An Environmental Examination Of Sedimentation In Lindisfarne Bay

Lindisfarne Bay is one of several small estuarine bays on the eastern side of the lower Derwent Estuary near Hobart in southeastern Tasmania. It once contained sandy beaches and was originally known as Beauty Bay being used as a place for recreational pursuits by residents of Hobart early after European settlement. Concomitant with urbanisation of the catchment, industrial activities, and indiscriminate land use practices has been the problem of sedimentation. Attempts to redress the situation have included land reclamation and public pressure to replace sand on the beaches. This study aims to quantify the extent of sediment deposition in Lindisfarne Bay and to identify sources of sediment from within the catchment by using the caesium radionuclide tracing and dating technique, by examining levels of heavy metals in the deposited sediment, and by relating these to the land use history of the Bay and estuary. Core samples were taken to a depth of up to 1.4 m from eight sites around the intertidal zone of the head of the bay by using a purpose-built coring device. These cores have been analysed for caesium-137 (137Cs) activity, heavy metal content (cadmium, copper, lead and zinc), and grain sizes of sediments. Sediment deposition rates are determined to be between 2.7 and 3.3 cm yr-1 by 137Cs analyses. Heavy metal concentrations indicated sedimentation rates compatible with 137Cs results and maximum levels at depths of 0.5 to 1.0m. Grainsize analyses formed an important part of the interpretation of sediment dynamics within the Bay by showing significantly different proportions of mean grainsize between the eastern side and the western side of the head of the Bay. An input of reference value for the average areal activity of 137Cs for the region was established to be 77 .6 mBq cm-2. The source of sediments has been alluded to by further areal activities of 137Cs distribution in the catchment Natone Hill emerged as a principal source of sediments. Before management options can be considered, it is argued that a sound knowledge of the sedimentation problem should be gained. Such an understanding has been achieved and laid a foundation for appropriate management practices such as minimising sediment escape from the catchment by reviewing the burning regime of Natone Hill and Gordons Hill, forbidding the use of off-road vehicles on Natone Hill, and minimising run-off from road sites as well as installing silt traps to major storm water outlets. Phasing out of Ministerial exemptions from the Environment Protection Act 1973 for sewage treatment plants along the lower Derwent Estuary and the Electrolytic Zinc Company of Australasia Limited is imperative for improving water quality and heavy metal loading to sediments within the bay. Once the necessary steps have been taken to prevent further sedimentation and pollution, a program of dredging and foreshore beautification could be implemented.