Derivation of cause-effect based sediment quality guidelines

Unlike Canadian Water Quality Guidelines, Canadian Interim Sediment Quality Guidelines (ISQGs) are not based on clear cause-effect relationships. They were derived using a modified U.S. National Status and Trends Program (NSTP) approach in which biological effects were compared with contaminant concentrations in field-collected sediments. Exceeding an assessment value indicates an increased likelihood of toxic effects, but correlation is not proof of cause, and it cannot be assumed that the contaminant present in excess of the assessment value is necessarily responsible for the observed effects. This has sometimes caused confusion and misinterpretation of the toxicological significance of sediment chemistry data. An alternative approach to computing sediment guidelines, based on estimating the bioavailable contaminant concentration and comparing this with concentrations known to be toxic, is proposed. Examples are given of guidelines for cadmium, copper, nickel, lead, and zinc computed using both methods for toxicity data collected from sediments in Canadian Shield lakes near smelters. The cause-effect based guidelines can be much higher than those computed using the NSTP approach. They can also be higher than toxic thresholds estimated from experiments with metal-spiked sediments. Only true cause-effect based guidelines should be used for predicting the cause of toxicity in sediments.