ABSTRACT

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.