Year

2013

Degree Name

Master of Environmental Sciences - Research

Department

School of Earth and Environmental Sciences

Abstract

For several years, dredging operations have been thought necessary to allow for boating movements and water quality improvement in Lake Illawarra. The aim of this study was to investigate some of the potential environmental impacts of land disposal of dredged materials from Lake Illawarra. There are a number of potential problems that may arise as a result of land disposal of such materials, particularly the transformation of sediments from anoxic to oxic conditions with a subsequent production of acid and release of trace metals.

Four sediment cores were collected from different parts of lake, (Koona Bay, Haywards Bay and Koonawarra Bay). They were analysed for physical and chemical properties including: particle size distribution, total metal concentrations, mineralogy, organic matter and carbonate content, extractability (biological availability) of metals as well as pH and conductivity. Also, two different methods of drying were undertaken, air drying and vacuum drying. No major differences were obtained between the differently dried samples possibly due to this fact that the vacuum oven did not exclude oxygen as was expected.

The results showed that fine-grained sediment (core 1 samples) contains higher percentages of organic matter which are likely to bind and retain metals. Half of core 2 is sandy and the other half is silty-clay, thus, the finer section shows some similarities to core 1. Cores 3 and 4 demonstrate relatively similar results due to having similar, sandy particle sizes. Shell materials were commonly found in the cores.

Overall, the results of the study show no evidence of acid sulfate generation from sulfides contained due to low percentages of pyrite present and also the neutralising capacity of the shell materials. Also, no unexpected trace metal concentrations have been observed and the extractabilities of the trace metals are generally low.

Future dredging will be essential in Lake Illawarra to remove excess sediments and decrease the impacts of siltation and also to improve water circulation within the lake.

While the results of this study indicate minimal impacts from trace metals releases and acidification through on land handling disposal of sediments, other sites where dredging may be required need investigation. In addition to land contamination by dredged material, other environmental impacts such as sediment resuspension, organism smothering, seagrass damage will need investigation. Careful selection of any sites for land disposal is also essential.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.