Toxicological constraints for rehabilitation of riverine habitats: a case study for metal contamination of floodplain soils along the Rhine.
Leuven, R. S. E. W.; Wijnhoven, S.; Kooistra, L.; de Nooij, R. J. W.; Huijbregts, M. A. J.
Ambitious international action programmes and ecological rehabilitation projects have improved the environmental quality of riverine habitats along the Rhine branches in the Netherlands (rivers Waal, Neder-Rijn and IJssel). A major problem that remains to be solved is the contamination that over the last century has been accumulated in river sediments and deposits. Respectively, 65, 45 and 35 % of soil samples from floodplains along the rivers Waal, Neder-Rijn and IJssel exceed environmental quality standards for one or more contaminants (mainly metals). Storing or cleaning of contaminated sediments goes with high remediation costs and impedes ecological rehabilitation projects. Physical reconstruction measures may alter exposure of organisms to contaminants, e. g. by changing the bioavailability through transitions of terrestrial to aquatic habitats and vice versa. The question arises whether contaminants accumulated in river sediment and deposits will hinder the recovery of riverine biodiversity. The present paper gives an account of various approaches for ecological risk assessment of contaminated floodplain soils and focuses on effects of metal contamination (mainly cadmium) on riverine biodiversity. A weight of evidence approach reveals that invertebrates, small mammals and birds of prey are at risk due to metal exposure. Comparisons of measured and predicted cadmium concentrations in various species show that available models may overestimate as well as underestimate exposure risks. Site-specific exposure models should take into account spatial and temporal variability of exposure. GIS-based ecological risk assessment and site-specific risk maps offer river managers opportunities to delineate high-risk areas and to balance costs and ecological benefits of soil sanitation measures in river rehabilitation projects. New approaches to the remediation of floodplain soil contamination are briefly discussed.