Ecological risk assessment of the east branch, Finniss River

Quantitative ecological risk assessment (ERA) is a means whereby the risk posed by a toxicant in any system can be evaluated by comparing the distribution of its measured or modelled concentrations (water quality data (WQD)) with available information on the range of concentrations that are known to adversely affect biota within that, or similar, habitats (dose-response data (DRD)). Initially, the WQD are compared with regulatory criteria (e.g. ANZECC and ARMCANZ, 2000). If they fail this test, then, on the assumption that both data sets comprise subsets of the entire range of concentrations, probability density functions are derived assuming a standard distribution form a typically log-normal. In this paper, AQUARISK has been used to estimate the risk posed by copper in effluent from the Rum Jungle mine site, pre- and post-remediation, and the proportion of taxa likely to be affected in the East Branch (EB) of the Finniss River downstream of the mine. In addition, the average concentrations required to achieve no more that 5% exceedence of the various criteria, or 67% tolerance by affected taxa, have also been evaluated. The data comprised total copper concentrations and, usually, pH and sulfate concentrations. Filtered copper concentrations were only reported after remediation. In addition, some information was given on the major cations, calcium and magnesium, and to a lesser extent sodium. If these parameters were highly correlated (P < 0.001) with sulfate within the period of assessed risk, then missing values were estimated from the sulfate concentrations, by linear regression, for the purposes of estimating bioavailable copper concentrations. As these parameters represent only a proportion of those that are likely to reduce bioavailability of copper, the final estimates are higher than in reality, and hence, allow for a reasonable, conservative risk assessment. The pre- and postremediation average bioavailable copper concentrations are 22,900 and 230 μg L-1 respectively. Hence, to achieve compliance, a further reduction in post-remedial bioavailable copper concentrations by a factor of 660 is required. To achieve the more reasonable target of recovery to 67% species diversity at this heavily impacted site, the reduction is more modest, ranging between factors of 3.2 to 6.8