Chapter 21 Geochemical behaviour of trace metals in freshwater sediments

Abstract

Abstract Pore-water concentration profiles provide insight into diagenetic processes taking place in aquatic sediments. Due to the high input of reactive organic matter and the relatively low availability of external oxidators, redox zones generally occur on a millimetre to centimetre scale in organic-rich freshwater sediments. Distinct peaks in dissolved trace metal concentrations measured immediately below the sediment—water interface, can be mainly attributed to degradation of organic matter. The resulting concentration gradients between the pore water and the water column indicate that upward fluxes of trace metals from the sediment may contribute to concentrations in the surface water. An excess of acid-volatile sulphide (AVS) over reactive metals (SEM) in the anoxic sediment layer is a strong indication that trace metals may be effectively scavenged by sulphides, thus reducing their pore-water concentrations. However, supersaturation of the pore water with respect to metal sulphides indicates intensive complexation of trace metals with dissolved organic compounds. Given the large vertical variation in redox potential with depth, mixed homogenized sediment samples (0–10 cm) are generally not suited for the assessment of potential toxicity of sediments with respect to trace metals. Temporal variation in pore-water chemistry reflects environmental and intrinsic changes. These may be related to fluctuation in temperature, the supply of degradable organic matter, microbiological activity in the sediment, diffusion of solutes, mixing and reaction rates. As a result of this variation in the intensities of biogeochemical processes, (bio)availability of trace metals may fluctuate considerably. Therefore, in dynamic depositional environments it is necessary to account for non-steady state to determine diffusive exchange between sediments and surface water, as well as potential toxicity of sediments with respect to trace metals.