Storm disturbance of sediment contaminants at a hot-spot in the Baltic Sea assessed by 234Th radionuclide tracer profiles.

Fly ash sludges from an abandoned metal smelter were dumped into the shallow inner part of the Mecklenburg Bay until 1971, representing the most severe heavy metal contamination hot-spot along the German coast. Half of the dumped Zn (455 t) and Pb (173 t) inventory was found to be spread from the originally 0.5 km2 hot-spot site to a now 360 km2 affected adjacent area. Wave-driven resuspension during gale events produced large pulses of contaminated sediments from this hot-spot due to the only 23 m water depth. Instantaneous sediment mixing down to 10 cm occurs during such a wave event as evidenced by activity profiles of the short-lived radionuclide 234Th in sediment cores. According to these estimated sediment exchange fluxes in the transport bottom area, each wave event may have mobilized Zn and Pb pulses on the order of several hundreds of kilograms from the dump site, sufficient to build up a plume in sediments of the outer bay area. With each centimeter (approximately 5 yr) of additional natural sediment capping, however, the amount of metal remobilization would decrease by about 50%.