Guest Author: Dr. Brian Redmond, Licensed Professional Geologist, Professor

Is it possible for water to be Radioactive? The answer, intriguingly, is yes, but this occurrence is quite rare and typically not a concern for most people.

Water (H₂O) is made up of hydrogen and oxygen. There are different types of hydrogen atoms, known as isotopes (same number of protons, different numbers of neutrons). Hydrogen has three isotopes, two of which are stable (not radioactive) and one of which is not stable (radioactive). Let's break them down:

1 Protium (Hydrogen-1 or ¹H)

The most abundant isotope of hydrogen, by far, is Hydrogen-1 whose nucleus has one proton and no neutrons; it is very stable and not a source of radiation. Note that protium is the name of the hydrogen isotope, not the name of the element which is, of course, hydrogen. Unless otherwise specified, the chemical symbol for hydrogen, H, is protium.

Remember: Protium is stable and not a source of radiation.

2 Deuterium (Hydrogen-2 or ²H)

There is another, much less common, stable, nonradioactive isotope of hydrogen: Hydrogen-2 aka Deuterium (the name of the isotope; it is still the element hydrogen). This isotope has one neutron along with the one proton in its nucleus, ²H. In chemical notation it is sometimes designated as D for deuterium. Note that "D" stands for a hydrogen isotope, not an element; there is no element designated as a capital D. Should one of the hydrogens in a water molecule happen to be deuterium, its chemical formula could be written as DHO or ¹H²H¹⁶O instead of H₂O or ¹H¹H¹⁶O (unless otherwise noted, a simple H can be assumed to be ¹H). Even D₂O is possible. Biologists sometimes use deuterated (water molecules with deuterium) as tracers to follow water in biologic systems.

The molecular 'weight' of a compound is the sum of the masses of the protons and neutrons in the elements making up the compound. The molecular weight of H₂O is 18 amu (atomic mass units), one each from the hydrogen and sixteen from the oxygen. But if one or both of the hydrogens is actually deuterium which has an atomic mass of 2, then the molecular weight of that water molecule will be one or two amu heavier than normal. Such water is known as heavy water. The concentration of 'heavy' water is only about 150 ppm in fresh water and about 200 ppm in seawater. Heavy water has value as it is an important moderator (it slows down neutrons in fission reactors) but it is not cheap nor is it easy to separate it out from sea water at those low concentrations.

‍Remember: Heavy water is water where all the hydrogen isotopes are in the form of deuterium.

3 Tritium (Hydrogen-3 or ³H)

Tritium is radioactive. If water has tritium, it can be considered radioactive.

Tritium or Hydrogen-3 or ³H has one proton (the definition of the element, hydrogen) and two neutrons. Such a combination is not stable and will eventually fall apart, changing into something else and emitting radiation. The instability of radioactive isotopes can be described in terms of half-life, how long it would take for half of the radioactive isotope to fall apart and emit radiation. In the case of tritium, that half-life is 12.3 years.

Tritium decays by emitting a Beta Particle (beta radiation, ß–) and turning into Helium-4 or ⁴He.

³H ––> ⁴He + ß–.

Should one of the hydrogens in a water molecule be tritium, then yes, water itself can be radioactive. Since there is no element designated with just the letter T alone (Ti is titanium, Th is thorium, etc.), a lone uppercase T can be used in the chemical formula of a water molecule to indicate that one of its hydrogens is tritium: HTO.

On an Earth billions of years old, one might wonder how there could be any tritium around at all. The answer is that it is continuously generated in Earth's atmosphere because of cosmic radiation (in this case, neutrons moving an appreciable fraction of the speed of light) which maintains a concentration of only 7.3 kg total tritium in all of the Earth's atmosphere. ¹⁴N + n ––> ¹²C + ³H Most of that tritium ends up as part of a water molecule; the water is tritiated.

There are other sources of tritium in our modern world. Nuclear explosions will produce it as will nuclear power reactors. One current issue is the release of tritiated water into the ocean from the Fukushima Daiichi nuclear power plant in Japan, damaged by a recent (2011) earthquake and tsunami.

‍Tritium is essential for hydrogen bombs. Such bombs are a form of hydrogen fusion but not like what happens in the sun. Hydrogen fusion in the sun is a continuous, three-step process. With a high enough temperature, millions of degrees, protium is fused to produce deuterium, the deuterium is fused to produce Helium-3. and the Helium-3 is fused to produce Helium-4 + protium along with copious amounts of energy.

Humans do not have the technology to maintain the required temperatures of millions of degrees for more than a tiny fraction of a second and those temperatures are produced by the explosion of an atomic bomb, the fission of uranium or plutonium. So, any hydrogen fusion has to be done in one very quick step which involves the fusion of deuterium and tritium. D + T ––> ⁴He + n + lots of energy.

The deuterium can be expensively extracted from sea water but getting tritium is a problem. It has to be manufactured by bombarding lithium with neutrons: ⁶Li + n –> ³H + ⁴He. Worse yet, Lithium-6 makes up only about 1.9% of lithium; most lithium is 7Li which can’t be used to make tritium. This sounds like a similar problem with atomic bombs which require U-235 which makes up only 0.7% of uranium, most being non-fissionable U-328. And don’t forget that you need an atomic bomb to trigger a hydrogen bomb.

Separating two very similar isotopes is very difficult and expensive not to mention that lithium is rare on Earth and in great demand for other things like lithium batteries. Finally, tritium has a very short shelf life because of its short half-life. That means that hydrogen bombs have a short shelf life, maybe a good thing. It would probably not be a good thing if someone figured out how to economically get tritium out of water, extremely unlikely, so no drinking-water treatment for tritium.

Since tritium is radioactive but very, very uncommon, its concentration in water is usually measured in terms of its radioactivity; it would be extremely difficult to measure such tiny concentrations any other way. Surface water typically has tritium concentrations of 10 to 30 pCi/L (0.4-1.2 Bq/L).

A Curie (Ci) is a measure of the radiation released from a gram of Radium which, although a mere pinch of radium, would be extremely dangerous. A picoCurie (pCi) is one millionth of one millionth of a Curie but still a significant amount of radiation. The amount (activity) of radiation is usually given in terms of a volume, a liter (L) of air or water.

The EPA limit for tritium in drinking water is 20,000 pCi/L, much, much higher than the ~ 20 pCi/L normally found in drinking water from a surface source. To put that into perspective, the EPA limit for Radon in indoor air is 4 pCi/L and the EPA Health Advisory, i.e. a proposed standard for radon, ranges from 300 to 4000 pCi/L. The 10-4 Cancer Risk for radon is 150 pCi/L. The 10-4 Cancer Risk is the concentration of a chemical in drinking water corresponding to an “excess estimated lifetime cancer risk of 1 in 10,000” (EPA, “2018 Edition of the Drinking Water Standards and Health Advisories”). The difference in the exposure limits between tritium and radon is partly because they emit different kinds of radiation: tritium emits beta radiation, and radon emits Alpha radiation; the different radiation types have different penetrating powers and punches. The difference between limits in air versus water is partly because humans breathe in much more air than they drink water and different organs in the human body are exposed to water than to air.

‍Note: Tritium emits beta radiation and radon emits alpha radiation.

Note: “The EPA's dose-based drinking water standard of 4 mrem per year is based on a maximum contaminant level of 20,000 picocuries per liter for tritium. In 1991, EPA used improved calculations to conclude a tritium concentration of 60,900 pCi/L would yield a 4 mrem per year dose. However, EPA kept the 20,000 pCi/L value for tritium in its latest regulations. (Source)

The tritium in surface water is in equilibrium (income = outgo) with the atmosphere which is to say, the tritium lost to radioactive decay in the water is replaced by more tritiated water from the atmosphere so that there is a fairly constant amount of tritium in the surface water.

However, should the surface water become groundwater, tritium lost to radioactive decay is no longer replaced by atmospheric tritium. This is analogous to Carbon-14 dating where the radioactive ¹⁴C ratio to stable ¹²C in living things is in equilibrium with the ¹⁴C in the atmosphere but once dead, the decaying ¹⁴C is no longer replaced. The older the groundwater or dead, buried carbon, the less tritium or ¹⁴C there will be. The difference is that ¹⁴C has a much longer half-life (~5700 yrs) than tritium (12.3 yrs) and so can be used to date older ¹⁴C or ³H-containing material. A rough rule-of-thumb is that you can date back ~ ten times the half-life of the isotope. Much more than that and the remaining amount of ¹⁴C or ³H is so low that it gets lost in measurement error.

Such a convenient relationship can be used by hydrologists to roughly measure the flow rate of the groundwater. As the groundwater moves away from the recharge area (where surface water becomes groundwater), it loses tritium at a known rate. Sample groundwater some distance from the recharge area and you can figure out how long it took the water to get there. This also means that your well water very likely has less tritium in it than the surface water does.

In conclusion, can water itself be radioactive? Definitely yes because of tritium but unless you are near a malfunctioning nuclear power plant (this was a concern in the 1979 Three-Mile-Island nuclear power accident near Harrisburg, Pennsylvania and the 1986 Chernobyl disaster in the Ukraine), it is unlikely to be a significant issue.

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