Soil-soil solution distribution coefficient of soil organic matter is a key factor for that of radioiodide in surface and subsurface soils.

We investigated the vertical distribution of the soil-soil-solution distribution coefficients (Kd) of 125I, 137Cs, and 85Sr in organic-rich surface soil and organic-poor subsurface soil of a pasture and an urban forest near a spent-nuclear-fuel reprocessing plant in Rokkasho, Japan. Kd of 137Cs was highly correlated with water-extractable K+. Kd of 85Sr was highly correlated with water-extractable Ca2+ and SOC. Kd of 125I- was low in organic-rich surface soil, high slightly below the surface, and lowest in the deepest soil. This kinked distribution pattern differed from the gradual decrease of the other radionuclides. The thickness of the high-125I-Kd middle layer (i.e., with high radioiodide retention ability) differed between sites. Kd of 125I- was significantly correlated with Kd of soil organic carbon. Our results also showed that the layer thickness is controlled by the ratio of Kd-OC between surface and subsurface soils. This finding suggests that the addition of SOC might prevent further radioiodide migration down the soil profile. As far as we know, this is the first report to show a strong correlation of a soil characteristic with Kd of 125I-. Further study is needed to clarify how radioiodide is retained and migrates in soil.