Mercury and arsenic mobility in resuspended contaminated estuarine sediments (Asturias, Spain): A laboratory-based study.

Estuarine sediments must be dredged to allow for navigation, and where these sediments are placed after dredging depends upon guidelines based only on the total concentration of contaminants. However, resuspension events could seriously affect the mobility and speciation of contaminants, including potentially toxic trace elements stored in sediments. The effects 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 estuaries in northern Spain based on samples collected from sites which have been impacted by decommissioned Hg and As mines and periodically subjected to dredging activities. Designed to mimic the resuspension of particles, each of the experiments revealed that the release of Hg and As species does not only depend on the total concentration in the sediments (16.3-50.9 mg kg-1, for As and 0.52-5.01 mg kg-1 for Hg). The contribution from porewaters and the subsequent reductive dissolution and/or desorption appear to be the main processes responsible for the abrupt increase in dissolved Hg and As species (maximum release of 427% and 125%, respectively). In some cases, As and Hg continued to remain at high concentrations in the water column even after the experiments were completed, thus testifying to their critical persistence in the dissolved form. Conversely, at the other sites, the restoration of pre-resuspension conditions was observed only a few hours after resuspension, mainly due to the role of Fe oxy-hydroxides which provides suitable surfaces for adsorption and/or co-precipitation involving dissolved Hg (maximum removal of -58%) and As (maximum removal of -25%) species. The results of this research could be helpful to take appropriate decisions regarding dredging especially at the Nalòn estuary, where the release of dissolved As(V) and MeHg appeared to be favoured by sediment resuspension.