Recovery of fish communities in the Finniss River, northern Australia, following remediation of the Rum Jungle uranium/copper mine site

Full text: The Finniss River in the wet-dry tropics of northern Australia has received acid rock drainage (ARD) contaminants from the Rum Jungle uranium/copper mine site over more than four decades. Annual-cycle loads of Cu, Zn, Mn and sulfate, calculated from daily water and flow measurements, have been determined both prior to and following mine-site remediation, that began in the early 1980's. The effects of varying contaminant loads on the relative abundances of seven fish species, sampled by enmeshing nets during dry seasons, were determined by non-metric multidimensional scaling (nMDS), in combination with cluster-analysis and other non-parametric statistical techniques. These analyses showed that: i) prior to remediation, the impacted region of the Finniss River in 1974 showed significantly dissimilar (P<0.001) and more heterogeneous fish communities, generally characterised by reduced diversity and abundance, compared to sites unexposed to elevated contaminant water concentrations and; ii) post-remediation, fish communities from the impacted region in both 1992 and 1995 were not dissimilar from those sampled at either contemporary (P=0.16) or pre-remedial unimpacted sites, indicating their recovery. Even though considerable contaminant loads are still being delivered to the impacted region of the Finniss River over the annual cycle, the recovery in fish abundances is consistent with (a) reductions of in situ contaminant water concentrations at the time of fish sampling, (b) reductions in annual-cycle contaminant loads of sulfate, Cu, Zn and Mn by factors of 3-7, c) greatly reduced frequencies of occurrence and magnitude of elevated contaminant water concentrations over the annual cycle, that was most pronounced for Cu, and (d) the absence of extensive fish-kills during the first-flushes of contaminants into the Finniss river proper at the beginning of the wet season, that were observed prior to remediation. As such, the results indicate that there has been ecological benefit to the Finniss River attributable to remedial works undertaken at the Rum Jungle mine site. Recovery in abundances of these fishes may also be due to their time-dependent evolution of tolerance to mine-waste contaminants over their long period of exposure