Factors affecting metal mobility and bioavailability in the superficial intertidal sediment layer of the Scheldt estuary

Abstract

Many factors and variables govern trace metal behaviour in soils and sediments in complex ways. Understanding metal behaviour in intertidal wetland systems is further complicated because it concerns highly dynamic systems that are continuously subjected to quickly changing environmental conditions governed by alternating low and high tides. We studied the effects of various influencing factors, such as hydraulic regime, organic matter and salinity on metal mobility and bioavailability in the superficial calcareous intertidal sediment layer of the Scheldt estuary. These sediments contain elevated levels of particularly Cd, Cr and Zn. Flooding regime and the supply of organic matter significantly affect pore water metal concentrations and hence potential mobility and bioavailability. Fe, Mn and Ni pore water contents in the upper intertidal sediment layer increased as a result of frequent flooding, whereas Cd, Cu and Zn contents decreased. Organic matter can act as a sink for metals, but it can also induce dissolution of metals which were previously bound to solid sediment compartments, especially Fe, Mn and Ni. Salinity particularly favoured Cd mobility and bioavailability in oxidised sediments, which was confirmed by field monitoring data, but it also affected Zn, Fe and Mn mobility. More detailed speciation analysis of metals in the pore water is needed to improve our understanding about the contribution and importance of various processes in determining the observed metal behaviour.