Empirical relationship for predicting radiocesium inventory and concentration in forest soils from ambient dose equivalent rates under quasi-equilibrium conditions

Following the Fukushima Daiichi Nuclear Power Plant accident, radiocesium (¹³⁴Cs and ¹³⁷Cs) cycling between forest trees and soils has approached a quasi-equilibrium state. In forest radioecology, efforts are increasingly being made to generalize soil-to-tree aggregated transfer factors (T_ag), defined as the ¹³⁷Cs activity concentration in aboveground tree tissues divided by the total soil ¹³⁷Cs inventory, across diverse sites. However, T_ag estimation requires time- and resource-demanding measurements of ¹³⁷Cs inventory in forest soils. Therefore, we proposed a practical and efficient method for estimating soil ¹³⁷Cs inventory using ambient dose equivalent rate (Ḣ∗(10)) as an alternative indicator. We established an empirical relationship between Ḣ∗(10) (1 m above ground) and ¹³⁷Cs inventory based on data collected from 14 sites in Fukushima Prefecture in 2023, including both evergreen coniferous and deciduous broad-leaved forests. The relationship followed a power-law form: 5.86 × 10⁵ × Ḣ∗(10)^0.874. Its predictions, based on Ḣ∗(10), were validated using measurements at 11 independent study sites made in 2024, not included in the model development. This empirical relationship is expected to facilitate the more time- and cost-efficient collection of T_ag data across diverse forest ecosystems. In addition, using orthogonal regression, we determined the upper 95% safety limit for estimating radiocesium activity concentration in forest soils using Ḣ∗(10). This upper limit is expected to help reduce the probability that concentrations estimated using Ḣ∗(10) exceed the thresholds specified in forest decontamination and forestry operation guidelines.