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Radioactive isotopes in Danish drinking water

4 Radioactivity in Drinking Water

Radioactivity in drinking-water is principally derived from two sources:

• the leaching of radionuclides from rocks and soils
• the deposition of radionuclides from the atmosphere.

Naturally occurring radionuclides from both these sources account for almost the entire radioactivity present in Danish drinking water. Traces of man-made radioactive fallout from atmospheric nuclear weapons tests (conducted up to 1980) are detectable in the environment but their contribution to drinking water radioactivity is negligible.

The naturally occurring radionuclides originate in the Earth's crust where uranium, thorium and potassium are widely distributed and detectable in all soils and rocks.

Uranium and thorium are radioactive, and each decays through a series of radionuclides to stable isotopes of lead, as shown in the decay schemes below. Only a very small percentage (0.0118%) of all potassium is the radioactive isotope potassium-40. It is not considered to be of radiological significance because potassium is an essential metabolic element and its levels in the body are in a state of equilibrium, and therefore do not vary significantly with dietary potassium levels.

Uranium series:

²³⁸U → ²³⁴Th → ²³⁴Pa → ²³⁴U → ²³⁰Th → ²²⁶Ra → ²²²Rn → ²¹⁸Po → ²¹⁴Pb → ²¹⁴Bi → ²¹⁴Po → ²¹⁰Pb → ²¹⁰Bi → ²¹⁰Po → ²⁰⁶Pb

Thorium series:

²³²Th → ²²⁸Ra → ²²⁸Ac → ²²⁸Th → ²²⁴Ra → ²²⁰Rn → ²¹⁶Po → ²¹²Pb → ²¹²Bi → ²¹²Po or ²⁰⁸Tl → ²⁰⁸Pb

where the symbols represent elements as follows:

Ac, actinium; Bi, bismuth; Pa, protactinium; Pb, lead; Po, polonium; Ra, radium; Rn, radon; Th, thorium; Tl, thallium; U, uranium.

The radionuclides in these decay series display a great range of radioactive half-lives from approximately 1010 years for ²³²Th to 0.0001 seconds for ²¹⁴Po. Every radionuclide emits either alpha or beta radiation but their radiological significance varies. The solubility of thorium, for example, is so low, that it is only found in water as a component of suspended mineral particles. The natural radionuclides primarily regarded as being of radiological interest in drinking water appear in the following table.

* Radon decay products emit both alpha and beta radiation

Only water supplies from groundwater sources are likely to contain significant concentrations of these radionuclides, and the concentrations are as variable as the nature of the soils and rocks themselves.

While groundwater may contain natural uranium and thorium series radionuclides, surface water may contain radioactive material deposited from the atmosphere, including both natural radionuclides and materials from man-made sources such as nuclear weapons tests and satellite debris.

Because all radionuclides of interest emit alpha or beta radiation, their levels in drinking water may be assessed by measurement of the total alpha and beta activities.

The table below shows reference concentrations in drinking water of the most common natural and man-made radionuclides. The reference concentrations are based on the parametric value of 0.1 mSv per year for the Total Indicative Dose, dosimetric data for adults (EC, 1998) and assumed intake of 730 litres per year.

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Version 1.0 April 2006, © Danish Environmental Protection Agency