Faculty of Science, Shinshu University, Matsumoto 390-8621, Japan.
Department of Disaster Prevention, Meteorology and Hydrology, Forestry and Forest Products Research Institute, Ibaraki, Japan.
Sci Total Environ. 2021 Nov 20;796:148929. doi: 10.1016/j.scitotenv.2021.148929. Epub 2021 Jul 8.
Forests are important sources of dissolved radiocesium (Cs) discharge downstream. To improve understanding of dissolved Cs discharge processes during rainstorms, we investigated the relationship between rainfall-runoff hydrological processes and the discharge of Cs leached from litter. Leaching tests were conducted with broadleaf litter collected in the area where saturated overland flow was generated during rainstorms in a broadleaf-tree-dominated forest. According to the leaching test results, the Cs leaching rate was higher in the early stage of the test and decreased afterward. There was no significant difference in the overall results between the agitation and non-agitation cases. The Cs leaching rate from litter after the 24-h test was up to 33.7%. A large proportion of the original Cs activity was present even after the tests, as leaching from litter during rainstorms in the headwater area could be an additional source of dissolved Cs in the stream water. If mixing of Cs originating from groundwater, soil water, and rainfall with the hydrological processes is assumed, differences between the observed and estimated Cs in the surface runoff water became larger under high flow conditions. This analysis indicates additional Cs loading on surface runoff water during rainstorms, where saturated surface area can expand as the surface runoff rate increases. Contact area between surface runoff and litter accumulated on the forest floor should increase and accelerate Cs leaching from the litter. Therefore, Cs leaching in the saturated surface area that is temporarily formed during rainstorms can play a principal role in dissolved Cs discharge during rainfall-runoff events. Contaminated litter in the temporally saturated region of forested headwaters is an important factor contributing to elevated levels of dissolved Cs during rainstorms in the Fukushima area.
森林是下游溶解放射性铯（Cs）排放的重要来源。为了更好地了解暴雨期间溶解 Cs 排放过程，我们研究了降雨径流水文过程与从凋落物中浸出的 Cs 排放之间的关系。利用在阔叶树种为主的森林中暴雨期间产生饱和坡面流的区域采集的阔叶凋落叶进行了浸出试验。根据浸出试验结果，Cs 的浸出率在试验初期较高，随后降低。搅拌和非搅拌两种情况下的总体结果没有显著差异。24 小时试验后，凋落叶中的 Cs 浸出率高达 33.7%。即使在试验之后，仍有相当一部分原始 Cs 活性存在，因为在源头区的暴雨期间，凋落叶的浸出可能是溪流水中溶解 Cs 的另一个来源。如果假设 Cs 源自地下水、土壤水和雨水的混合以及水文过程，那么在高流量条件下，观测到的和估算到的 Cs 在地表径流水之间的差异会更大。这种分析表明，在暴雨期间，地表径流水的 Cs 负载量会增加，因为随着地表径流量的增加，饱和地表面积可以扩大。地表径流与森林地表积累的凋落物之间的接触面积应该会增加，并加速凋落物中 Cs 的浸出。因此，在暴雨期间暂时形成的饱和地表区域中的 Cs 浸出可能在降雨径流事件中对溶解 Cs 的排放起到主要作用。森林流域暂时饱和区域中的污染凋落物是福岛地区暴雨期间溶解 Cs 水平升高的一个重要因素。
