Abstract

 Whereas freshwater and marine sediments are among the most important sites of chemical contamination known, their biogeochemical complexity prevents simple, general approaches to risk assessment from being applicable for this type of matrix. Predictive models may give a rough indication of the likelihood that a given level of contamination will cause ecological damage but these cannot be expected to consistently or precisely predict risk for actual systems. In retrospective studies of specific sites, assessing the degree of ecological damage and assigning causation to one or more agents are the main challenges. New developments in weight of evidence approaches offer improvements by being more quantitative, logical and transparent, though there is not yet a general consensus on how WOE should be practiced, or on how large a role 'best professional judgement' should play. Significant challenges remain, for example, with regard to contaminant bioavailability and the interpretation of various widely-used effect endpoints. In addition, quantifying the economic and ecological costs and benefits of re-mediating contaminated sediments is an area where further progress is needed.