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 en vironmental records contained in lake sediments. Since the radionuclid e records are themselves the outcome of the transformation of atmosphe ric fallout by mediating transport processes from the catchment, throu gh the water column and postdepositional migration via pore waters, re liable models of these processes are crucial to accurate dating. The l arge quantities of data on Pb-210 and Cs-137 in lake sediments accumul ated 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 simpl e 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 phas es. The relatively solubility of Cs-137 in the water column is demonst rated by the reduced Cs-137/Pb-210 inventory ratios in sediments compa red to values expected from direct fallout. Using sediment records fro m a wide range of Cumbrian lakes, calculations based on simple models indicate that the particulate fraction of weapons fallout Cs-137 in th e water column ranged from 5-22%, and was proportional to the square r oot of the sedimentation rate (determined by Pb-210). The K-D value fo r 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 measu rements in the water column.