Monitoring the radiocesium (¹³⁴Cs and ¹³⁷Cs) at the bottom of agricultural ponds in Fukushima Prefecture is crucial to ensure the safety of irrigation water and to support the resumption of agricultural activities. In a previous study, we developed a plastic scintillation fiber (PSF) measurement technique for quick and easy estimation of radiocesium concentration in bottom sediment in the field.
 The conversion factor is needed to convert the γ-ray count rate obtained by placing the PSF statically at the bottom of a pond (Fig. 1) into radiocesium concentration, and this factor has conventionally been set assuming that radiocesium was distributed mainly on the 10-cm thick surface layer of the bottom sediment. However, it has been suggested that the depth distribution of radiocesium in bottom sediments may change over time.
 In this study, we examined the necessity of evaluating radiocesium concentrations considering changes in the depth distribution of radiocesium. Sediment sampling in 2015–2019 showed that 90 % of radiocesium was contained decreased over time with cases distributed at 10 cm from the surface and increased over time with cases distributed at 15 and 20 cm from the surface (Fig. 2). Since 2015, a strong correlation between PSF count rates and radiocesium concentrations in bottom sediments was observed when 15 or 20 cm from the surface of the sediment were considered (Fig. 3), which is consistent with the trend in Fig. 2. However, even when only 10 cm from the surface were considered, a coefficient of determination was obtained that maintained sufficient accuracy (Fig. 4). After 5 years after the accident, a conversion factor that considers changes in the depth distribution of radiocesium is expected to improve the estimation accuracy even further.

This study is a part of the results titled “Technical Guidance on Measurement Methods of Radiation Distribution in Water Beds,” a research commissioned by the Fukushima Prefecture Federation of Land Improvement Associations.