Vertical Migration of ¹³⁷Cs in Japanese Orchards after the Fukushima Daiichi Nuclear Power Plant Accident

Abstract

We investigated the vertical ¹³⁷Cs distribution in soil among five sod culture orchards with different soil textures over six years after 2011, when ¹³⁷Cs fallout was released by the Fukushima Daiichi Nuclear Power Plant accident, to confirm the long-term ¹³⁷Cs downward-migration into soil. At each orchard, soil cores were collected to a depth of 30 cm and subdivided into intervals of 3 to 9 cm. The ¹³⁷Cs within the 3 cm of topsoil decreased from 84–94% during the first 7 months after deposition in 2011 to 41–75% in 2017. From 2012 onward, the vertical ¹³⁷Cs profiles in the soils were explained by a two-component negative exponential model composed of a rapid and a slow component with a change of slope at a depth of 6 to 9 cm. It took 4 years after deposition to show a significant difference in the value of the average ¹³⁷Cs migration distance (M_d) among the orchards. The speed of ¹³⁷Cs migration in the orchards during the 6 years after the accident year was 0.44 to 0.97 cm year⁻¹ based on the M_d. There was a significant positive correlation between M_d and fine sand content in the 3 cm of topsoil between M_d and the ratio of the total carbon content (TC) at a depth of 3 to 6 cm to that in the top 3 cm of soil. Furthermore, the percentage of exchangeable ¹³⁷Cs (ex-¹³⁷Cs) to ¹³⁷Cs at 3 to 6 cm depth increased significantly in proportion to the ratio of TC at 3 to 6 cm depth to TC at 0 to 3 cm depth in soil collected in 2013. These findings indicate that one of the mobile forms of ¹³⁷Cs was ex-¹³⁷Cs combined with TC and that the fine sand content and TC influenced the ¹³⁷Cs downward-migration in the 3 cm of topsoil in the orchard in which organic matter accumulated by sod culture.