Spatial variations in ¹³⁷Cs of soil and crop within a cultivated field in Fukushima, Japan

Obtaining an accurate representative value for a field to evaluate the annual fluctuation of radiocaesium (¹³⁷Cs) concentration is critical. This study clarifies within-field spatial variability in two cultivated fields affected by fallout from the Fukushima Dai-ichi Nuclear Power Plant accident. We investigated the spatial patterns of ¹³⁷Cs in soil (two sites) and broccoli (one site), along with related parameters including the transfer factor (TF), exchangeable ¹³⁷Cs (Ex¹³⁷Cs), the ratio of Ex¹³⁷Cs to total ¹³⁷Cs in soil (Ex¹³⁷Cs%), and exchangeable potassium (ExK) using geostatistical analysis (semivariogram). In addition, we estimated the relative error of the mean at a 95% confidence interval for selected sample sizes to support monitoring design. The coefficients of variation were 19% and 29% for soil ¹³⁷Cs and broccoli inflorescence ¹³⁷Cs, respectively. Semivariograms indicated limited detectable spatial dependence for soil ¹³⁷Cs and Ex¹³⁷Cs at the sampling resolution, although crop row-aligned distributions were noted for soil ¹³⁷Cs. With five sampling points, relative errors were estimated at 16-17% and 21-31% for soil ¹³⁷Cs and Ex¹³⁷Cs, respectively, whereas five samples produced relative errors of 25-29% and 21-27% for broccoli inflorescence ¹³⁷Cs and TF, respectively. These uncertainty estimates provide practical guidance for selecting sampling intensity based on monitoring objectives and for interpreting interannual changes. Further studies are needed to establish robust sampling recommendations for cultivated fields.