Investigation on metal geochemical cycling in an anthropogenically impacted tidal river in Belgium

The geochemical behavior of metals in water and sediment was investigated in the tidal section of the Zenne River in Belgium. Twelve-hour sampling campaigns were performed in October 2013 and March 2021 at the mouth of the Zenne River, under dry and rainy weather conditions respectively. Water samples were collected every hour while the passive samplers of Diffusive Gradients in Thin-films (DGT) were deployed continuously during a tidal cycle. In addition, bottom sediments were sampled at the tidal station and water samples were taken upstream and downstream of that station to identify the metal sources. The highest concentrations of Fe, Mn, Pb, Cr, Ni and Zn appear at low tide, indicating the Zenner River as a main source. However, for Co, Cd and Cu, other sources including upstream transport may explain their behavior during a tidal cycle. Fe, Pb and Cr are essentially transported in the particulate phase (<10 % dissolved) while the other metals in the dissolved phase (20 to 90 %). Rainfall and wind gust events also play an important role in trace metal distribution, increasing sediment resuspension and metal desorption. A good agreement was found between the time-averaged dissolved and DGT-labile metal concentrations with the exception of Cu and Fe, which form strong organic Cu complexes and Fe colloids respectively. The sediments of the tidal Zenne are contaminated by trace metals, thus acting as a secondary pollution source to the river. The reductive dissolution of Mn and Fe oxyhydroxides and the release of associated trace metals are the main mobilization mechanisms. Knowledge of the upstream and downstream levels in the water column, the benthic fluxes, which are based on turbulent diffusion, and the partitioning between dissolved and particulate phases allow to explain the metal concentration variations during the tidal cycle.