Using 210Pb measurements to estimate sedimentation rates on river floodplains

Abstract

Growing interest in the dynamics of floodplain evolution and the important role of overbank sedimentation on river floodplains as a sediment sink has focused attention on the need to document contemporary and recent rates of overbank sedimentation. The potential for using the fallout radionuclides 137Cs and excess 210Pb to estimate medium-term (10–10 2 years) sedimentation rates on river floodplains has attracted increasing attention. Most studies that have successfully used fallout radionuclides for this purpose have focused on the use of 137Cs. However, the use of excess 210Pb potentially offers a number of advantages over 137Cs measurements. Most existing investigations that have used excess 210Pb measurements to document sedimentation rates have, however, focused on lakes rather than floodplains and the transfer of the approach, and particularly the models used to estimate the sedimentation rate, to river floodplains involves a number of uncertainties, which require further attention. This contribution reports the results of an investigation of overbank sedimentation rates on the floodplains of several UK rivers. Sediment cores were collected from seven floodplain sites representative of different environmental conditions and located in different areas of England and Wales. Measurements of excess 210Pb and 137Cs were made on these cores. The 210Pb measurements have been used to estimate sedimentation rates and the results obtained by using different models have been compared. The 137Cs measurements have also been used to provide an essentially independent time marker for validation purposes. In using the 210Pb measurements, particular attention was directed to the problem of obtaining reliable estimates of the supported and excess or unsupported components of the total 210Pb activity of sediment samples. Although there was a reasonable degree of consistency between the estimates of sedimentation rate provided by the 137Cs and excess 210Pb measurements, some differences existed and the various models used to interpret excess 210Pb measurements could produce different results. By using the 137Cs measurements to provide independent validation of the estimates of sedimentation rate provided by the different models used with the excess 210Pb measurement it was shown that the CICCS and Composite CRS models appeared to generally provide the best results.