Internal Report 643
The current trigger value for the above background water radium-226 (226Ra) activity concentration at the Magela Creek compliance site is 10 mBq·L-1 (in the total fraction). This has been derived from human radiation protection considerations, based on the potential for bioaccumulation of 226Ra in mussels downstream of the Ranger mine and subsequent ingestion by humans. The potential for radiological impact on the environment has not been considered in previous derivations of trigger values for 226Ra. The aim of this report is to provide a review of the trigger value for 226Ra, including assessment of the potential impacts on human health and also taking into consideration the potential radiological impacts on the environment.
The current 226Ra trigger value for Magela Creek is based on data collected and assessed in the 1980s and, as the major source of mine related doses to people from the ingestion of traditional food items, with new data available it was considered timely to conduct this review into the appropriateness of the current value. Data from more than ten years (2001-2013) of bioaccumulation monitoring in Mudginberri Billabong mussels are statistically evaluated and concentration ratios for Mudginberri Billabong mussels are calculated and used in subsequent assessments of radiation doses to human and the environment.
Our assessment is based on the above background total water 226Ra activity concentration in Magela Creek. It assumes that a 10 year old child consumes 2 kg (wet weight) of large mussels per year. The resulting ingestion dose to humans, as well as internal and external doses to mussels from 226Ra are assessed. It is shown that human radiological protection is more limiting on total water 226Ra activity concentration in Magela Creek than environmental radiation protection considerations.
A mine origin increase in total water 226Ra activity concentration of less than 3 mBq·L-1 will lead to a mine origin ingestion dose to humans of less than 0.2 mSv per year. The same increase in total water 226Ra activity concentration of 3 mBq·L-1, will lead to a maximum additional dose rate to a small number of mussels (1%) of 50 µGy·h-1. This is much lower than the 400 µGy·h-1 benchmark dose rate, which, according to the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), would not have any detrimental effect at the population level in an aquatic environment.
Thus, the operational limit for the above background water total 226Ra activity concentration downstream of Ranger mine should be revised and set at 3 mBq·L-1.