Sedimentation of trace metals in the Gulf of Bothnia

In the Gulf of Bothnia the rapid land uplift exposes former accumulated sediments to erosion and ‘old’ material is mixed in the material flow towards the central deep areas of both the Bothnian Sea and the Bothnian Bay. High concentrations of arsenic, mercury and cadmium are accumulated in the recent sediments of a few small scale nearshore basins in the vicinity of the outlets of individual industries. The almost complete lack of accumulation in the coastal areas where the depth is less than some 60 m contribute to a continuous transport of the bulk of the elements towards the central basins. Therefore a clear decreasing relation of the concentrations of elements to the distance from the source was not found in the sediments. Concentrations of arsenic, lead, mercury and cadmium are higher in the Bothnian Bay central basin sediments than in the Bothnian Sea sediments. For most of the other elements studied almost equal concentrations are found in both sea areas. A metal budget calculation based on discharge and sedimentation data indicates that the amount of the annual total load of lead is found in the annually accumulating sediments, whereas the estimated sedimentation of mercury, cadmium, copper and zinc are not in balance with the load.