Changes in the geochemistry and ecotoxicity of a Zn and Cd contaminated dredged sediment over time after land disposal.

The management of dredged sediments is of environmental concern worldwide since they may be overloaded with myriads of pollutants. For inland waters' sediments, disposal on land is a common practice. For the long-term risks assessment of such a management, a better understanding of the fate of pollutants over time and an assessment of possible associated biological consequences are needed. Here, we studied the geochemical distribution of Fe, Mn, Zn and Cd in sediment dredged from the Scarpe canal (Nord-Pas-de-Calais Region, France). Analyses were carried out immediately after dredging and 12, 18 and 24 months after disposal in field conditions. In parallel, ecotoxicity of sediment leachates was assessed using standardized bioassays. The results reflected an initial oxidation of sulphides (first year) followed by changes explained by a reversible binding of metals to organic matter in winter and to Fe oxihydroxides in summer. The water-leachable fraction represented less than 2% of the total metal and its ecotoxicity was higher for deposited sediments than for the fresh one. After first year of disposal, sediment ecotoxicity remained stable. A long-term natural attenuation of metals within disposed sediment seemed unlikely since their speciation seemed to fluctuate seasonally without any time trend over years.