Annelies Van de Moortel

Metal Accumulation in Intertidal Marshes: Role of Sulphide Precipitation

We assessed short-term temporal and spatial variation of metal contents in the upper 100 cm sediment profile of intertidal marshes vegetated by common reed (Phragmites australis) along the Scheldt estuary in Belgium. The upper 0–100 cm sediment profile was sampled in three reedbeds at 56, 94, and 131 km from the river mouth. Sampling was repeated five times, at approximately two month intervals. Sediment properties such as texture and chloride, carbonate and organic matter content differed among locations. Metal accumulation, which is primarily due to association of metals with organic matter and clay in the surface sediment layer, seemed to be supplemented by an accumulation of sulphide precipitates deeper in the sediments. The depth at which sulphide precipitation significantly contributed to metal accumulation depended on the sampling location, and varied from less than 5 cm in clayey, organic sediments to more than 1 m in sandy sediments. Temporal variation of Cu, Cd, Pb, and Zn concentrations could only be linked to newly formed sulphides or sulphide oxidation at the sites with the lowest sulphide content. At sampling sites containing high sulphide amounts, variations should be primarily attributed to metal exchange and the presence of mobile metal complexes. Litter decomposition at the end of the growing season could hereby play a significant role.

Distribution and mobilization of pollutants in the sediment of a constructed floating wetland used for treatment of combined sewer overflow events

Sediments in combined sewer overflow treatment systems may exhibit elevated pollutant concentrations. Concentrations measured in the sediment of a floating treatment wetland ranged from 0.17 to 1.6 (cadmium), 28 to 142 (copper), 10 to 33 (chromium), 50 to 141 (manganese), 5 to 20 (nickel), 50 to 203 (lead), and 185 to 804 (zinc) mg/kg dry matter and 7.4 to 17 (iron), 2 to 8 (total nitrogen), and 1.3 to 4.4 (total phosphorus) g/kg dry matter. During overflow events, the entering water volumes can disturb the sediments. A greenhouse experiment was set up to evaluate the possible mobilization of pollutants through disturbation. The disturbation did not result in an increased mobilization of cadmium, copper, chromium, nickel, lead, zinc, nitrogen, phosphorus, and organic carbon towards the pore and surface water. Calcium concentrations in the surface water increased for all sediments, as a result of release from the exchangeable sediment pool and dissolution of carbonates. Geochemical speciation modeling indicated that, in the pore water, the free ion form was the most abundant for calcium, iron, manganese, cadmium, and nickel, with its fraction increasing with time.

Factors Affecting Metal Accumulation, Mobility and Availability in Intertidal Wetlands of the Scheldt Estuary (Belgium)

We studied factors affecting the accumulation, mobility and availability of metals in intertidal wetlands of the Scheldt estuary (Belgium), both in greenhouse experiments and under field conditions. The surface layer of the intertidal Scheldt sediments was found to be significantly contaminated with trace metals. The metal concentrations can be predicted from clay or organic matter concentrations. They were somewhat higher than predicted at sites within a range of a few kilometres from specific point-sources of metals, whereas they were lower than predicted at sites which are regularly subjected to flooding by high salinity water. In the deeper sediment layers, sulphides seem to play an important role in the metal accumulation. Salts significantly increased the metal mobility in the oxidised sediment layers in the brackish part of the estuary, which was especially observed for cadmium (U). The decomposition of stems and leaves of reed plants and willows in the upper sediment layer was found to both increase and decrease the metal mobility. The concentrations in the reed litter itself increased significantly during decomposition under field conditions. The hydrological regime to which metal-polluted sediments were subjected, affected the metal concentrations in the pore water to an important extent. Flooding increased the mobility of iron (Fe), manganese (Mn), nickel (Ni) and chromium (Cr) and decreased the mobility of Cd, copper (Cu) and zinc (Zn) in a calcareous substrate. Fluctuating hydrological conditions resulted in fluctuating metal concentrations in the pore water. Reduction and oxidation of Fe and Mn, decomposition of carbonates and the formation and re-oxidation of sulphides were hereby the main controlling processes.