Investigation of sulfidic sediments in a coastal lake impacted by urban development

Lake Coombabah is a 2 km2, shallow, turbid estuarine system, surrounded by urbanized development situated within the Gold Coast Broadwater tidal system on the east coast of Australia; it is one of the fastest growing urban areas in the developed world 5. The region is an example of a system that demands urgent attention to the relationship between the environment and sustained development. The western shores of the Lake are heavily urbanised as is the upper catchment. Point and non-point source pollutants enter the system from urban sources (such as runoff from residential and commercial areas), acid sulfate soil disturbance, waste disposal, and municipal and industrial discharges. Residential developments in the area may affect the lake's sediment and water quality through their construction and post construction phases including; increased sediment loads from erosion during construction and accelerated urban run-off . In addition the low lying estuarine deposits surrounding the lake are known to contain acid sulfate soil layers and have experienced varying levels of lower intensity disturbance such as the construction of infrastructure (roads, pipes, a sewage treatment plant) and golf courses. The sediments of Lake Coombabah were investigated to determine if urbanisation and soil disturbance had affected the rate of sediment accumulation, the nature of their sulfidic component and any associated environmental implications for the health of the lake, a protected fisheries habitat. Lake sediments were sampled and tested for heavy metal levels, the level and nature of sulfides present and geochemical interactions associated with organic matter, pH and redox status. Results are provided on radiocarbon dated shells, implied sedimentation rates, Immediate Sediment Oxygen Demand (ISOD) determinations and levels of heavy metals, organic carbon, chromium reducible sulfur, acid volatile sulfides and sulfur. Stability field diagrams for the Fe-S system in different parts of the lake are presented. Interpretations are made in terms of their implications for the water quality of the lake and the impacts of urbanisation. The authors wish to acknowledge the Cooperative Research Centre for Coastal Zone, Estuary and Waterway Management for their financial support and encouragement.