Abstract Because of limited environmental monitoring data, the regional-scale impact of the deposition of fogwater radiologically contaminated by the Fukushima Daiichi Nuclear Power Plant (F1NPP) accident remains unclear. To redress this situation, we present an observational report of the radiocesium concentration in fogwater and its deposition in a Japanese forest during the early stages of the F1NPP accident (March 2011). The data were acquired by using a passive collector to capture fogwater above the forest canopy on a monthly basis. In addition, the radiocesium concentrations in monthly throughfall and stemflow were measured under the canopies of four tree species. The 137Cs activity concentration in fogwater during the observational period was 45.8 Bq L−1, which was twice as high as that present in bulk precipitation. The ratio of 137Cs in throughfall to that in bulk precipitation (TF/BP ratio) ranged from 1.0 to 2.5. The high TF/BP ratios may have been caused by the high radiocesium concentration in fogwater deposition. Based on this assumption, we assessed the TF/BP ratio according to the 137Cs activity concentrations of throughfall and bulk precipitation measured in various mountainous regions in East Japan. Our results reveal that the TF/BP ratio is high at some sites and that it increases with elevation. Sites with a high TF/BP ratio were almost entirely situated in areas of fogwater deposition, as predicted by an atmospheric dispersion model. In addition, sites with a high TF/BP ratio were above the cloud base at the time when plumes with high atmospheric 137Cs activity concentrations passed through the areas. Thus, these measurements of radiocesium in fogwater during the early stages of the F1NPP accident provide evidence that fogwater with high radioactive contamination was deposited in the forested mountain areas of East Japan. Given the major impact of fogwater deposition of radiocesium, its role should be considered carefully to better understand radiocesium cycling in forest ecosystems.

中文翻译：

---

福岛第一核电站事故期间日本东部森林山脉中放射性铯的雾水沉积：区域放射性污染的关键过程

摘要 由于环境监测数据有限，福岛第一核电站（F1NPP）事故放射性污染雾水沉积的区域尺度影响尚不清楚。为了纠正这种情况，我们提供了一份关于 F1NPP 事故早期（2011 年 3 月）雾水中放射性铯浓度及其在日本森林中沉积的观测报告。这些数据是通过使用被动收集器每月捕获森林冠层上方的雾水而获得的。此外，在四种树种的树冠下测量了每月通过和茎流中的放射性铯浓度。观测期间雾水中的 137Cs 活度浓度为 45.8 Bq L−1，是大量降水中的 137Cs 活度浓度的两倍。贯穿沉降中的 137Cs 与大量沉淀中的 137C 之比（TF/BP 比）在 1.0 到 2.5 之间。高 TF/BP 比率可能是由雾水中的高放射性铯浓度引起的。基于这一假设，我们根据在日本东部各个山区测量的直通降水和大量降水的 137Cs 活动浓度评估了 TF/BP 比率。我们的结果表明，某些地点的 TF/BP 比率很高，并且随着海拔的升高而增加。正如大气扩散模型所预测的那样，具有高 TF/BP 比率的站点几乎完全位于雾水沉积区。此外，当具有高大气 137Cs 活动浓度的羽流通过这些区域时，具有高 TF/BP 比的站点位于云底之上。因此，这些在 F1NPP 事故早期对雾水中放射性铯的测量提供了证据，证明具有高放射性污染的雾水沉积在日本东部的森林山区。鉴于放射性铯雾水沉积的主要影响，应仔细考虑其作用，以更好地了解森林生态系统中的放射性铯循环。