The mobilisation potential of trace metals in aquatic sediments as a tool for sediment quality classification

Abstract

Heavy metal contents and distributions in aquatic sediments in the Belgian rivers Dommel, a well oxygenated, non-navigable river with sandy bottom sediments, and Bovenschelde, a suboxic, frequently dredged river with muddy anoxic sediments, were assessed. To that aim two methods were used: firstly, pore water and total sediment analyses and secondly, sequential extraction experiments, providing the following operationally defined speciation forms: exchangeable and carbonate fraction, reducible fraction, oxidizable fraction and the residual fraction. In the anoxic sediments of the Bovenschelde the exchangeable and carbonate fraction is almost negligible for all elements except Mn, whereas 50 to 90% of the metals are bound to the residual fraction. In contrast, in the sandy sediments of the Dommel, the results are quite different for Cu, Zn and Cd: the exchangeable fraction is a dominant fraction for Zn, Cu is almost completely present in the oxidizable fraction and Cd is now distributed over all fractions except the residual one, which is still the most important for Fe and often also for Pb. Trace metal concentrations in porewaters are much higher in the Dommel compared to the Bovenschelde. In addition, the potential pH shift in these sediments due to oxygenation was calculated: the acid producing potential is slightly higher than the acid consuming capacity in the Dommel sediments in contrast to the sediments of the Bovenschelde. Combination of these results suggests an enhanced mobilisation potential of heavy metals in the sediments of the Dommel compared to the Bovenschelde. A sediment quality classification based on the mobilization potential of heavy metals is proposed. The sequential extraction method described in this paper can be used as an additional tool for evaluating the remediation needs if the different axes of the TRIAD show different signals. As a regular tool, however, the technique requires too much knowledge and is too expensive and time-consuming.