Stable lead isotope ratios and metals in freshwater mussels from a uranium mining environment in Australia’s wet-dry tropics

Concentrations of Fe, Mn, Cu, Zn, U and Pb, and stable Pb isotopes ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb were measured via inductively coupled plasma mass spectrometry in sediments, water and freshwater mussels (Velesunio angasi) from two catchments in the Alligator Rivers Region, Australia. Sediment U and Pb concentrations were higher in Magela Creek downstream than upstream of the Ranger U mine due to the mineralised nature of the catchment and potential local input of sediment from the mine site. Water metal concentrations were highest in Georgetown Creek, which is a tributary of Magela Creek and part drains the Ranger mine site, but there was little difference in concentrations between the Magela Creek upstream and downstream sites. Metal concentrations in mussels collected immediately upstream and downstream of the mine site also showed little difference, whereas Pb isotope ratios displayed a very distinct pattern. The ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁷Pb isotope ratios were more uranogenic downstream than upstream of the site and also more uranogenic than ratios measured in Sandy Billabong, a reference billabong in a catchment not influenced by U mineralisation. Isotope ratios were also more uranogenic in younger mussels, potentially due to the increasing footprint of the mine site over the past decade. The most uranogenic ratios were found in mussels from Georgetown Creek and at a site approximately 2 km downstream. At Mudginberri Billabong, approximately 12 km downstream of the Ranger mine, the relative contribution of uranogenic Pb to the total Pb concentration in mussels was small and overwhelmed by the input of industrial Pb with a Broken Hill type Pb signature. Whereas metal uptake by and thus concentrations in mussel flesh are influenced by water chemistry, mussel condition and metabolic rates, Pb isotope ratios are independent of these factors and provide a powerful means of source apportionment of contaminants in mussels and waterways, in particular in an U mining environment.