Trace-element contamination of a shallow wetland in Western Australia

The arsenic, chromium, iron and lead contamination of sediments in Lake Yangebup was investigated, focussing on the potential for arsenic to be remobilized during resuspension of contaminated sediments. Despite a distinct source of arsenic, chromium and iron on one shore, horizontal trends in sediment concentrations were weak. Lead concentrations were homogeneous. This indicated a continual remobilization of these trace elements through the lake; a likely mechanism in such a shallow lake is resuspension of bottom sediments. An arsenic maximum was found 10 cm below the surface of the sediment in a settled flocculant layer that predominantly comprised decaying plankton. Remobilization of arsenic from this layer during resuspension was simulated with elutriate tests under both oxic and anoxic conditions. Under oxic conditions, the amount of arsenic released was correlated to the initial concentrations of sediment arsenic, indicating that porewater was the source of arsenic. However, in the anoxic experiments, no such correlations were evident, suggesting that biological or chemical processes were interfering with the redox potential and thus with the release of arsenic. Processes such as stratification, oxygen depletion and reaeration, desorption and adsorption in this shallow lake interact on timescales shorter than those that dominate contaminant cycling in deeper water bodies.