Dynamic behaviour of 210Pb, 210Po and 137Cs in coastal and shelf environments

Abstract

The continental margins of the southern North Sea and the northwestern Mediterranean Sea were chosen as the main subject areas for the study of some of the key processes operating in water and in sediment: chemical scavenging, particle transport, sediment resuspension and deposition. The dynamic behaviour of the radioactive tracers, 21OPb, 210po and 137Cs, was examined in the study areas to identify mechanisms and time scales of those key processes, in order to predict the fate and impact of the particle-reactive chemical pollutants in the coastal and shelf systems of the North Sea and the Mediterranean Sea. Determination of sediment accumulation and mixing based on the 210Pb and 137Cs profiles gave an estimate of annual deposition flux (0.12 g/cm2/yr) in the Oyster Ground, an organic-rich fine grained deposition area in the southern North Sea. A similar study was also carried out in the Gulf of Lions area (northwestern Mediterranean Sea), which showed that the intensive sediment reworking is restricted to the uppermost sediment column and most of the sediment supplied by the Rhone river is deposited close to the river mouth with only a small amount being transported towards the nearby deep basin of the Mediterranean Sea. The spatial distribution of 210Pb and 137Cs and the mixing rates of the bottom sediment in the southern North Sea were studied. Profiles of the two radioactive nuclides show intensive sediment mixing and subsurface maxima of 210Pb in most sediment cores down to 40 cm depth in the sediments. Diffusion model calculations revealed that sediment mixing in this area could not be described by diffusive processes. Applying a single event subsurface egestion model gave good agreement between model curves and measured profiles. It is concluded that the subsurface maxima of excess 210Pb are related to the non-diffusive biogenic mixing and that this kind of mixing plays an important role in determining the fate of the sediment in the coastal and shelf environments of the study area. A study of the disequilibrium and distribution of 210Pb and 21Opo showed the importance of scavenging processes in the southern North Sea. The observed low concentrations of total 210Pb in the study area were related to a high concentration of suspended matter, high resuspension rates and low atmospheric input. An excess of 21OPo, both in dissolved and particulate form, indicated an additional flux of 210Po from the coastal and shelf sediments because of high resuspension rates. Mass balance calculations from a box model revealed a shorter residence time and higher uptake rate for dissolved 210Po than for dissolved 210pb, which indicates a high recycling efficiency. Comparison of data on the 210Pb concentrations in water and sediment showed that a regular excess of 210po as observed in the water column is balanced by a small deficit of 210Po in the sediments. Special attention was also given to possible seasonal influces on distributions of 210Pb and 210po in the water column of the Dutch coastal zone. Measurements of water samples revealed that the concentrations of dissolved 210Pb and 21Opo were low during the summer, whereas the particulate 210Pb and 21Opo were relatively high during the spring and winter. The observed seasonal variation in the distribution of the two radionuclides was considered to be the result of high effective scavenging in summer and enhanced resuspension in winter. A onedimensional seasonal model was developed to account for the observed seasonal variation. The derived time-variation of the total 210Pb and 21OPo is in close agreement with the field data. It is concluded that 210Pb deposition from the atmosphere at the sea surface plays a key role in controlling the distribution of 210Pb and 21Opo in the study area, but is not sufficient to account for all 210Pb and 21Opo that is present. About 10-25% of the total amount of 210Pb and 21Opo is supplied by lateral advection. In order to assess the accuracy of the mass budget of particle transport and deposition, the flux of atmospheric deposition of 210Pb in the Netherlands was studied. The observed total 210Pb deposition shows strong variation on a short time scale as well as regional variations. There is a good correlation between the daily 210Pb deposition and the precipitation, which indicates the presence of a strong seasonal effect during the year: the deposition flux is higher in the summer than in the winter. The dry fallout flux, estimated from the relation of 210Pb flux with precipitation, is about 16-38% of the total deposition of 210Pb. Although strong seasonal variations were observed in the daily total 210Pb flux, the annual deposition rate of 210Pb did not differ significantly from year to year. For a good representation of the flux, however, measurements on a longer time scale arerequired to overcome the influence of strong seasonal variation and regional variability.