The distribution of heavy metals in a highly regulated river: the River Murray, Australia

Dams and weirs are efficient traps for sediments and associated pollutants. They interrupt the downstream movement of material, leading to changes in sediment composition. The lower reach of the River Murray, in southeast Australia, is regulated along its 830 km length by a series of ten weirs constructed between 1929 and 1935. Large amounts of sediment have accumulated in each weir pool, and in response to flow regulation the river has initiated a series of channel adjustments. Surface sediment samples taken along 154 km of the river between Locks 2 and 4 reveal the impact of these structures on the textural and geochemical composition of the sediment. Downstream of each weir, surficial sediments were found to be well-sorted medium sands, while poorly sorted fine sands, silts and clays were found downstream (above the next successive weir). Concentrations of sediment-associated chromium (Cr), copper (Cu), nickel (Ni), lead (Pb) and zinc (Zn) were up to five times background levels, presumably as a result of increased agricultural and urban development. Peak heavy metal loadings in the sediment were found in the depositing areas above each weir. It thus appears that maximum environmental disturbance occurred some distance from urbanization. Since heavy metal loads are amplified by changes in sediment texture, the spatial concentrations of these pollutants reflect sediment-transport factors associated with the presence of weirs. We also calculate, for this section of the River Murray, the long-term heavy metal concentrations arising from unabated pollutant runoff from urban areas, and the results provide cause for concern.