Using multi-element comparisons to discriminate between natural and anthropogenic post-depositional additions in estuarine sediments

Abstract

In most areas of the world, high population centres and heavy industry are located along river systems and provide a catchment wide source of heavy metal pollution along the length of the river system. These anthropogenically derived heavy metal pollutants enter the freshwater river systems and are transported downstream as part of the suspended sediment load. Such metals combine with the suspended sediment such that there is a higher concentration of heavy metals with the finer grained, clay-rich component of the transported sediment. In such cases, the ultimate depositional environment of contaminated sediments bears little relation to the distant source regions of anthropogenic pollution. In an arid country like Australia, there are numerous coastal estuaries that are supplied with sediment transported down catchments that lack both industry and significant population centres. However such coastal estuaries are undergoing rapid population growth, even though in such regions the upstream catchment is notably lacking in population centres and industry. This provides an opportunity to study heavy metal additions and contamination in a post-depositional estuarine saline environment, in which relatively uncontaminated freshwater sediments are input into the system, mainly during seasonal flood events. This study demonstrates that transported sediments, derived from upcatchment, undergo post depositional transformations across the freshwater-marine boundary. As a result of these terrigenous, biogenic, authigenic and anthropogenic changes, estuarine sediments do not necessarily reflect the original source composition. Conservative elements, especially rare earth elements, were used to differentiate between original and post-depositional additions and transformations to the sediments of an estuarine system on the south coast of New South Wales, Australia, to show that metal contamination does not correlate wholly with sediment grain size, but also correlates with proximity to local townships surrounding part of an estuary. Metals such as Pb have been discriminated in terms of original, transported sediment compositions and local pollutant sources. Furthermore, analyses of baseline, spatial and temporal data has generated a methodology and conceptual framework for assessing anomalies, even when concentrations are low within apparently 'pristine' settings. This thesis identifies both natural and anthropogenic processes and contributions to estuarine sediments, and presents a methodology by which metal contamination of sediments and the potential future risks from remobilisation can be quantified.