Secondary mobilisation of heavy metals in overbank sediments.

Heavy metal mobility was studied in overbank sediments of the Grote Beek river in Central Belgium. The geochemical signature of heavy metals in fine-scale sampled overbank sediments was compared with data on heavy metal emission into the river. The influence of acidification, organic and inorganic complexation on heavy metal mobility in overbank sediments was studied by single and sequential extractions and leaching tests. As confirmed by these tests, the elevated CaCl(2) content of the river water significantly enhanced the mobilisation of especially Cd, while Zn was mobilised to a lesser extent. The mobilisation of As on the other hand decreased in the presence of elevated CaCl(2) concentrations. Based on the results of single extractions, two highly contaminated zones with a different Cd mobility were observed in one of the overbank profiles. A detailed investigation of Cd leaching behaviour in the zone of Fe-accumulation during pH(stat) leaching tests, suggested that it was related to the association of Cd with Fe-oxides, while adsorption was the dominant binding form of Cd in the clay-rich part of the overbank sediment profile.