Cadmium remobilization during sediment resuspension can lead dissolved concentration exceeding EU water framework maximum
The EU water Framework (2008/105/EC) delimited total dissolved concentrations thresholds of one or a group of hazardous chemical elements in water, that if exceeded, indicate poor water quality. In parallel, French (JORF n°184 10-08-2000) and Italian (Italian Ministerial Decree 173/2016) authorities have delimited contaminants bulk sediment content thresholds and suspended matter monitoring for the management of dredging operation and the mitigation of their impact on water quality. As a matter of fact, the metals accumulated in sediments can be remobilized by their resuspension  through natural and anthropogenic activities (such as dredging), and degrade water quality. Among these metals, Cd is a non-essential, harmful trace metal for biota, causing serious problems towards organisms due to its high toxicity [2,3]. Through this work, we aimed at studying the kinetic and amplitude of Cd remobilization as a function of the origin of the resuspended sediment, its initial Cd content and sediment/water ratio. For that purpose, three Mediterranean coastal sediments were studied, presenting Cd contents considered as not harmful by the guidance of the corresponding country (France or Italy). In lab, controlled continuous resuspension experiments allowed us to demonstrate that the kinetics of the processes were similar in each experiment, but the amplitude of the transfers was specific to each sediment. Indeed, Cd systematically undergone a removal from water by adsorption onto particles in the very first minutes of mixing. Then this element was desorbed from particles after a few hours to reach higher (up to 22.1-fold higher) dissolved Cd concentrations after 2 weeks of mixing than in the initial waters. Finally, with high sediment/water ratio mimicking dredging activities (125 gWW.L-1), after 2 weeks of mixing, dissolved Cd concentration was up to 2.4 times higher than the maximum concentration permitted by EU water Framework. Such results point out a risk of water quality degradation that is not considered in sediment management practices (only considering the content metal in the sediment and only regulating / monitoring the resuspended particles concentration.)
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