7.6.4 Reference levels and screening values for drinking water

For radiological characteristics, the most appropriate measure of water quality that could be regarded as a health-based reference level is the annual dose to a person due to ingestion of radionuclides in the drinking water. However, the dose is not a directly measurable quantity. Assessment of water quality is based on the measurement of radionuclide concentration (screening) followed, if necessary, by the calculation of dose and its comparison to a reference level.

Australian reference level for drinking water

For radiation protection purposes, the Australian national reference level for commodities including drinking water is 1 mSv/year (ARPANSA 2017). This reference level is in line with the IAEA General Safety Requirements Part 3 reference level for exposure due to radionuclides in commodities (IAEA 2014), co-sponsored by the WHO and the Food and Agriculture Organization (FAO) of the United Nations. The established Australian and international reference level for radionuclides has been adopted in these Guidelines.

The reference level does not represent a level at which protective measures must occur, but is the level at which measures to reduce exposure should be considered. When the existing or potential dose from the radionuclide content exceeds this reference level, a decision on the need for and the degree of protective measures (e.g. remedial actions and/or protective actions) should be based on advice from the relevant health authority or drinking water regulator. It should also include a cost–benefit analysis where the resulting public benefit should be balanced against the overall costs of achieving a reduction in exposure.

There may be some circumstances where there is no practical alternative but to accept a dose that exceeds the reference level of 1 mSv/year, together with a potential slight increase in the theoretical risk to health as a consequence. The decision will be dependent on the situation at the time, and consideration should take into account a balance of the overall risks, including the risk of not having a supply of drinking water (WHO 2018). However, if doses from the use of a particular water supply will exceed 10 mSv/year, immediate action should be taken to reduce the existing or potential exposures (ARPANSA 2017).

Australian screening values for drinking water

Screening values of 0.5 Bq/L are recommended for both gross alpha and gross beta activity. Potassium-40 is a natural beta emitter, which occurs in a fixed ratio to stable potassium (0.0276 Bq of potassium-40 per milligram of stable potassium). Metabolic regulation of potassium levels in the body means that potassium-40 does not accumulate in the body but is maintained at a constant level independent of intake (Palmer BF, 2015). Therefore, the contribution of potassium-40 in drinking water to total beta activity can be considered negligible. Potassium-40 is subtracted following a separate determination of total potassium. If the screening values are not exceeded, there is no need for further assessment. The screening values are set so that when converting the activity concentration to an annual dose the reference level of 1 mSv/year is not exceeded. If either or both screening values of 0.5 Bq/L for gross alpha and 0.5 Bq/L for gross beta analysis are exceeded, further investigation is necessary to identify the nature of the radioactivity.

Screening values are conservative and should not be interpreted as mandatory limits or as an indication that exceeding these levels means that drinking water is unsafe. For more information on the application of the screening values in practice refer to Information Sheet 2.2. Refer to Chapter 10, Table 10.7 for specific guidance on the radiological quality of drinking water.