Background levels of metals in St. Lawrence river sediments: Implications for sediment quality criteria and environmental management

Preindustrial sediments dredged from the St. Lawrence Seaway are often considered potentially toxic relative to interim sediment quality criteria. The aim of the present study was to better document the background levels of target metals that were once used to define the minimal threshold of these criteria. Three extractions were performed on sediment samples to evaluate the distribution and potential bioavailability of trace metals. The results showed that the background levels established for the criteria are representative of contaminant concentrations observed in preindustrial sediments but not of the postglacial marine clays underlying these sediments. Chromium, nickel, and copper concentrations in postglacial marine clays, when solubilized by strong acids, exceeded the minimal effect threshold, whereas chromium and nickel frequently exceeded the toxic effect threshold and therefore are problematic in terms of applying sediment quality assessment criteria. The results suggest that these trace metals are mostly associated with inert silicates in postglacial marine clays and are unlikely to be bioavailable to aquatic organisms. Postglacial marine clays should be better considered differently than are sediments in regard to the sediment quality criteria implementation. Accordingly, the use of tools to identify the postglacial material may become necessary if a particular management disposition should be established for this sedimentary material. Total recoverable aluminum concentrations in sediments, in conjunction with other physical characteristics, was shown to be an interesting tool to identify this specific material. The normalization of metals concentrations with total recoverable aluminum concentrations would take account of natural mineralogical and textural variability of freshwater sediments in the quality assessment process.