Year

2011

Degree Name

Doctor of Philosophy

Department

Department of Chemistry

Abstract

Sediments have been recognised as an important source and repository for many contaminants that enter the aquatic environment. In addition to influencing surface water quality, contaminated sediments also represent a potential hazard to benthic biota and the aquatic food chain. Accurately defining the extent of environmental risk posed by sediment-associated contaminants is becoming an important priority for regulators and industries worldwide.

Sediment quality guidelines (SQGs) are intended to delineate between sediments that cause adverse biological effects and those which do not. In the case of metals, however, an absence of toxicity is frequently observed in sediments with metal concentrations many times greater than the SQG trigger values. Factors that modify metal partitioning between dissolved and particulate phases in sediments (e.g. particle size, organic carbon, sulfide, pH) and thereby influence metal bioavailability and toxicity are believed to be responsible for the disparity. Further, the significance of exposure pathways (dietary particle ingestion and dissolved metal uptake) to a range of representative benthic organisms must also be understood to derive SQGs that offer protection for a full range of organism behaviours and benthic habitats.

Laboratory-based experimentation was undertaken to assess the sensitivity of benthic organisms to copper in sediments and the appropriateness of current SQGs for copper. Detailed information was obtained on the equilibration of copper-spiked sediments and the influence of sediment properties on the partitioning of copper between different sediment compartments (solid phases and dissolved copper in the pore water and overlying water) before and during tests. The research focused primarily on acute toxicity to sediment-dwelling estuarine and marine organisms, including the epibenthic amphipod, Melita plumulosa and the bivalve, Spisula trigonella, for numerous sediments with varying metal-binding properties.

The 10-day acute EC50s of the particle-ingesting amphipod varied from 200 to 20,000 mg/kg for sediments with copper-binding affinities ranging from low (sandy) to very high (mineralised). Similar patterns of acute toxicity across the varying sediment types were observed for the bivalve, despite having a dissolved metal uptake exposure pathway. Of the factors expected to influence copper binding, elevated acid-volatile sulfide, was demonstrated to temporarily ameliorate the bioavailability of copper, and sediment pH had a minor effect on dissolved copper toxicity. Far more influential were the sediment particle size and organic carbon content. Concentration-response relationships based on the total particulate copper and organic carbon (POC) concentrations of the less than 63 μm sediment fraction effectively normalised the influence of sediment properties on the measured copper effects threshold for all sediment-types tested. Acute and chronic effect thresholds were derived (14 and 3.6 mg Cu/g POC, respectively) using a species sensitivity distribution of twelve organisms that incorporated an understanding of bioavailability and exposure pathways. The utility of this new effects threshold for assessing sediment quality is discussed with respect to the current SQGs.

Recommendations for future development of predictive and protective SQGs for metals includes an emphasis on chronic effects data, understanding the influence of cooccurring contaminants, addressing analytical techniques specific to identifying mineralised copper phases and field validation of laboratory derived effects-based SQG concepts.

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