Abstract
Coastal areas are sites of discharge of anthropogenic compounds, such as trace metals. In seawater, trace metals have a strong affinity for particulate organic matter or clay mineral and tend to accumulate in sediments. However, natural events and human activities can cause disturbances in surface sediments involving modification of chemical balances and contamination of surrounding waters. Here, we investigated the dynamics of trace metals in the Sfax coastal area (Gulf of Gabes, southern Mediterranean Sea), a shallow coastal ecosystem impacted by tides and submitted to urban/industrial effluents. We presented the spatial distribution of trace metals concentrations, their potential mobility in sediments and evaluated the potential sources of target elements in surface waters. The highest concentration levels in surficial sediments (3.51 µg/g) and surface waters (0.25 µg/L) were found for Cd. The latter showed a great affinity (50%) for the exchangeable phase while other elements (Cu, Cr and Ni) were found in most residual phases, reducing the environmental risk. Pb and Zn, associated Fe/Mn oxyhydroxides revealed potential inputs from urban and industrial effluents. Multivariate statistical analysis suggested that dissolved trace metals in surface waters were probably derived from effluents/wadis but also from sediment resuspension processes, induced by natural (tides, hydrodynamics) or anthropogenic (dredging) events. Overall, this study highlights the importance of the interactions between sediment and water column for the trace metal dynamics in very shallow coastal environments with an exacerbated pattern for Cd.
Highlights
Marine coastal areas, which represent high value eco-socio-systems, are sites of discharge and accumulation of anthropogenic compounds, such as trace metals
We investigated the dynamics of trace metals in the Sfax coastal area (Gulf of Gabès, southern Mediterranean Sea), a shallow coastal ecosystem impacted by tides and submitted to urban/industrial effluents
The sampling area can be divided into two distinct zones (Figure 1) In the surface sediments of the southern coast (S1-9), distributions of each element presented strong heterogeneities (Table 1) The lowest trace metal concentrations were recorded at S2 (Co, Cr, Cu, Fe, Ni, Pb, Sb, Sn and V) and S9 (As, Cd, Mo, U and Zn) and the highest ones were observed at S6-7
Summary
Marine coastal areas, which represent high value eco-socio-systems, are sites of discharge and accumulation of anthropogenic compounds, such as trace metals. Some metals which have bound to the sediment can be remobilized and released back into the water column via hydrodynamics, biogeochemical processes and anthropogenic activities [4]. Due to their physico-chemical properties, trace metals exhibit different affinities for the various solid-phase fractions of the sediment ( exchangeable ions, metal carbonates oxides, sulfides, organo-metallic compounds, ions in crystal lattices of minerals, etc.), which influence their transfer and bioavailability toward the water column [5,6]. In shallow coastal systems, the dynamics of trace metals could be influenced by the interactions between water column and sediment
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