Sediment records of fallout radionuclides and their application to studies of sediment-water interactions

The fallout radionuclides Pb-210 and Cs-137 are widely used to date environmental records contained in lake sediments. Since the radionuclide records are themselves the outcome of the transformation of atmospheric fallout by mediating transport processes from the catchment, through the water column and postdepositional migration via pore waters, reliable models of these processes are crucial to accurate dating. The large quantities of data on Pb-210 and Cs-137 in lake sediments accumulated through their widespread dating applications may be used to study transport models. Their advantages as tracers of transport processes include widespread dispersal through the environment, relatively simple and well known input functions, and ease of measurement. One of the principle factors controlling the transport of any species through the water column is its distribution between aqueous and particulate phases. The relatively solubility of Cs-137 in the water column is demonstrated by the reduced Cs-137/Pb-210 inventory ratios in sediments compared to values expected from direct fallout. Using sediment records from a wide range of Cumbrian lakes, calculations based on simple models indicate that the particulate fraction of weapons fallout Cs-137 in the water column ranged from 5-22%, and was proportional to the square root of the sedimentation rate (determined by Pb-210). The K-D value for weapons Cs-137 in the water column is estimated to be in the range 1-2x10(5) L kg(-1). This is comparable with K-D values for Chernobyl Cs-137 in these lakes (Smith et al. in press) obtained from direct measurements in the water column.