EXCHANGE OF TRACE-ELEMENTS AT THE SEDIMENT-WATER INTERFACE DURING EARLY DIAGENESIS PROCESSES

The exchange processes at the sediment-water interface can significant ly influence the transport of trace contaminants in a river. In order to investigate the importance of these processes in the tidal part of the River Elbe, a laboratory 'simulation' apparatus (LABOSI) was used that allows the incubation of undisturbed sediment cores with a relati vely large area (1700 cm(2)) in a closed system under defined conditio ns (temperature, pH, pE, etc.). Together with measured depth profiles of the pore-water and the solids of the incubated cores before and aft er the experiment, this method allows all relevant processes to be inv estigated in all three phases(sediment, overlying water, gas) simultan eously. Under oxic conditions, a flux of heavy metals (Cu, Cd, Zn) int o the water phase was observed as a result of the mineralization of or ganic matter in the thin oxic layer of the sediment (thickness about 3 mm) as well as a flux of manganese caused by diffusion from deeper an oxic layers. Up to 40% of Cd, Zn and Ni and 15% of Zn deposited in the sediment by settling of contaminated suspended particulate matter can be remobilized from the sediment surface by the processes of early di agenesis. Within 14 days, an enrichment of arsenic in the sediment sur face caused by diffusion from deeper anoxic layers could be detected. Under anoxic conditions and when the other electron accepters were con sumed, Fe(II) was released and accompanied by a strong remobilization of As, Co and Cr and an accelerated release of Mn(II). This release wa s soon stopped when the water phase became oxic again, and the trace e lements were removed from the water phase by co-precipitation and/or a dsorption on the freshly formed hydrous ferric oxides.