Assessment of sediment ecotoxicity and genotoxicity in freshwater laboratory microcosms

One possible fate of dredged sediments is disposal in a submerged gravel pit, but this practice may be a source of contamination of the aquatic environment. In an attempt to assess the risks for the aquatic ecosystem, a microcosm test was conducted in the laboratory on three sediments of channel sections to be dredged. After a 3-day period of stabilization, pelagic organisms (microalgae, daphnids, duckweeds, pond snails) and benthic organisms (amphipods, chironomids) were introduced into microcosms. Survival and growth of organisms were monitored during 28 days. Genetic damage was measured weekly in pond snail hemocytes through the comet assay. The sediments contained high levels of metals (up to 740 mg copper/kg and 1220 mg zinc/kg), but were moderately contaminated by PAHs (total PAH content < 10 mg/kg) and PCBs (total PCB content < 0.6 mg/kg). Only moderate effects on the biota were observed, except for genetic damage on pond snail hemocytes which was significant in all microcosms. The absence of pronounced toxic effects on organisms was not expected since concentrations of some sediment toxicants were above threshold values reported in the literature, especially for metals in two sediments, and for PAHs in the three sediments. Speciation, influence of organic matter and presence of sulfides might explain the low toxicity observed. By contrast, assessment of DNA damage revealed to be more sensitive and enabled a ranking of sediments coherent with their toxic load. In the present work, the microcosm protocol previously set up with a formulated sediment appears to be fitted to natural sediments.