Abstract：
Following the Fukushima Daiichi Nuclear Power Plant disaster in March 2011, the Japanese government initiated an unprecedented decontamination programme to remediate 137Cs-contaminated soils and allow population return. This programme involved the removal of topsoil under farmland and residential land, and its replacement with \"fresh soil\" composed of granitic saprolite. However, decontamination was limited to these two land uses, without remediating forests, which cover 70 % of the surface area in the affected region. In this unprecedented context, the specific impact of this unique decontamination programme on 137Cs transfers in river systems remains to be quantified at the catchment scale. In this study, based on the analysis of a sediment core collected in June 2021 in the Mano Dam reservoir draining a decontaminated catchment, the effects of soil decontamination on particle-bound 137Cs dynamics and sediment source contributions in response to a succession of extreme precipitation events were retrospectively assessed. The sequence of sediment layer deposition and its chronology were reconstructed through the analysis of several diagnostic properties (organic matter, elemental geochemistry, visible colourimetry, granulometry) and contextual information. During abandonment (2011-2016), cropland contribution decreased (31 %). Concurrently, 137Cs activity and deposition flux decreased (19 and 29%year-1, respectively). Following decontamination (2017), sediment transfer increased (270 %) in response to increased contributions from decontaminated cropland and \"fresh soil\" (625 % and 180 % respectively). Meanwhile, forest contributions remained stable. In contrast, 137Cs activity dropped (65 %), although 137Cs deposition flux remained constant. Forests acted as a stable source of 137Cs. Accordingly, 137Cs deposition flux after decontamination (2016-2021) was similar to that observed during the 5-years period of land abandonment (2011-2016), as a result of the regrowth of spontaneous vegetation over farmland, protecting soil against erosion. Future research should further investigate the impact of longer land abandonment that prevailed in some regions decontaminated lately on the 137Cs fluxes in the rivers.
摘要：
在2011年3月福岛第一核电站灾难之后，日本政府启动了一项前所未有的净化计划，以修复受137Cs污染的土壤并允许人口返回。该方案涉及清除农田和居住用地下的表土，并将其替换为由花岗岩腐岩组成的“新鲜土壤”。然而,净化仅限于这两种土地用途，没有修复森林，覆盖受影响区域表面积的70%。在这种前所未有的背景下，这一独特的净化方案对河流系统中137Cs转移的具体影响仍有待在集水区规模上量化。在这项研究中,根据对2021年6月在马诺大坝水库中收集的沉积物岩心的分析，该水库排干了一个净化的集水区，回顾性评估了土壤去污对一系列极端降水事件对颗粒结合的137Cs动力学和沉积物源贡献的影响。通过分析几种诊断特性（有机物，元素地球化学,可见比色法,粒度)和上下文信息。放弃期间（2011-2016年），耕地贡献下降(31%)。同时,137Cs活性和沉积通量下降（分别为1年的19％和29％）。净化后（2017年），沉积物转移增加（270％），以响应去污农田和“新鲜土壤”的增加（分别为625％和180％）。同时,森林贡献保持稳定。相比之下,137Cs活动下降（65%），尽管137Cs沉积通量保持恒定。森林是137Cs的稳定来源。因此，净化后的137Cs沉积通量（2016-2021年）与5年土地废弃期间（2011-2016年）观察到的相似，由于农田上自发植被的再生，保护土壤免受侵蚀。未来的研究应进一步调查最近净化的某些地区普遍存在的更长的土地废弃对河流中137Cs通量的影响。
