Time changes of dose equivalent rate above the soil surface as indication of natural attenuation processes

Abstract. Eight sites in the Fukushima contaminated area were surveyed for long-term changes in D-shuttle dose equivalent rate above the soil surface during 2015–2017. D-shuttle readings in most cases decreased faster than if due to radioactive decay only. More rapid decrease can be explained by natural attenuation processes, such as erosion of the topsoil, the vertical migration of radionuclides in the soil and the deposition of cleaner sediments transported by surface runoff. According to the time dependencies of D-shuttle dose rate readings integral rate constants of the natural attenuation were estimated using the exponential trend-line. Estimated rate constants of natural attenuation ranged from 7.3×10-3 to 0.48 yr−1, while the correspondent a half-dose rate decrease was 1.4–95 years.