]]
Supervising Scientist Environmental Monitoring Program
Mudginberri Billabong is the first major, permanent waterbody downstream (12 km) of the Ranger mine. Local Aboriginal people harvest aquatic food items, in particular fish and mussels, from the billabong and hence it is essential that they are fit for human consumption. Thus, concentrations of metals and radionuclides in the tissues and organs of aquatic biota specifically attributable to inputs from Ranger into Magela Creek must remain within acceptable levels. Enhanced body burdens and bioavailability of mine-derived solutes in biota may provide early warning of mine-related changes to aquatic ecosystems while reaching limits which, in principle, might harm the organisms themselves; hence the bioaccumulation monitoring programme serves an ecosystem protection role in addition to the human health aspect.
Mussel bioaccumulation data were obtained from Mudginberri Billabong intermittently from 1980 to 1999. Since 2000 there has been regular (annual) sampling from Mudginberri and from 2002 a control site (Sandy Billabong, Nourlangie Creek catchment) (Ryan et al 2005).
Forktail catfish have been identified as the most reliable species to monitor for uranium uptake, primarily because there is a reasonable historical dataset, they are sufficiently abundant in numbers in both billabongs and they are a popular food for the local Aboriginal people. Collection of forktail catfish, sediment and water occurs every two years at both Mudginberri and Sandy Billabongs.
Uranium (U) concentrations in freshwater mussels from Mudginberri and Sandy (post 2000) Billabongs are plotted on the charts together with U concentrations in water and sediment samples collected concurrently. Data for mussels from the early to mid 1980s may suffer from QA/QC problems (specifically relating to adventitious contamination of samples by metals) arising from the outsourcing of chemical analyses or from within the laboratories of ERA that processed the samples at this time (Jones 2005). The concentrations of uranium in mussels from both Mudginberri and Sandy Billabongs are very similar from 2000 onwards, with no evidence of an increasing trend in concentration over time or with mussel age (the latter a feature of radium concentrations in mussel soft tissues, see discussion below). Uranium in mussels has been reported to have a short biological half-life (Allison & Simpson 1989), a conclusion that is supported by the data, with the uranium concentrations in mussel flesh being low.
The lack of any increase in U in mussel tissues through time, with essentially constant levels observed between 1989 and 1995, and consistently low levels from 2000 to the last sample taken in May 2007, indicates absence of any mining influence. The decrease in U concentrations in water, sediment and mussels after 2000 in Mudginberri Billabong may be an artefact of changes in the sampling regime or analysis methods.
Early research has shown that 226Ra activity concentrations in mussel flesh are very high (Davy & Conway 1974). A concentration factor for radium in mussels of 19 000 has been determined (Johnston 1987) and, consequently, the major contributor to radiological dose from the ingestion of traditional foods is 226Ra in mussels (Martin et al 1998).
Due to the long biological half-life for radium in mussels, concentrations increase with mussel age (Johnston et al 1984). Concentrations also appear to be related to growth rates, water chemistry and location (and associated sediment characteristics) within a billabong. When comparing data from amongst years and billabongs, concentrations of Ra in mussels from Mudginberri Billabong are seen to be higher, age-for-age, than in mussels from Sandy Billabong. This may be attributable to three factors: (i) naturally higher catchment concentrations of Ra in Magela Creek compared with Nourlangie Creek catchment, (ii) lower concentrations of Ca in Mudginberri Billabong waters compared with Sandy (Ca can act as an antagonist to the uptake of Ra by aquatic organisms); and (iii) finer sediment particle sizes in Mudginberri compared with Sandy (finer sediments tend to contain higher Ra concentrations) (Ryan et al 2005). The need to better characterise sediment is now recognised and a more extensive and refined sediment sampling and size fractionation protocol was used for sampling following the 2006–07 wet season (results currently being analysed).
The average annual committed effective doses calculated for a 10-year old child who eats 2 kg of mussel flesh, based upon average concentrations of 226Ra and 210Pb from Mudginberri Billabong mussels collected between 2000 and 2005 amounts to ~ 0.2 mSv and is about two times higher than the average for Sandy Billabong for the same time period (Supervising Scientist 2007). Even in the unlikely case that the difference in doses between the two billabongs was exclusively mine-related, the mine contribution would still amount to only 10% of the recommended public dose limit (ICRP 1991).
The generally consistent relationship between age and Ra concentration observed for mussels amongst years and for each billabong currently provides a robust baseline against which any future mine-related change in Ra concentrations can be detected. Statistical tests using ANCOVA show no significant difference in the Ra-Age relationship amongst collection years, validating the use of mean, pooled data against which data from the current year may be compared.
Mussel collection for 2006 and 2007 has occurred and samples are currently being analysed and interpreted for 226Ra.
Time series concentrations of uranium (U) in the flesh of forktail catfish collected from Mudginberri and Sandy Billabongs biennially are summarised, together with uranium concentrations measured in water and sediment.
The concentrations of uranium in the flesh of forktail catfish are low (<0.02 mg/kg) with no significant variation over time. The large range in the error bars indicates that sample contamination was a significant issue in 1999 and 2001. However, refinement of sample processing methods and analytical procedures reduced the influence of contamination in subsequent years.
Allison HE & Simpson RD 1989. Element concentrations in the freshwater mussel, Velesunio angasi, in the Alligator Rivers Region. Technical memorandum 25. Supervising Scientist for the Alligator Rivers Region, AGPS, Canberra.
Davy DR & Conway NF 1974. Environmental studies, Northern Territory Uranium Province 1971-73. In The Alligator Rivers Area Fact Finding Study. Four Reports. Australian Atomic Energy Commission Report, eds NF Conway, DR Davy, MS Giles, PJF Newton & DA Polard, AAEC/E305, vol III, Lucas Heights NSW.
ICRP 1991. 1990 Recommendations of the International Commission on Radiological Protection. ICRP Publication 60,Vienna.
Jones DR (ed) 2005. Summary of presentations and key issues raised at the Bioaccumulation and Bushtucker workshop, 14 October 2005. Internal Report 508, December, Supervising Scientist, Darwin. Unpublished paper.
Johnston A, Murray A, Allison H, Cusbert P & Martin P 1984. The use of the freshwater mussel as an environmental monitor for radium. In Alligator Rivers Region Research Institute Research Report for 1983–84. Alligator Rivers Region Research Institute. Supervising Scientist for the Alligator Rivers Region, Australian Government Publishing Service, Canberra, 33–37.
Johnston A 1987. Radiation exposure of members of the public resulting from operations of the Ranger Uranium Mine. Technical memorandum 20, Supervising Scientist for the Alligator Rivers Region, AGPS, Canberra.
Martin P, Hancock GJ, Johnston A & Murray AS 1998. Natural-series radionuclides in traditional north Australian Aboriginal foods. Journal of Environmental Radioactivity 40, 37–58.
Ryan B, Martin P, Humphrey C, Pidgeon R, Bollhöfer A, Fox T & Medley P 2005. Radionuclides and metals in fish and freshwater mussels from Mudginberri and Sandy Billabongs, Alligator Rivers Region, 2000–2003. Internal Report 498, November, Supervising Scientist, Darwin. Unpublished paper.
Supervising Scientist 2007. Annual report 2006–2007. Supervising Scientist, Darwin NT.