Metal concentrations were measured in sediments of the mine-affected Molonglo River to determine current metal concentrations and distribution along the river. Compared with an uncontaminated site at 6.5 km upstream of the Captains Flat mine, sediments collected from the river at ≤12.5 km distance below the mine had a significantly higher percentage of finely divided silt and clay with higher concentrations of cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn). The measured metal concentrations in the mine affected sites of the river were in the following order: Zn = 697-6818 > Pb = 23-1796 > Cu = 10-628 > Cd = 0.13-8.7 µg/g dry mass. The highest recorded metal concentrations were Cd at 48, Cu at 45, Pb at 240, and Zn at 81 times higher than the background concentrations of these metals in the river sediments. A clear sediment metal-contamination gradient from the mine site to 63 km downstream was established for Cd, Cu, Pb, and Zn in the river sediments. Compared with sediment metal concentrations before a major flood in 2010, only Zn concentrations increased. For all of the mine-affected sites studied, Cd and Zn concentrations exceeded the (ANZECC/ARMCANZ, Australian and New Zealand guidelines for fresh and marine water quality. Australian and New Zealand Environment and Conservation Council/Agriculture and Resource Management Council of Australia and New Zealand, 2000) interim sediment-quality guidelines low values for Cd (1.5 µg/g dry mass) and the high value for Zn (410 µg/g dry mass). Existing metal loads in the riverbed sediments may still be adversely affecting the river infauna.

The diffusive gradients in thin films (DGT) technique has shown to be a useful tool for predicting metal bioavailability and toxicity in sediments, however, links between DGT measurements and biological responses have often relied on laboratory-based exposures and further field evaluations are required. In this study, DGT probes were deployed in metal-contaminated (Cd, Pb, Zn) sediments to evaluate relationships between bioaccumulation by the freshwater bivalve Hyridella australis and DGT-metal fluxes under both laboratory and field conditions. The DGT-metal flux measured across the sediment/water interface (±1 cm) was useful for predicting significant cadmium and zinc bioaccumulation, irrespective of the type of sediment and exposure. A greater DGT-Zn flux measured in the field was consistent with significantly higher zinc bioaccumulation, highlighting the importance of performing metal bioavailability assessments in situ. In addition, DGT fluxes were useful for predicting the potential risk of sub-lethal toxicity (i.e., lipid peroxidation and lysosomal membrane damage). Due to its ability to account for multiple metal exposures, DGT better predicted bioaccumulation and toxicity than particulate metal concentrations in sediments. These results provide further evidence supporting the applicability of the DGT technique as a monitoring tool for sediment quality assessment.

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Metal uptake and induced toxic effects on Hyridella australis were investigated by establishing 28 day exposure-dose-response relationships (EDR) of transplanted H. australis at four sites along a sediment metal contamination gradient in the mine affected Molonglo River, NSW. Laboratory exposure of this organism to the same sediments, collected from in situ sites, was run concurrently. Metal concentrations in whole organisms, individual tissues and sub-cellular tissue fractions were measured as organism metal dose. Total antioxidant capacity (TAOC), lipid peroxidation (MDA) and lysosomal membrane destabilisation (LMS) were measured as biological responses. H. australis accumulated significantly higher tissue zinc concentrations compared to the other metals. In situ organisms at the mine affected sites accumulated more metals than organisms in laboratory microcosms. Accumulated zinc, cadmium and the total metal concentrations in whole organism tissues reflected exposure-dose relationships. Sub-cellular analysis showed that most of the accumulated metals, both in the field and laboratory exposed organisms, were detoxified over 28 days exposure. Clear exposure and dose dependent responses of decreased TAOC and measurable increases in MDA and LMS with increased metal exposure and dose were evident in H. australis caged in the river. In contrast, a dose-response relationship was only evident for cadmium in laboratory exposed organisms. Organisms caged at mine affected sites showed stronger EDR relationships than those exposed in laboratory microcosms as they were exposed to additional sources of dissolved zinc and cadmium. Exposure in laboratory microcosms underestimated metal uptake and effects, thus assessment of metal contaminated sediments should be undertaken "in situ".