Trends in the spatial and temporal distribution of metals (Cd,Cu, Zn and Pb) and organic compounds (PCBs and PAHs) in Dutch coastal zone sediments from 1981 to 1996: a model case study for Cd and PCBs

The spatial and temporal distribution of trace metals (Cd, Pb, Zn and Cu), polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) was studied to assess trends in the sediment of the Dutch coastal zone (<63-μm fraction) from 1981 to 1996. In general, the distributions showed a distinct zonation, with highest concentrations close to the coast near freshwater outlets and where dredged material had been dumped. Over time, the zone with high concentrations narrowed and moved towards the coast. The median of the percentage of the <63-μm fraction in the surface sediments did not change significantly in the Dutch coastal zone between 1981 and 1996. In that period, the median concentrations of Cd, Cu, Zn and Pb fell by 71, 38, 49 and 53%, respectively, in the area close to the coast (<20 km) north of the outlet of the Rhine. South of this outlet the median Cd, Cu and Zn concentrations fell by 45, 29 and 17%, respectively, and in the open sea area (>20 km offshore) the median concentrations for Cu and Zn fell by 35 and 18%, respectively. The largest reduction (80%) of the median concentration of PCBs occurred in the surface sediment in the open sea area (>20 km offshore) between 1986 and 1996. During this period the median PCBs concentration fell by nearly 70% close to the coast. The median PAH concentration fell by only 26% north of the outlet of the Rhine (<20 km from the coast) between 1986 and 1996. In the other areas there was no significant change. Possible reasons are that the main sources of PAHs are in the North Sea itself and that the load of PAHs from dredged material dumped in the Dutch coastal zone increased during the period studied. The reduction of the concentrations of metals and organic micropollutants in the surface sediments of the Dutch coastal zone is explained by a conservative physical mixing model, in which the previously polluted silt fraction in the surface sediments is replaced by less polluted suspended matter. There is a time lag of several years between the trend in the load of Cd and PCBs and the observed field concentration. The half life of the silt fraction and associated substances in the surface sediments is 1.9. Taking the uncertainties in the loads into account, the calculated surface sediment concentrations for Cd and PCBs agree fairly well with the field measurements. It is concluded that there are three main reasons for the decline in the metal, PCB and PAH concentrations in the highly dynamic surface sediments of the Dutch coastal zone: (1) the decrease in the load from various sources; (2) the sedimentation and mixing of less polluted suspended matter into the active sediment layer; and (3) the washing out of previously deposited more heavily polluted particles.