EVALUATING THE MOBILITY OF CS-137, PU-239+240 AND PB-210 FROM THEIR DISTRIBUTIONS IN LAMINATED LAKE-SEDIMENTS

Post-depositional mobility of Cs-137, Pu-239+240 and Pb-210 was assessed in six small lake basins by comparing sedimentary nuclide profiles with their known fallout history. Laminae couplets, when present, were determined to be varves because the Cs-137 and (239+240)pu 1963 fallout peaks are present in laminae couplets corresponding to years 1962-1964. There is no evidence of mobility of Pb-210, because 1) mass accumulation rates based on Pb-210 agree with those based on Cs-137 and Pu-239+240 peak depths and with those based on varve counts, and 2) Pb-210 ages agree with varve ages. Significant mobility of Cs-137 is evident from the penetration of Cs-137 to depths 15-20 cm deeper than Pu-239+240. Deep penetration of Cs-137 in spite of a sharp gradient below the peak is interpreted by a numerical model to suggest that Cs-137 is present in two distinct forms in these sediments, 67-82% as an immobile form and 18-33% reversibly adsorbed with a K-d of approximately 5000. The profiles can be interpreted equally well assuming a small portion of the total Cs-137 Was present as an extremely mobile phase (K-d << 5000) in the months to years following peak fallout, slowly becoming more strongly adsorbed. High NH4+ concentrations in porewaters may enhance diffusion of the mobile form of Cs-137, but not of the immobile form of Cs-137 that defines the sharp gradient. Mobility of Cs-137 is likely also enhanced by the low clay content and the high porosity of these sediments. Thus the first detection of Cs-137 is the sediments cannot automatically be assumed to correspond to a date of 1952 (initial testing of thermonuclear weapons), although the depth of the peak can be assumed to correspond to 1963 (the year of maximum fallout from testing of thermonuclear weapons). Pu-239+240 is a more reliable sediment chronometer than Cs-137 because it is significantly less mobile.