, and combines it with a proton, emitting gamma radiation, and becoming Ar-40; this is where all that atmospheric argon comes from which means the Earth must have a lot of K-40.  Indeed, the most abundant radioisotope in the typical human body is K-40, closely followed by C-14.  

Potassium, the element, is quite common in the Earth’s crust, especially in the form of the mineral, orthoclase feldspar (KAlSiO4), very common in granite. It is the 8th most abundant element in the Earth and makes up 3.1% of the crust.  It is also, unsurprisingly, one of the more common cations (K+) found dissolved in water.  

Potassium consists of two stable isotopes: K-39 at 93.1% and K-41 at 6.9%.  Today, after several half-lives, K-40 makes up about 0.012% (120 ppm) of all potassium which might seem like a small amount but there is a lot of potassium out there and even a tiny percentage adds up.  Dissolved potassium, including a tiny proportion of K-40, is in drinking water but most of our potassium comes, not from water, but from food, especially potassium-rich food like bananas.  We need that potassium and cannot avoid the K-40 that comes with it.  

Since no human activity generates significant K-40 and the risks associated with exposure to it are very low compared to other risks in our lives and we can’t avoid it anyway, we live with it – keep the bananas.  It would assuredly not be a problem in drinking water and if it were, it would fall under the EPA’s maximum contaminant level standard of 4 millirems/year beta and gamma radiation for public drinking water.  This would correspond with the National Primary Interim Drinking Water Regulations (NIPDWR) gross beta limit of 4 pCi/L in water, not to be confused with the 4 pCi/L limit for radon (an alpha emitter) in air. 

Uranium and Thorium Daughters

Three unstable isotopes very important in generating geothermal energy within the Earth throughout its long history are 238U, 232Th, and 235U.  232Th has a much longer half-life than 238U and, although four times as abundant as 238U today, generates geothermal heat comparable to that of 238U.  235U is even less abundant, being only about 0.7% as abundant as 238U and, although it has the shortest half-life of the three isotopes, is the least important of the three in generating geothermal heat (radiation).  

There is no natural process on Earth that produces significant amounts of more of these isotopes; their abundance continues to decline since the Earth formed.  Based on their half-lives, about half of the 238U the Earth originally had has now decayed and disappeared.  The longer-lived 232Th has diminished by about 5%.  235U has decreased most, to about 1% of its original abundance.  This suggests that the decay of 238U has generated the most radiation (geothermal heat) throughout Earth’s history and continues to do so today.  

The uranium and thorium isotopes begin a long chain of decays, starting with an alpha decay.  The daughter isotopes produced are still not stable and so a chain of decays ensues, ultimately ending in different stable isotopes of lead.  Some of the unstable daughter isotopes are too neutron-rich and so those isotopes undergo beta decay.

Decay of 235U Decay of 238 U Decay of 232 Th
Isotope Half-life Isotope Half-life Isotope Half-life
92U235 713 million yrs 92U238 4.51 billion yrs 90Th232 14.1 billion yrs
90Th231 25.6 hrs 90Th234 24.1 days 88Ra228 6.7yrs
91Pa231 34,300 yrs 91Pa234 6.7 hrs 89Ac228 6.13 hrs
*89Ac227 21.2 yrs 92U234 248,000 yrs 90Th228 1.91 yrs
90Th227 18.2 days 90Th230 76,000 yrs 88Ra224 3.64 days
88Ra223 11.7 days 88Ra226 1620 yrs 86Rn220 56 sec
86Rn219 4.0 days 86Rn222 3.82 days 84Po216 0.15 sec
84Po215 0.0018 sec 84Po218 3.05 mths 82Pb212 10.6 hrs
82Pb211 36.1 mths 82Pb214 26.8 mths *83Bi212 60.5 min
**83Bi211 2.15 mths *83Bi214 19.7 mths 84Po212 0.0000003 sec
81Tl207 4.78 mths 84Po214 0.000164 sec 82Pb208 stable
82Pb207 stable 82Pb210 22yrs    
    83Bi210 5.0 ays *33.7% of 83Bi212 decays to:
*1.2% of 89Ac227 decays to: 84Po210 138 days 81Tl208 3.1 min
87Fr223 21 min 82Pb206 stable 82Pb208 stable
88Ra223 etc.        
**0.30% of 83Bi211decays to: *0.04% of 83Bi214 decays to: Present-day Ratios
84Po211 0.52 sec 81Tl210 1.3 min Th/U=3.9
82Pb207 stable 82Pb210 etc. 92U235/92U238=0.7257%

The type of radiation emitted by a particular decay can be determined by looking at the change in the atomic mass.  If the atomic mass drops four mass units and the elemental number (subscript on left) decreases by two, changing the element, it is an alpha decay and alpha radiation is emitted.  If the parent isotope and daughter isotope remain the same element but the atomic mass increases by one, it is a beta decay (β) and beta radiation is emitted.  Beta radiation is usually accompanied by gamma radiation of various unique energies.  

The daughter isotopes in the 232Th decay chain all have relatively short half-lives so that once 232Th begins the decay chain, it quickly (in a geologic sense) ends up as stable 208Pb.  This means that it is very unlikely that any of the radioactive thorium daughters would be present in water in any significant amounts.  

Most of the radioactive daughters in the 235U chain are also short-lived with one exception: 231Pa (Protactinium).  Since the abundance of protactinium in the Earth’s crust is on the order of a few parts per trillion, protactinium is unlikely to be a problem in water unless the water is influenced by a rich, natural uranium deposit.  

The 238U decay chain, however, has several radioactive daughters with modestly long half-lives beginning with 234U at 248,000 years.  Such a half-life allows the 234U to accumulate in significant amounts such that it makes up 0.0056% of uranium today.  230Th has a similar story, making up about 0.02% of all thorium today.  Note that 230Th is not derived from the much more abundant 232Th but from 238U.  Their characteristics are the same as discussed in the previous sections on uranium and thorium.  

That leaves Radium-226 at 1620 years and its immediate daughter, Radon-222 which, although it has a very short half-life, 3.82 days, is important because unlike any other element in the uranium and thorium decay chains, it is a gas and thereby considerably more mobile.  Radium and radon deserve their own sections but before that is a short discussion on secular equilibrium, especially as applied to 238U.  

Secular Equilibrium

If you were to start fresh with nothing but uranium, there would be no uranium daughters.  However, as time passed and some of the uranium entered the decay chain, uranium daughters would begin to accumulate.  Eventually, a given unstable daughter would reach an equilibrium in which, as fast as it decayed, it would be replaced with more of that isotope by the decay of its immediate parent.  Daughters with longer half-lives would accumulate to a greater extent than those with shorter half-lives.  Such a condition is secular equilibrium which assumes that there is no addition or loss of material to or from the outside; everything remains locked in place in the rock.    

This means that in the U-238 decay chain, the ratio of the abundance of, say, Radium-226 to U-234, would remain the same over long periods of time.  The amount of the original U-238 would slowly diminish as Pb-208, the end of the decay chain, slowly began to appear but the intermediate daughter ratios would remain the same.  Such a condition makes it possible to date uranium-containing rocks by determining the ratio of U-238 to Pb-208; the more lead to uranium, the older the rock.  

Most uranium-containing rocks and uranium ores have reached secular equilibrium and so you would expect to find daughter isotopes present.  However, mining can break that secular equilibrium as can some geologic processes which naturally separate the uranium from some of its daughters.  

If you want to use uranium for something like a nuclear reactor, you would want to concentrate and purify the uranium by separating it from everything else including it daughters.  Such uranium would no longer be in secular equilibrium which means that the uranium in the fuel rods used in nuclear power plants would not have uranium daughters such as radium or radon; starting with fresh uranium, there simply would not have been enough time for such daughters to form.  Used fuel rods may become loaded with fission products but not decay products like radium and radon.  

As an illustration of the significance of secular equilibrium, or its lack, consider two examples.  In the late 1980s when concerns about indoor radon were rising, I was contacted by a woman who had a basement full of mineral samples including several crates of uranium ore.  She was concerned that the uranium ore might be generating unacceptable amounts of radon in her house.  Testing with a very sensitive scintillator confirmed that the ore was generating significant gamma radiation and radon testing confirmed that she had elevated radon levels in her house.  The uranium ore was clearly in secular equilibrium.  I recommended that she move her uranium ore samples to a detached garage.  

Also in the late 1980s, I was contacted by a man who had an extensive collection of vaseline glass.  Vaseline glass (aka Depression glass) became popular in the 1930s.  It has a soft yellowish-green color similar to that of vaseline, hence, the name.  The color is generated by including a small amount of uranium in the glass.  The owner of the collection was concerned that the uranium in the glass might be producing radon gas.  The scintillator confirmed gamma radiation from the uranium in the glass but the radon test showed normal, acceptable levels.  

The reason that radon was not a problem in this case is that purified uranium was used to dope the glass.  There were no uranium daughters yet present in any significant amounts – the uranium was not in secular equilibrium.  

The relevance of all this to uranium daughters in water is what to expect of potential sources. As already mentioned, nuclear power plants would not be expected to be a source of either radium or radon, nor would facilities which store things like atomic bombs, whether uranium or plutonium. Natural uranium deposits/minerals and uranium mines, however, and their tailings (uranium mine waste) would be expected to have associated radium and radon, some of which could enter surface and groundwater. Such sources are in secular equilibrium.


It can happen that radium is naturally extracted from a uranium-containing rock and enters the groundwater.  An example of such a case is Saratoga Springs, New York.  There is a State park there whose mineral spring water contains significant amounts of radium, up to ~ 800 pCi/L.  To put this into perspective, the EPA limit for radium in drinking water is 5 pCi/L or a total of 15 pCi/L for all alpha emitters (radium is an alpha emitter).  The radium is probably leaching from nearby alkaline volcanic rock which also contains some uranium but much less uranium than radium is leaching into the water.  Radium is more soluble in water at higher pHs; the spring water has a higher pH.  

During WWII, the U.S. Radium Corporation had a plant in Bloomsburg, Pennsylvania (not far from the nuclear power plant in Berwick mentioned in the discussion on 131I) that manufactured radium paint.  Such paint would, powered by the energy from the radium, glow in the dark which was important for the dials in night-fighter airplanes.  Women employees would lick the ends of their fine paint brushes to keep them sharp as they painted the fine figures on the dials.  Many of them would later develop radium-related cancers.  Today, the former plant is an archived (doesn’t require further treatment) superfund site.  Presumably, none of the radium made it into the local groundwater.  This demonstrates that there are both natural and manmade sources of radium which can impact the environment.  Fortunately, such sources are not common.  

Ingested radium tends to end up in the bones and teeth and is especially linked with bone cancer; it has a biologic half-life of somewhere between 40 to 50 years.  Small amounts of radium can be found in some food such as Brazil nuts but if water is a suspected source, it should be tested.  Should a water test exceed the gross alpha limit of 15 pCi/L, a more specific test for radium can determine whether radium is the problem.  Many water treatments can remove radium from water but they would all concentrate radium in the waste which could be a disposal problem.


The uranium and thorium decay chains produce three different radon isotopes: 219Rn from 235U, 222Rn from 238U, and 224Rn from 232Th.  The radon from thorium, also known as thoron (isotopes can have their own names), has such a short half-life (< 1 minute) that it can be ignored.  The other two radon isotopes have half-lives ~ four days which seems quite short.  

You might think that you could ignore them too as they don’t last very long but unlike all of the other elements in the decay chains, radon is a gas.  It is mobile enough that some of it can escape from the rock in which it is generated and get into buildings in significant amounts before it decays.  

Because 235U is so much less abundant than 238U, the radon from the 235U (aka actinon) is much less abundant than the radon from 238U.  This means that almost all of the radon that might be in a house or in drinking water is 222Rn; unless otherwise indicated, ‘radon’ refers to 222Rn.  

Being an alpha emitter, in order to trigger a cancerous change, radon has to get into the body where its alpha particles can hit living tissue.  Its most important route into the body is in the air where it can be inhaled to irradiate the lungs which is why radon exposure is associated with lung cancer.  Statistically, radon exposure is linked to 15% of lung cancer in the U.S.  The other 85% is linked to cigarette smoking.  The EPA limit for radon in indoor air is 4 pCi/L.  Outdoor air typically has radon levels of about 0.2 pCi/L.  

Radon can enter the body through drinking water but has not yet been definitively linked to any particular cancer through such exposure.  Being a nonreactive noble gas, radon probably remains in the water in the body.  Water has a biologic half-life of from one to two weeks by which time most of the radon will have decayed.  Maybe the water itself blocks some of the alpha radiation.  

The EPA recommends a limit of 4000 pCi/L radon in water.  That there is a limit at all is because the radon can come out of the water and into the air where it can be inhaled.  If you think your water has too much radon in it, it can be removed by simple aeration, preferably while still outside the house.  It can be unintentionally removed from the water in the house and put into the indoor air when the water is used in washing machines and showers; what better way to aerate the water indoors?

Radon Daughters

Radon is not the end of the decay chain.  Although all of the radon daughters are solids, charged particles, not a gas like radon, they are created in the air, in water, or in the body, wherever the radon happens to be when it decays.  If in the air, the charged particles will float for a while until they encounter a surface to which they will cling.  Smoking puts particulates into the air, particulates to which the radon daughters can cling, keeping them in the air longer where they can be inhaled.  This is why there is a synergistic effect between radon and cigarette smoking.  The risk of developing lung cancer is greater than the sum of the separate risks from radon and radon daughter exposure and cigarette smoking.  

Should the radon decay in water, the daughters will remain in the water until they encounter a surface to which, as charged particles, they will cling until they reach the end of the decay chain.  While simple aeration removes the radon from water, activated charcoal will take out the radon daughters.  

If you happen to have a public water supply, radon and radon daughters in water are likely to be much less of a problem for a variety of reasons.  If the water supply is from a surface reservoir, the water is probably already pretty well-aerated which would remove the radon.  

If from a groundwater source, there is still the transmission time to consider.  How long does it take the water to travel from the public water source to the consumer?  The longer it takes, the more radon decays.  As for the daughters, the longer the distribution system, pipes, etc., the more chances the daughters have to encounter a surface to which they can cling.  

Finally, public water sources in the U.S. are required to periodically test for alpha and beta emitters in water and if those levels are too high, to do something to reduce them.  Private well owners don’t have those advantages and should have at least one gross alpha and gross beta test.  

The Decay of Uranium Diagram

To ensure safe drinking water (and environmental water in general), you must be aware of potential threats of all types, including radionuclides, to the continued purity of the water but do recognize that there is no such thing as zero risk, that it is not feasible to completely remove all traces of every radionuclide from water.  Agencies like the EPA set exposure limits based on what is prudent and reasonably attainable for drinking water while reducing health risks to an acceptable minimum.  

Keep it all in perspective 

I once spent over four hours talking with a man who was quite upset over his indoor air radon level of 4.2 pCi/L which is 0.2 pCi/L over the recommended limit.  The entire four hours he was chain-smoking.  

Cartoon of smoker worried about radon

Radionuclides is a scary word, especially when linked with cancer, another scary word.  Radon is worse yet: it is an invisible radioactive gas that can cause cancer.  Cars should be even scarier as far more people die from car accidents than from exposure to radionuclides.  

Nevertheless, being aware means testing your drinking water for things like radionuclides.  If you don’t test, you won’t know there is a problem.  If there is a problem, it can probably be treated.  If you don’t know how, find an expert who does know how.  Act but don’t over-react. 

Step 1: Get Tested for Radon in Air and Radionuclides in Drinking Water

Radon in Air Test

Water Testing from KnowYourH2O Partners:

Radionuclides Drinking Water -  National Testing Laboratories

Advanced Radiological Water Test - Tap Score