Measurements in forest catchments in Fukushima Prefecture indicate that cesium-137 (¹³⁷Cs) leaching from forest litter affects dissolved ¹³⁷Cs levels in river water. Existing simulations have been unsuccessful in reproducing this phenomenon due to mechanistic limitations in the models connecting the forest ecosystem and the river system. This paper introduces a new coupled watershed-geochemical model to address this. We connected a forest ecosystem compartment model to the 3D watershed model General-purpose Terrestrial Fluid-Flow Simulator to describe ¹³⁷Cs transfer between forests and the river system. The compartment model included nonleachable and leachable ¹³⁷Cs sites in the organic layer. The latter sites model how leachable ¹³⁷Cs stocks increase with ambient temperature owing to decomposing organic matter. The model was tested by simulating dissolved and particulate ¹³⁷Cs discharges from a forested catchment upstream of the Ohta River, Fukushima Prefecture. The results for dissolved ¹³⁷Cs concentrations in river water for 1 January 2014, to 31 December 2015, correlated well with catchment measurements. The simulations reproduced dissolved ¹³⁷Cs concentration peaks that occurred during three typhoon events and the seasonal variations under baseflow conditions. The results support the theory that leaching from the organic layer in forests is a primary factor affecting river water dissolved ¹³⁷Cs concentrations. The wider contribution of this study is it introduces a coupled watershed-geochemical model, applicable not only for evaluating the dynamics of ¹³⁷Cs, but also those of other substances that adsorb onto sediments and soil particles, and leach from forest organic matter.

中文翻译：

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模拟森林集水区溶解和颗粒 137Cs 排放的流域地球化学模型

福岛县森林集水区的测量表明，从森林枯枝落叶中浸出的铯 137 ( ¹³⁷ Cs) 会影响河水中溶解的^{137 Cs 水平。}由于连接森林生态系统和河流系统的模型存在机械限制，现有模拟未能成功再现这一现象。本文介绍了一种新的流域地球化学耦合模型来解决这个问题。我们将森林生态系统隔室模型连接到 3D 流域模型通用陆地流体流动模拟器，以描述森林和河流系统之间的^{137 Cs 转移。}隔室模型包括有机层中不可浸出和可浸出的¹³⁷ Cs 位点。后一个站点模拟了可浸出的程度^{由于有机物质的分解， 137} Cs 库存随着环境温度的增加而增加。该模型通过模拟福岛县太田川上游森林集水区的溶解和颗粒^{137 Cs 排放进行了测试。}2014 年 1 月 1 日至 2015 年 12 月 31 日河水中溶解的^{137 Cs 浓度的结果与集水区测量值具有很好的相关性。}模拟再现了在三个台风事件期间发生的溶解¹³⁷ Cs 浓度峰值以及基流条件下的季节性变化。结果支持这样的理论，即森林有机层的淋溶是影响河水溶解的主要因素¹³⁷铯浓度。本研究更广泛的贡献在于它引入了一个流域-地球化学耦合模型，该模型不仅适用于评估¹³⁷ Cs 的动态，还适用于其他吸附在沉积物和土壤颗粒上以及从森林有机质中浸出的物质的动态。