Land use changes and metal mobility: Multi-approach study on tidal marsh restoration in a contaminated estuary

Inundation of formerly embanked areas in order to combine flood control and tidal marsh restoration will be applied increasingly. However, areas suitable for the implementation are often found to be contaminated. Re-inundation of metal contaminated soils can have consequences on total metal concentrations as well as metal mobility. In this study, metal mobility in a tidal marsh restoration project was evaluated based on the modified BCR sequential extraction method, concentrations of acid volatile sulfides (AVS) and simultaneously extracted metals (SEM) and metal concentrations in plants. The results obtained from the sequential extraction suggest an increase in metal mobility following inundation due to the reduction of Fe and Mn oxides and the subsequent release of associated metals. However, the differences in results between sequential extraction and [SEM–AVS] may indicate that redistribution of the metals to the mobile fraction can be caused by sample processing. High AVS concentrations in newly deposited sediments in the restored marsh may indicate that the formation of insoluble metal–sulfide complexes will reduce metal mobility on the longer term. Processes following inundation of metal contaminated land are complex and different conditions prevailing in other sites or estuaries can result in different behavior of the trace metals. More in situ research is needed to get a better insight in the risks involved.

► Metal mobility was investigated in a tidal marsh restoration project.
► Sequential extractions, SEM–AVS and metal concentrations in plants were studied.
► Results suggest a short term increase in metal mobility due to inundation.
► Metal–sulfide precipitation is expected to reduce mobility on the longer term.