Environmental status of sediments in the lower Hawkesbury-Nepean River, New South Wales

Like many other estuarine systems, the lower Hawkesbury-Nepean River is under increasing environmental pressure from continued development. Hornsby Shire Council instigated an investigation of sediment quality in this river as part of an Estuary Management Program to determine sources and the regional extent of sediment contamination, and to assist in the prioritisation of any required remedial actions. The outcomes and recommendations of this investigation have been incorporated in the Lower Hawkesbury Estuary Management Plan to be implemented progressively over a 7-10 year time frame. Sediments from 52 locations in the lower Hawkesbury-Nepean River and tributaries were analysed for trace metals, major elements, nutrients, TOC, organic contaminants and grainsize, and 16 of these locations were selected to determine concentrations of tributyltin (TBT) in the sediments. Organic booster biocides (diuron, chlorothalonil, irgarol and dichlofluanid), which augment Cu-based antifoulants since TBT was banned in 1989, were analysed in sediments from four locations near marinas, including a pristine reference location. Analysis for these compounds in sediments is rarely undertaken in Australia and this study represents the first of its kind for sediments in the lower Hawkesbury-Nepean River. Concentrations of diuron up to 40g/kg were detected in sediments near a marina compared with 1g/kg at the reference location, suggesting an impact from booster biocides used in antifoulants on sediments in areas of high boating activity. Regionally, only few heavy metals and no organic contaminants exceed ANZECC/ARMCANZ sediment quality guideline values in sediments of the lower Hawkesbury-Nepean River. However, sediments near marinas and riverside settlements in upper Berowra and Cowan Creeks contain elevated concentrations of TBT and heavy metals and may pose adverse effects on aquatic biota.