抄録
A new alternative method to calculate the migration rate of excess 210Pb from undisturbed soil profiles was developed on the basis of the same advection equation used in the previous work. In this method, no assumptions are made to evaluate migration rate and travel time. The authors have already described two different methods with some assumptions. The method can numerically calculate the migration rate of excess 210Pb at every horizon of interest, and estimate their changes through the ground. The numerical approximation method was considered to be the most appropriate one of the three, which were outlined and illustrated. Undisturbed soil samples were collected from three sites in Kyoto City, Japan and the contents of 210Pb and 226Ra were analyzed with a γ-ray spectrometry to obtain excess 210Pb profiles in the ground surface. Each method was applied to these observed profiles and the results were compared. Irrespective of the estimating methods, the average migration rates were almost the same for each site, and were found to be within a range of about 1-2mm/y. The average deposition rate of 210Pb for three sites was also estimated to about 140MBq/km2·y.
