SIMULATION OF THE EFFECTS OF DREDGING ON THE MOBILITY OF MERCURY AND ARSENIC IN RESUSPENDED CONTAMINATED ESTUARINE SEDIMENTS (ASTURIAS, SPAIN)

Metal ore extraction was active in the Nalón River drainage basin until the 1970s. The mining activity, abandoned before the introduction of environmental laws, led to the contamination of soil, water and sediments, where high concentrations of potentially toxic elements (PTEs) can be found. Estuarine sediments must be dredged in order to allow for navigation, and their destination must be managed following guidelines based only on the total concentration of contaminants. The effect of resuspension on the cycling of mercury (Hg) and arsenic (As) between the sediment and water column was investigated in a mesocosm study. Four experiments were conducted in three Asturian estuaries based on samples collected from sites impacted by Hg and As and periodically subjected to dredging activities. Designed to mimic the resuspension of particles, all the experiments revealed that the release of Hg and As species do not depend on the total concentration in the sediments. In summary, it is evident that following a resuspension event there is an increase in particulate and dissolved Hg and As which can be released from pore waters or from the dissolution of iron (Fe) and manganese (Mn) oxi-hydroxides depending on the redox conditions. In some cases, the concentrations of As and Hg in the water column remained high even at the end of the experiments, after 24 hours. Conversely, in other sites, the effects of resuspension lasted only a few hours and the restoration of pre-suspension conditions were observed. Part of the Hg and As released from porewaters and/or by desorption during resuspension was presumably removed by the subsequent adsorption onto the settling fine particles and/or was coprecipitated in association with oxi-hydroxides of Fe and Mn.