Diagenetic processes and sediment-water exchanges of heavy metals in the Mejerda River Delta (Gulf of Tunis)

An in situ benthic flux study of iron (Fe), manganese (Mn), lead (Pb), zinc (Zn), cadmium (Cd), copper (Cu) and cobalt (Co) was conducted at three stations at the Mejerda River outlet in the Gulf of Tunis, at depths of 10, 20 and 40 m in March and August 2012. Simultaneously, three sedimentary cores were taken at the same locations to evaluate the diffusive flux of these heavy metals and to determine the early diagenetic mechanisms occurring below the sediment-water interface and their impact on heavy metal mobility. The concentrations of Fe2+ in pore waters were similar to those observed in the rest of the Gulf of Tunis; flux, both benthic (275 A mu mol m(-2) day(-1)) and diffusive (9 A mu mol m(-2) day(-1)), confirmed that sediment was a source of Fe2+ for the water column, at least for the 10- and 20-m stations. The diffusive flux of Mn2+ was greater in August (11-14 A mu mol m(-2) day(-1)) than in March (1-2.6 A mu mol m(-2) day(-1)), representing 8-15 % of benthic flux and indicating that, as for Fe, sediment is a source of Mn2+ for the water column, especially in August. Mn2+ is produced before Fe2+, which corresponds to the overall pattern of early diagenesis. In March, the production of Mn2+ and Fe2+ takes place in the lower layers of the sediment, probably due to agitation and resuspension occurring at the sediment-water interface. The concentrations of Pb2+ and Zn2+ were higher in this study with reference to the corresponding concentrations in the coastal areas, revealing an anthropogenic activity on the Mejerda River, Ghar El Melh Lagoon (old Mejerda outlet) and the Khlij Channel as the main vectors.