Drinking water supplies in regional Queensland are often sourced wholly or partially from groundwater. Groundwaters have greater potential for increased concentrations of naturally occurring radioactive isotopes owing to the relatively long contact time with sub‑surface minerals. The use of groundwater in Australia has increased over time, particularly where availability of surface water sources is reduced because of climate change and prolonged droughts.
The Australian Drinking Water Guidelines (ADWG) provide guidance on assessing the radiological quality of drinking water. The assessment process suggested by the ADWG, begins with a preliminary screening test for determination of gross alpha and beta radioactivity. The published guideline value of 0.5 Bq L-1 for either gross alpha or beta activity is based on the activity concentration of assumed representative radionuclides, Ra-226 (alpha) and Ra-228 (beta), that correlate to reference radiological doses. Where gross alpha or beta activity exceeds the 0.5 Bq L-1 guideline value, further testing is required to identify the nature of the radioactivity. The activity concentrations for Ra-226 and Ra-228 (guideline values of 0.5 Bq L-1 also apply to each) should be determined as the next step, as they are the most significant naturally occurring radionuclides in Australian water supplies. The ADWG notes that other radionuclides should be identified and quantified if necessary, to ensure all gross alpha and beta activity is accounted for, but in practice, this can be challenging for water suppliers to understand when this is necessary.
There is little published information available for activity concentrations of other radionuclides in Australian drinking waters and consequently their potential contribution to doses from consumption. An earlier study performed by Queensland Health1 showed that the radiological guideline values in ADWG are appropriate for assessing drinking water in regional Central and Northern Queensland, and supported the assumptions that Ra-226 and Ra-228 are generally the highest contributors to radiological dose is supported.
This study was expanded to include drinking water, sourced or supplemented with groundwater, for regional towns across Southern Queensland and includes samples from two bores at a commercial mining site in Northwestern Queensland that were being considered as a drinking water source. Results from this study include activity concentrations for radionuclides Pb-210, Po-210, Ra-226, Ra‑228, Rn-222, U-234 and U-238. These results were used for a comprehensive dose assessment to further test the validity of assumptions underpinning radiological water quality assessment recommendations in the ADWG.
Our dose assessment shows that groundwater sources from major regional areas in Southern Queensland, in general, support the original findings from the earlier study. For the two samples from Northwestern Queensland, both samples exceeded the ADWG radiological reference levels. The first sample with majority of estimated radiological dose from consumption attributed to Ra-226 and Ra‑228, typical of Australian drinking water. However, the second sample was atypical of Australian drinking waters, with the majority of the estimated radiological dose from consumption attributed to Po-210. This finding supports the need to identify and quantify radionuclides other than Ra‑226 and Ra-228, to ensure all contributors to radiological dose is accounted for.