Assessment of ambient dose equivalent rate distribution patterns in a forested-rugged terrain using field-measured and modeled dose equivalent rates

Measuring radioactivity in forested mountainous areas is challenging owing to the limited accessibility on foot and the canopy blocking of airborne survey signals. Meanwhile, the influence of multiple environmental factors on radioactivity, in particular on ambient dose equivalent rates (ADER) in a forested area, are not well understood. In this study, we surveyed the ADER using hand-held and backpack-type scintillators in a forest of deciduous and evergreen trees in Iitate Village, Fukushima Prefecture, Japan, on multiple dates. The village is located 35 km northwest of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP). The observations showed that, 7.5 years after the FNDPP accident, the ADER still exceeded the maximum permissible level (0.23 μSv h⁻¹) of the Japanese standard limit. The ADERs were distributed unevenly in the forest, ranging from 1 μSv h⁻¹ to 3.73 μSv h⁻¹, with statistically significant variabilities among survey dates. The spatial variabilities of the ADER among the survey dates were depicted by semivariograms. The effects of the complex topography on the ADER were examined using multivariate adaptive regression splines against five selected topographic parameters. Although the topographic effect of individual parameters varied between survey dates, when all topographic parameters were considered, the model predictions yielded a positive correlation (R² > 0.54). We discovered ground wetness as the source of ADER variations among survey dates based on climatic records. Additionally, we estimated the ADER from radiocesium concentrations in soil samples and checked their consistency with the measured ADER. It was found that about half of the ADER estimated from the soil samples were within the range of field measurements. Although challenges remain in the modeling of radioactivity pathways under natural conditions, this study shows that combining topographic features, meteorological factors, and near-ground and high-altitude surveys are the keys to understanding radiation behavior in the environment.