Relative importance of natural and anthropogenic influences on the fresh surface water chemistry of the Hawkesbury–Nepean River, south-eastern Australia

Fresh surface waters from the Hawkesbury–Nepean River, the major river supplying water to the Sydney region in south-eastern Australia, were sampled monthly during 1991 and analysed for major ions (Na, K, Ca, Mg, Cl, SO4 and HCO3), nutrients (NO3 and PO4), organic carbon and trace metals (Al, Fe, Cu, Zn, Pb, Cd, Ni, Co and Mn). The chemical composition of the river during 1991 was consistent with other studies of the river from 1977 to 1996. The major ion composition in the river is predominantly influenced by sea-salt aerosols in rainwater (headwaters) and connate sea-salt in groundwater (mid-lower reaches), with a cationic dominance order of Na≫Mg>Ca>K (equivalents) and an anionic order of Cl≫HCO3>SO4. This is typical of the headwaters of other permanent coastal rivers (freshwater) in south-eastern Australia with a similar catchment lithology. These results differ markedly from the most common natural major ion assemblages established for world rivers (i.e. Ca>Mg>Na>K and HCO3>SO4>Cl), which tend to be predominantly influenced by chemical weathering of rocks and minerals. The mean concentrations of major ions, nutrients, organic carbon and trace metals in the freshwater reaches of the Hawkesbury–Nepean River increased by factors of 2.5–4.4, 14–18, 2.2 and 1.6–11, respectively, with increasing distance from the headwaters. Increases in major ion concentrations are attributed mainly to the increasing influence of saline groundwater inflows from regions of Wianamatta shale. Conversely, concentrations of nutrients, organic carbon and trace metals (except Fe and Al) increased as a consequence of anthropogenic inputs, particularly point discharges from sewage treatment plants (i.e. showing distinct, but variable, concentration peaks), as well as diffuse urban and/or agricultural runoff during storm events. The temporal variability of the mean concentrations of all measured parameters in this study was related to variability in water discharge. The mean concentrations of the major ions decreased by a factor of 1.5–3.0 with increasing water discharge, whereas the concentrations of nutrients, organic carbon and trace metals increased by a factor of 2.0–3.0, 1.6 and 1.3–2.0, respectively. This study provides the first survey of trace metal concentrations in the freshwater reaches of a permanent coastal river in Australia using `clean' sampling and handling techniques. The concentrations of Cu, Zn, Pb, Cd and Ni measured in the headwaters of the Hawkesbury–Nepean River were amongst the lowest reported in the literature for riverine (freshwater) systems, and will form a benchmark for assessing the effects of increasing urbanisation in the catchment.