Use of the freshwater mussel, Velesunio angasi, in the monitoring and assessment of mining impact in top and streams

A number of features of freshwater mussels make them well suited as indicators of past environments and present-day environmental health. Radium and thorium isotope loads, and activity ratios such as Ra-228/Ra-226 and Th-228/Ra-228, are strongly age-dependent (ARRRI, 1985, 1987a). Determination of this dependency for a site can give information on pollution events even several years later. For this type of investigation, radioisotope activity ratios are more sensitive and reliable indicators than concentrations or loads of individual radionuclides. An example of the archival potential of the Ra-228/Ra-226 ratio in mussels for demonstrating downstream effects in Magela Creek of hypothetical releases of process water from the Ranger uranium mine is shown. The Ra-228/Ra-226 activity ratio decreases with age due to the disproportionate decay of the two isotopes: 6 year half-life for Ra-228 vs 1600 year half-life for Ra- 226. Hence, the ratio is lower in older mussels as the Ra-228 activity incorporated early in life has partially decayed. If there had been an event several years ago, this would be observed in the age-dependence of the ratio. Uranium provides an unusual example of metal absorption and bioaccumulation. The overall evidence suggests that, although U has a short half-life in mussel protoplasm, the transport mechanism to extracellular granules is impeded in a way that does not occur with many other divalent ions. This may be because U22+ is mainly only nominally divalent at physiological and most naturally-occurring pH values. The transport of metals from an intracellular environment to an extracellular phosphate granule probably involves a decomplexation step. For most divalent metals, extracellular pH is unlikely to be sufficiently high to cause significant hydrolysis. For U, however, extensive hydrolysis occurs at pH > 4 (Moulin et al., 1995), lowering the effective charge on the U species, and restricting its incorporation (as an unhydrolysed UO22+ ion) into the phosphate crystal structure. At the physiological pH of mussel body fluid (approximative, 7.8; Brown et al., 1996), the fraction of U present as UO22+ is around 0.3% (calculated from Moulin et al., 1995). The low percentage of the metal readily fixed in the granules, coupled with (in most cases) a low ambient concentration of total soluble U, may provide a high kinetic barrier to retention in the granules