Use of radioisotopes to elucidate sedimentary history in relation to estuarine dredging and river discharge.

Radioisotopes have been used extensively to evaluate the sedimentary history especially sediment accumulation rates. This paper focuses on the importance of radioisotopes for the understanding of the sedimentary history in a modern estuary. Pb-210, Cs-137 and Ra-226 are used in this study on the sediment cores from the Scheldt estuary (Belgium, the Netherlands). The Scheldt estuary is an important waterway subject to intensive anthropogenic activities such as maintenance dredging of the shipping channel and harbors. The estuary has undergone major deepening modifications since the 1960s. Radioisotope measurements revealed strong deviations of the Pb-210 profiles from what is expected under natural estuarine sedimentation conditions which can be reflected on, for instance, the Pb-210 activity is highest at the surface and declines gradually from the top of a sediment core (most recent deposits) down to in depth (older deposits). In the intertidal area, the highest Pb-210 activity is observed not at the top of the sediment cores but at some distance below the surface. The top layer with declining Pb-210 activity can be subdivided into two phases, a relatively slow upward decreasing activity followed by a relatively rapid decreasing trend toward the surface. According to the recorded Cs-137 activity, this upper declining layer with two phases can be dated approximately started after 1963 and around 1986 respectively. The abnormality observed in the Pb-210 activity corresponds well with the estuarine dredging periods, i.e, the period 1968-1971 and the period 1992-1999. It is known that the Ra-226 is brought into the estuarine system as a by-product of the phosphate industry. The distribution and the fluctuations of the 226Ra activity reflected variations in river discharges, hydrodynamic flows and estuarine resuspension mechanism as well as dredging practice. Hence, use of radioisotopes in this study elucidates that sedimentary history and sediment accumulation behaviour are strongly subject to anthropogenic activities such as estuarine maintenance dredging, river discharge and estuarine hydrodynamic flows.