Supervising Scientist, Darwin, 2005
ISBN 0 642 24395 6
ISSN 0 158-4030
After seasonal road works in 1999 along the Gunlom Falls access road, an inspection of the site by an oss employee discovered that the road machinery had exposed tailings directly opposite the old South Alligator Mill tailings dam area. This finding initiated a gamma survey that was conducted in 1999 followed by an airborne gamma survey that was flown in 2000. Data were evaluated for the Rockhole tailings site and the remaining portion of the South Alligator Valley to quantify the magnitude and extent of residual radiological contamination in the South Alligator Valley from the mining activities in the 1950s and 1960s.
The old South Alligator Mill tailings area may be a source of radionuclides and heavy metals washing into the South Alligator River in the wet season, and Rockhole Mine Creek is known to be polluted by acid rock drainage emanating from an old mine adit associated with the abandoned workings of the Rockhole mine site.
As part of the radiological investigation and dose assessment studies, samples of freshwater mussels, Velesunio angasi, were collected from the South Alligator River, near and at the confluence of Rockhole Mine Creek, and adjacent to the exposed tailings. Collection sites for mussels, sediments and water were selected following consultations with Parks Australia North (PAN) staff and local Aboriginal people.
In total, 177 mussels, 4 water and 6 sediment samples were jointly collected by eriss, PAN and Aboriginal Traditional Owners from sites either potentially exposed (downstream) or unexposed (upstream) to mine waste contaminants. Radionuclide activity concentrations and activity ratios were determined by alpha and gamma spectrometry. Concentrations of a number of elements (including heavy metals) and stable lead isotopes were determined by ICP-MS.
The age of each mussel was determined by placing the shell over an incandescent light source and counting the (annual) dark bands (annuli) (Figure 3.7). The dried and ground flesh of the mussels were combined according to age class and site, and a composite sample of each age class was cast in epoxy resin for determination of radioisotopes of lead ( 210Pb), thorium ( 228Th) and radium ( 226Ra & 228Ra) by gamma spectrometry. Mussels ≤ 1 year of age, an age class with insufficient mass for analysis by gamma spectrometry, were analysed by alpha spectrometry.
Figure 3.7 A freshwater mussel, Velesunio angasi, from the South Alligator River. Counting of the distinctive dark bands (annuli) is used to determine the age of the mussels.
A sub-sample of dried and ground composite material of each age group from each site was analysed by ICP-MS for aluminium (Al), arsenic (As), barium (Ba), cadmium (Cd), calcium (Ca), cobalt (Co), copper (Cu), iron (Fe), lead (Pb), magnesium (Mg), manganese (Mn), nickel (Ni), potassium (K), rubidium (Rb), selenium (Se), sodium (Na), strontium (Sr), uranium (U) and zinc (Zn). Sub-samples of a range of age class composites from each site were analysed for stable lead isotopes with ICP-MS.
The radionuclide and metal concentration measurements indicate that some contamination, although relatively small, has occurred downstream of Rockhole Mine Creek and the tailings area. In particular the 226Ra/ 228Ra activity ratios in the mussel flesh indicated that mussels from the potentially exposed sites have accumulated relatively more 226Ra compared to mussels of the same ages from sites upstream of Rockhole Mine Creek and the tailings area.
Applying the mussel population statistics (age, weight and proportional age representation) from the collection program to a hypothetical collection of freshwater mussels harvested for consumption by local Aboriginal people the committed effective dose for a 10-year-old child from the ingestion of freshwater mussels was estimated. The dose of 0.08 mSv, assuming the child ate 2 kg of mussels in a year (wet weight), is based upon the average activity concentrations of 226Ra and 210Pb from all sites and age groups in the collected samples from the upper South Alligator River. It also includes the dose originating from natural 226Ra accumulated in the mussels.
The corresponding doses arising from 238U intake are extremely low. This is mainly because both the uranium concentration factor, and thus the uranium concentration in mussel flesh, and the dose conversion factor are much lower. For example, assuming a 10-year old child ate 2 kg (wet weight) of flesh from mussels from where the tailings enter the South Alligator River, the committed effective dose from the intake of uranium would only be 0.0001 mSv.
The combined dose from the 226Ra, 210Pb and 238U are very low compared with the incremental dose limit of 1 mSv per year to the natural background for a member of the public that applies to a practice such as an operating uranium mine.