Evaluation of contribution rate of the infiltrated water collected using zero-tension lysimeter to the downward migration of Cs-137 derived from the FDNPP accident in a cedar forest soil

The vertical distribution of Cs-137 in forest soil is important for predicting air dose rates and future cycling in forest eco-systems. However, there are many unexplained questions about the mechanisms of its downward migration. In this study, the Cs-137 flux by rainfall infiltration was observed for three years from August 2017 using zero-tension lysimeters in a mature cedar forest where monitoring of the vertical distribution of Cs-137 has been conducted since 2011. As a result, the Cs-137 concentration in infiltrated water through the litter layer, 5 cm and 10 cm showed a tendency to be high in summer, but no such seasonal variation was found at 20 cm. Although the Cs-137 inventory in the litter layer has been exponentially decreasing, the annual Cs-137 fluxes in infiltrated water through the litter layer were almost the same in three years, and about 0.14-0.17% of the deposition density of Cs-137. Comparing these Cs-137 fluxes with the apparent amounts of downward migration of Cs-137 estimated from the change in the vertical distribution of Cs-137, the contribution rate of the infiltrated water to downward migration of Cs-137 from litter to soil was calculated to be 8.5-17.7%. Similarly, the contribution rate in mineral soil layers was calculated to be 0.6-0.8% on a measured basis and estimated to be 3.0 +/- 0.2% after correcting the amount of collected water, which is a problem with zero-tension lysimeter. It indicates that rainfall infiltration can explain a small part of the downward migration of Cs-137, thus further studies are required to clarify the contribution rate of remaining mechanisms such as advection-diffusion, colloidal transport, physical mixing, bioturbation, and growth and death of plant roots.