Abstract

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.

K. Sakuma, S. Hayashi, K. Yoshimura, H. Kurikami, A. Malins, H. Funaki, H. Tsuji, T. Kobayashi, A. Kitamura & K. Iijima
Water Resources Research 58 (8), e2021WR031181 (2022)

DOI: 10.1029/2021WR031181
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