Processes affecting the distribution and speciation of heavy metals in the Rhine/Meuse estuary

Abstract

When rivers drain areas with a high population density the sediments are often contaminated with heavy metals, such as chromium, cobalt, nickel, copper, zinc, cadmium, mercury and lead. The extent and seriousness of sediment contamination is most pronounced in the lower reaches of rivers, where river water meets seawater. In these regions often a large preferential sedimentation of contaminated suspended solids occur. The question arises if these contaminated sediments pose a serious threat to aquatic life. A better knowledge of the behaviour of heavy metals in sediments will contribute to make a better estimation of the risks of these polluted sediments to aquatic life. In this thesis attention is focused mainly on the chemical behaviour of heavy metals in estuarine sediments. Two appropriate sedimentation areas were selected for study. One area was lake Ketelmeer, a fresh water sedimentation basin of the river IJssel (branch of the river Rhine). The other area was the Rhine/Meuse estuary, including the harbour area of Rotterdam. The study consisted of collecting and analysing of water and sediment samples from the Rhine/Meuse estuary and lake Ketelmeer and of interpreting of the data obtained. In addition some experiments were carried out. Answers were sought to the following research questions : What are the main diagenetic processes in fresh water and saline water sediments and how do they affect the distribution and speciation of heavy metals? To what extent do physical processes (e.g. advection, diffusion) and chemical processes (eg. mineralization, precipitation) determine the concentrations of heavy metals in pore water? Can these processes be quantified? Are the heavy metals desorbed from or adsorbed onto the particulate matter when fresh riverine water and saline water mix in the estuary?