Dynamics of heavy metals in reedbeds along the banks of the river Scheldt

The pollution status of recent sediment deposits in the intertidal reedbeds along the banks of the river Scheldt was assessed, to be able to predict potential metal accumulation in controlled flooding areas and new wetlands. Moreover, the most important factors that affect metal accumulation and mobility in these marshes were identified. The superficial intertidal sediment layer was found to be significantly contaminated with trace metals. The metal concentrations could be predicted from clay or organic matter concentrations. They were somewhat higher then predicted at sites within a range of a few km from specific point-sources, whereas they were lower than predicted at sites which are regularly subjected to flooding by high salinity water. In the deeper sediment layers, sulphides also seem to play an important role in the metal accumulation. An increased salinity increases the metal mobility in oxidised sediment layers in the brackish part of the estuary. This was mainly observed for Cd. It was reflected in increasing metal concentrations in the reed plants, which were however primarily related to the clay and organic matter concentrations in the sediments. The decomposition of stems and leaves of reed and willow can both increase and decrease the metal mobility in the upper sediment layer. The concentrations of most metals in the reed litter itself increased strongly during decomposition under field conditions. The hydrological regime to which metal-polluted sediments are subjected, affects the metal concentrations in the pore water to an important extent. Flooding of a calcareous sediment led to an increased mobility of Fe, Mn, Ni and Cr and a decreased mobility of Cd, Cu and Zn. Alternating hydrological conditions resulted in fluctuating metal concentrations in the pore water. Reduction and oxidation of Fe and Mn, carbonate breakdown and the formation and re-oxidation of sulphides were the main controlling processes