Even years after the Fukushima accident, some wild plants remain dangerously radioactive. This paper reveals why measuring total soil contamination fails to predict radiation levels in Koshiabura buds. The aggregated transfer factor is used to track radiocesium moving from soil to wild plants, yet values have varied wildly since the Fukushima accident.

To solve this mystery, the authors investigated how spatial scale and vertical soil distribution affect these values for Koshiabura buds. Wild edible plants are vital to Japanese mountain culture, but prolonged contamination has damaged this tradition. Koshiabura buds show exceptionally high radiocesium concentrations compared to other plants, leading to widespread transport restrictions.

The Tag value represents the ratio of radionuclide concentration in plants to deposition in the soil. For Koshiabura buds alone, these values span a range of one or two orders of magnitude, making predictions difficult. Calculating these factors usually relies on averaged data from large administrative districts rather than specific collection points.

However, using spatially averaged data might introduce large errors because different plants prefer specific micro-habitats. Plants have unique root systems that determine how they uptake nutrients and potentially radiocesium from specific soil depths. Since radiocesium decreases exponentially with depth, calculating transfer based on total soil deposition might be inappropriate.

This study aims to clarify variation causes by examining both spatial scale and the depth dependency of soil deposition. Researchers tested three spatial scales and examined correlations between bud concentration and deposition in each soil layer. Figure 1 tracks the annual change in radiocesium concentration within Koshiabura buds at Iitate Village, with data normalized against the geometric mean value for 2016.

The chart plots individual measurements as blue dots alongside the yearly geometric means shown as orange circles, revealing substantial variability in contamination levels across different years. This visualization highlights that even within a single administrative district, broad variation in activity concentration can be expected among wild edible plants.

When looking at administrative district averages, bud concentration increased with soil deposition, but with considerable variation. Using point-scale measurements showed a stronger positive correlation, yet the overall trend remained similar. Figure 3 tracks the annual change in the aggregated transfer factor for Koshiabura buds from 2014 to 2020, using publicly available data from Iitate Village.

The authors plot individual measurements as blue dots alongside orange circles representing the geometric mean to illustrate substantial year-to-year variability. This visualization highlights that even within similar administrative districts, one can expect broad fluctuations in cesium-137 activity concentration over time.

The transfer factor values showed no significant trend to increase or decrease over the years studied. Despite natural attenuation, the lack of decrease suggests a stable, continuous transfer of radiocesium from soil to buds. Table 1 compares the aggregated transfer factor of Cesium-137 in Koshiabura buds calculated using deposition data from four different spatial scales.

The authors report that while the geometric mean values remain within the same order of magnitude, ranging from 5.2 times ten to the power of negative three up to 9.1 times ten to the power of negative three, the variability changes across datasets. Specifically, the Geometric Standard Deviation decreases from 3.5 at the municipal scale to 2.0 or 2.6 when using administrative district or local measurements.

Variability in the transfer factor decreased when narrowing the spatial scale from municipality to administrative district. Surprisingly, directly measured data showed similar variability to the administrative district averages, failing to eliminate uncertainty. This figure compares how radioactive cesium levels in Koshiabura buds correlate with the amount of cesium deposited in different soil layers.

The authors found a strong positive relationship between bud contamination and cesium in the top organic horizon, as shown by the tight clustering of data points along the trend line in panel (a). In contrast, the connection to cesium deeper in the mineral soil is much weaker, suggesting that the plant's shallow root system primarily draws contaminants from the surface layer.

The shallow root system of Koshiabura likely drives its uptake, with most roots found in the top five centimeters of soil. Consequently, bud concentration reflects the contamination status of the organic matter rather than the deeper mineral soil. The abundance of radiocesium in the organic horizon correlates significantly with the activity concentration in the buds.

Broad-leaved deciduous forests, the primary habitat of Koshiabura, show significantly greater variation in radiocesium retention ratios. Figure 5 illustrates the temporal decline in the ratio of radiocesium retained within the organic soil horizon across three distinct forest types: pine, Japanese cedar, and broad-leaved deciduous.

The authors fit an exponential function to the data points from each ecosystem, represented by the dashed line, while the shaded gray area indicates the 95% confidence interval for these trends. This visualization highlights how the retention of radioactive contamination decreases over time following the accident, a dynamic that varies significantly between evergreen coniferous and deciduous environments.

Microbial activities and litter decomposition differences cause considerable variation in radiocesium accumulation in deciduous forests. This variation explains why using total soil deposition fails to reduce uncertainty in predicting bud contamination.

Understanding the organic horizon's contamination status could reduce uncertainty in determining bud activity concentrations. A new transfer factor based on the organic horizon inventory showed a strong significant positive correlation with bud concentration. The marked variation in the ratio of organic horizon inventory to total soil deposition is the primary cause of the wide transfer factor range.

The key isn't total soil radiation, but specifically how much radiocesium sits in the top organic layer where shallow roots feed. This explains the massive unpredictability in contamination levels.