A decision framework for the remediation of contaminated rivers

The understanding of the behaviour and fate of contaminants in river sediments is essential to deal with their management. Especially in systems involving solid-solution interactions it is important to know how heavy metal mobility is influenced by geochemical and biological processes. In the present study, a framework is presented that allows making a quick evaluation of the risk caused by heavy metal contaminated sediments, based on simple physico-chemical analyses and tests. River system dynamics and geomorphological properties of rivers were also taken into account when different management scenario’s were considered. The decision framework was established based on the investigation of 20 contaminated river sediments from Flanders (North Belgium). Besides physico-chemical sediment characteristics such as total concentrations of heavy metals, CEC, organic matter content and grainsize, attention is also paid to the evolution of sediments properties and associated heavy metal release during oxidation, and the acid neutralizing capacity of the sediments after oxidation. In general, for the cases analyzed in this study, pH was the most important factor explaining heavy metal release during oxidation. This pH-decrease is on its turn caused by the oxidation of sulphides and the decay of organic matter. Clay-rich samples mostly displayed a higher acid neutralizing capacity than sandy samples, which resulted in a less important pH-decrease during oxidation and a more limited release of heavy metals. A simple resuspension experiments allowed to determine the potential release of heavy metals upon dredging and oxidation of the river sediments. It is of primary importance to recognize the rivers where removal of the contaminated sediments is urgent, but also to evaluate the possible dispersion of contaminants caused by dredging operations, in order to establish appropriate management options.status: publishe