Predictive ability of sediment quality guidelines and design of a tier1 risk assessment framework for dredged sediments: how to deal with confounding factors in practice ?

Many tiered frameworks designed for contaminated sediment risk assessment rely upon sediment quality guidelines (SQG) at the first tier. In case of multiple contaminations, results can be aggregated in indices such as mean quotients. It can thus be decided e.g. to dispose on dredged materials in open water without further investigation, provided SQGs, or specific values of indices derived from SQGs, are not exceeded. Thus, the relevance of SQGs, and indices as well, is critical for environment protection. In the context of the development of a tiered framework for dredged materials assessment for the St Lawrence River, we assessed various indices based on the SQGs available for this stream and a database matching chemistry and toxicity tests. As the overall efficiency of any of the tested indices remained rather low, factors such as sediment grain size, nutrients, metal-binding phases, which could explain type II errors (false negatives), were examined. This lead to the design of a modified tier I, where SQGs are used in combination with decision rules based on some explanatory factors. This work is supported by Environment Canada and the Ministère du Développement Durable, de l'Environnement et des Parcs du Québec.