The use of sediments to assess environmental impact on a large coastal catchment-the Hawkesbury River system

Sediment samples (140) acquired from the estuarine and fluvial sections of the Hawkesbury River between Windsor (130 km from the coast) and Broken Bay have been analysed for texture (gravel, sand, mud), heavy metals (Cd, Co, Cu, Fe, Mn, Ni, Pb, Zn) organochlorine/pesticides (DDT, DDD, DDE, HCB, chlordanes, aldrin, lindane, heptchlor, H-epoxide, dieldrin) and nutrients (organic P, available P, inorganic P, total P, TOC, TKN).  Generally low heavy metal and organochlorine/pesticide concentrations in the sediments of the main river channel reflect low intensity land use for the majority of the catchment, whereas impacted areas are related to urbanisation and increased industrial and recreational activity. High nutrient sediment concentrations are due to large sewage loads to some tributaries of this river.  Contaminant sources are varied. Light industry and intense boating activities are the probable source of toxicants for the most contaminated area of the Hawkesbury River in southeast Pittwater in terms of heavy metals and organochlorine/pesticides.  Elevated metallic and organic contaminant levels associated with a marina and water-related recreation facilities inside parkland (upper Cowan Creek) demonstrate the potential for such activities to affect pristine environments . Increased urbanisation and industrialisation in the upper Berowra catchment provide a source of heavy metals and nutrients, probably through a large sewage treatment plant, as well as via stormwater drainage. Sewage effluent discharge from isolated urban areas (Berowra Waters and, to a lesser extent, Brooklyn) on the banks of Hawkesbury River results in enrichment in metallic and organic contaminants and nutrients and illustrates the threat such developments can pose to the estuarine environment.  These toxicants, especially in the upper reaches of the river, are chemically reactive and are, therefore, potentially mobile and bio-available.  The flow of rivers draining large parts of the upper Hawkesbury catchment (South and Cattai Creeks) is almost entirely industrial and domestic effluent during periods of low precipitation. However, sediments reflect minimal enrichment in heavy metals and organochlorines/pesticides, owing to generally low industrial activity, but the high sewage-derived organic content of the water results in marked enrichment in sediment nutrients and, importantly, the more bio-available fraction.  The reservoir of such nutrients in bed sediments of these rivers may have important implications for the continual reoccurrence of algal blooms and eutrophication in the upper reaches of the Hawkesbury River.  Sediments provide information on source and dispersion of contaminants and a long-term integrated assessment of environmental impact in a large dynamic and complex ecosystem. The successful management of fluvial and estuarine resources requires consideration of water quality information and a more holistic view, including sediment information.

Tags: Earth Sciences, GA Publication, Journal, NSW, Published_External, marine
Organization: Australian Ocean Data Network