7.6.2 Estimation of the dose from radionuclides in water

To estimate the dose to members of the public from the ingestion of radionuclides in drinking water, the parameters required are the concentration of the radionuclides in water (measured in Bq/L), the daily consumption rate of water (L/day) and the dose coefficient for the particular radionuclide.

The World Health Organization (WHO) has estimated that adults consume 2 L of water/day. This figure is believed to be an appropriate average figure for Australia (see Section 6.3.3), giving an annual consumption of 730 L for each adult Australian. Therefore, the amount of each radionuclide ingested/year from the water supply is the concentration of that radionuclide in the water (Bq/L) multiplied by 730.

The annual dose from an individual radionuclide consumed in water is calculated using the following equation:

Annual dose (mSv/year) = dose per unit intake (mSv/Bq) x annual water consumption (litre/year) x radionuclide concentration (Bq/L)

Usually, a water supply contains more than one radionuclide; therefore, the doses arising from each individual radionuclide are summed to give the total dose. For information on specific radionuclides, refer to the relevant fact sheets for Specific alpha and beta radionuclides, Radium, Radon-222, and Uranium.

Consideration of consumption rates and age specific dose coefficients may be required when calculating the annual dose for specific groups such as infants, children and individuals with increased water consumptions. Increased consumption may occur for specific groups such as pregnant or lactating women, outdoor workers and athletes.

Age specific dose coefficients for the ingestion of radionuclides are available from ICRP publication 119 (ICRP 2012). Where doses have been calculated for specific groups within a population (based on either adjusted consumption rates or age related information) and the values exceed the reference level of 1 mSv/year, the information should be brought to the attention of the relevant health authorities or drinking water regulators. A decision can then be made on the requirement for further assessment.

Where the consumption rate of water may be higher than the nominal rate of 2 L per person per day, an annual dose can be calculated for a specific situation, for example using local or regional drinking water consumption rates. Where national or regional consumption rates are known, the annual dose is calculated to take into account the known consumption rates (refer to the equation above to calculate and adjust the annual dose). This will influence the operational response (see Information Sheet 2.2).