Mudginberri Billabong is the first major permanent waterbody downstream (12 km) of the Ranger mine (see map). Local Aboriginal people harvest aquatic food items, in particular fish and mussels, from the billabong and hence it is essential that these food sources are fit for human consumption. Consequently, concentrations of metals and/or radionuclides in the tissues and organs of aquatic biota attributable to inputs to Magela Creek from Ranger should remain within acceptable levels. Enhanced bioavailability of mine-derived solutes to biota could also lead to body burdens that may harm the organisms themselves. Hence the bioaccumulation monitoring program serves an ecosystem protection role in addition to the human health aspect.
Mussel bioaccumulation data were obtained intermittently from Mudginberri Billabong from 1980 to 2001. From 2002, regular (annual) sampling from Mudginberri and Sandy Billabong, a control site in the nearby Nourlangie catchment, was initiated. Only data from 2000 onwards where methods were standardised and control sites included are presented and discussed on this website. Data obtained prior to 2000 are available in earlier Supervising Scientist annual reports.
Very detailed assessment of fish tissue, organ and bone data was conducted over a period of several years to both establish a baseline for metals and radionuclides in fish as well as to determine if the levels were likely to be significant from organism or human health perspectives. Forktail catfish were identified as the most effective fish species to monitor uranium and radionuclide because they are sufficiently abundant in numbers in both billabongs to support an ongoing sampling program, they are a popular food for the local Aboriginal people, and there is a reasonable historical dataset. The concentrations of metals (including uranium) and radionuclide activities in all species of fish sampled were very low and in many cases approached detection limits. Hence fish are not considered to be at risk from either bioaccumulation (organism health) or pose a human health risk via the ingestion route, from the very low levels of metals and radionuclides currently being exported from Ranger.
Accordingly the annual sampling of fish was removed from the routine bioaccumulation monitoring program after the 2007 collection. In the possible event that local Aboriginal people become concerned about consuming fish from Mudginberri Billabong, or a in the event that a significant increase in minesite influence is detected from the water quality monitoring program, a specific sampling program will be undertaken, the scope of which will depend on the nature of the situation.
Uranium concentrations in freshwater mussels, water and sediment samples collected concurrently from Mudginberri and Sandy Billabongs are shown in Figure 1. There is some evidence of a natural increase in uranium concentration with age in mussels from both Mudginberri and Sandy Billabongs. However, the concentrations in mussels from both sites are very similar from 2000 onwards, with no evidence of an increasing trend in concentration over time in Mudginberri Billabong (Analysis of Covariance testing, p <0.05). Notwithstanding some bioaccumulation with age, uranium in mussels is reported to have a short biological half-life, a conclusion that is supported by the data in Figure 1, with the uranium concentrations in mussel flesh being low.
Figure 1 Mean concentrations of uranium measured in the dried flesh of mussels, sediment and water samples collected from Mudginberri Billabong and control billabongs since 2000.
The lack of any increase in concentration of U in tissues of mussels from Mudginberri Billabong through time, with essentially constant levels observed between 1989 and 1995 (Supervising Scientist Annual Report 2006-2007, pp 25-28, section 2.2.3), and consistently low levels from 2000 to the last sample taken in October 2008, indicates absence of any mining influence.
Concentrations of Ra in mussels increase with age (Figure 2) and also appear to be related to growth rates, seasonal soft body weights, water chemistry and location (and associated sediment characteristics) within a billabong. When comparing data between years and billabongs (Figure 2), concentrations of Ra in mussels from Mudginberri Billabong are 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 calcium (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).
Figure 2 226Ra activity concentrations in the dried flesh of freshwater mussels collected from Mudginberri Billabong 2000–2008 and Sandy Billabong 2002–2008. Mussels were not collected from Sandy Billabong in 2007. The error bars are ± 1 standard deviation and are the result of instrument counting error.
The average annual committed effective dose 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 2008 is approximately 0.2 mSv. (The committed effective dose indicates the radiation dose received when taking into account the long-term nature of the dose (i.e. the person is "committed" to receiving the dose over several years) as well as the type of radiation involved (alpha, beta, gamma) and the type of tissues irradiated.) The average for Sandy Billabong mussels collected between 2002 and 2008 is approximately 0.1 mSv. A longitudinal study of the Magela Creek catchment conducted in 2007 measuring uptake of radium and uranium in mussels showed that the source of the radium was largely a natural feature of the catchment, rather than being mining-related (Supervising Scientist Annual Report 2007-2008, pp 64-71, section 3.3). However, even if it was assumed that the difference in doses between the two billabongs was exclusively mine-related, the mine contribution would still amount to only 10% of the public dose guideline limit (ICRP 1996).
The generally consistent relationship between age and Ra concentration observed for mussels amongst years and for each billabong (Figure 2) currently provides a robust baseline against which any future mine-related change in Ra concentrations can be detected.
In 2008, a longitudinal study of radium and uranium uptake in mussels from within Mudginberri Billabong was undertaken. Results for this project are currently being analysed and are reported in the Supervising Scientist Annual Report 2008-09.
As discussed above bioaccumulation of metals and radionuclides in fish was discontinued after the 2007 dry season collection, and as such, no more recent results are reported here.
Time-series concentrations of uranium in the flesh of forktail catfish collected from Mudginberri and Sandy Billabongs are summarised, together with U concentrations measured in water and sediment, for 2000–2007 collections in figure 3. The concentrations of U in the flesh of forktail catfish are low (<0.02 mg/kg) with no significant variation over time. This is consistent with the low concentrations of uranium in sediments (<1.4 mg kg-1) and water (<0.05 μg L-1) of Mudginberri Billabong for the corresponding period.
Figure 3 Mean concentrations of uranium measured in the flesh of forktail catfish, sediment and water samples collected from Mudginberri and Sandy Billabongs was collected from 2000 to 2007.
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.
ICRP 1991. 1990 Recommendations of the International Commission on Radiological Protection. ICRP Publication 60,Vienna.
ICRP 1996. Age-dependent doses to members of the public from the intake of radionuclides: part 5. Compilation of Ingestion and Inhalation dose coefficients. International Commission on Radiation Protection Publication 72, Pergamon Press, Oxford.
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.