Watershed‐Geochemical Model to Simulate Dissolved and Particulate 137Cs Discharge From a Forested Catchment

• Published in: Water resources research 2022-08, Vol.58 (8), p.n/a
• Main Authors: Sakuma, Kazuyuki, Hayashi, Seiji, Yoshimura, Kazuya, Kurikami, Hiroshi, Malins, Alex, Funaki, Hironori, Tsuji, Hideki, Kobayashi, Takamaru, Kitamura, Akihiro, Iijima, Kazuki
• Format: Article
• Language: English
• Subjects: Ambient temperature; Aquatic ecosystems; Base flow; Caesium; Caesium 137; Catchment area; Catchments; Cesium; Cesium 137; Cesium isotopes; Cesium radioisotopes; Connecting; Contaminants; Decomposing organic matter; Discharge; Dissolved 137Cs; Dynamics; Ecosystems; Environment models; Fallout; Fluid flow; Forest catchment; Forest compartment model; Forest ecosystems; Forest litter; Forest watersheds; Forests; Fukushima nuclear accident; Geochemistry; Hurricanes; Leaching; Modelling; Nuclear energy; Nuclear power plants; Organic matter; Radioisotopes; River; River ecology; River water; Rivers; Seasonal variation; Seasonal variations; Sediments; Simulation; Simulation models; Simulators; Soil contamination; Soil particles; Stocks; Storms; Terrestrial ecosystems; Three dimensional models; Typhoons; Watershed modeling; Watersheds
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1029/2021WR031181

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. Plain Language Summary Cesium‐137 (137Cs) derived from the Fukushima Daiichi Nuclear Power Plant is a critical radioisotope remaining from the fallout owing to its high activity, long half‐life (30.1 y), and potential impacts on humans and ecosystems. Approximately 70% of all land in Fukushima Prefecture is covered with forests, where 137Cs is predominantly present. The behavior of dissolved 137Cs in the environment must be investigated because this form of 137Cs is readily absorbed by living beings. Researchers have reported that dissolved 137Cs concentrations in river water under baseflow conditions vary seasonally and rise during storms, indicating that leaching from organic matter in forests is a dissolved 137Cs source. We developed a new simulation model for the dynamics of