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

The Finniss River in the wet-dry tropics of northern Australia has received acid rock drainage (ARD) contaminants from the Rum Jungle uranium/copper m ine site over more than four decades. Annual-cycle loads of Cu, Zn, Mn, and sulfate, calculated from daily water and flow measurements, have been dete rmined both prior to and following mine-site remediation, that began in the early 1980s. The effects of varying contaminant loads on the relative abun dances of seven fish species, sampled by enmeshing nets during dry seasons, were determined by nonmetric multidimensional scaling(nMDS), in combinatio n with cluster-analysis and other nonparametric statistical techniques. The se analyses showed that (i) prior to remediation, the impacted region of th e Finniss River in 1974 had significantly dissimilar (P < 0.001) and more h eterogeneous fish communities, generally characterized by reduced diversity and abundance, compared to sites unexposed to elevated contaminant water c oncentrations and (ii) postremediation, recovery in fish communities from t he impacted region was indicated because they were not significantly dissim ilar from those sampled at contemporary(P = 0.16) unimpacted sites, that we re also similar to preremedial unimpacted sites. Even though considerable c ontaminant loads are still being delivered to the impacted region of the Fi nniss River over the annual cycle, the recovery in fish diversity and abund ances is consistent with (a) reductions of in situ contaminant water concen trations at the time of fish sampling, (b) reductions in annual-cycle conta minant loads of sulfate, Cu, Zn, and Mn by factors of 3-7, (c) greatly redu ced frequencies of occurrence and magnitude of elevated contaminant water c oncentrations over the annual cycle, that was most pronounced for Cu, and ( d) the absence of extensive fish-kills during the first-flushes of contamin ants into the Finniss river proper at the beginning of the wet season, that were observed prior to remediation. As such, the results indicate that the re has been ecological benefit to the Finniss River attributable to remedia l works undertaken at the Rum Jungle mine site. Recovery in abundances of t hese fishes may also be due to their time-dependent evolution of tolerance to mine-waste contaminants over their long period of exposure.