1986 Volume 20 Issue 2 Pages 81-90
Two kinds of soils undisturbed for the past 100 years were obtained as core samples at several sites in Japan: one that have been exposed to rainfall and the other that have not at least these 100 years. These soil samples were determined for 226Ra and 210Pb. The deficiency of 210Pb relative to 226Ra was observed in all the soil samples without rain. This is due to exhalation of 222Rn. The surpluses of 210Pb activities in the soil exposed to rain relative to those in the soil without rain at the same site were observed in the top 30cm of the core. The 222Rn exhalation rates and the supported 210Pb profiles were evaluated using a steady state vertical diffusion model of 222Rn. The 222Rn exhalation rates in the soil samples are estimated to be 0.6-2.0 atoms cm-2s-1. The 210Pb deposition rates were determined from the inventories of excess 210Pb relative to the supported 210Pb in the soil samples. These rates are 1.1-2.6dpm cm-2y-1, which are closed to the directly observed values for 2 years at nearby locations. The 210Pb deposition rates estimated regardless of 222Rn loss from the excess 210Pb relative to 226Ra are underestimated by about 10-50% for the soils studied. It is shown that the correction for the 222Rn loss is particularly important for the soils with small activity ratios of 210Pb/226Ra (<10) at the surface.