Kazuyuki Sakuma

Soil and groundwater contamination at some sites impacts downstream populations when contaminants migrate from groundwater to rivers. Predictive modeling is challenging since it is required to include detailed subsurface structure and groundwater flow models within the site, as well as watershed-scale models for large-scale transport. Now that climate change impacts are major concerns at many sites, it is important to have the capability to represent the water balance change and its impact on contaminant transport both at the site and watershed scale in a consistent manner. This study introduces a new simulation framework to couple a detailed 2D site/hillslope-scale groundwater model to the 3D watershed-scale model to describe contaminant transport from groundwater to river water within the catchment. Within the site, we estimate the contaminant discharges to the river from contaminant sources based on the Richards equation and advection-dispersion equation. The discharges are then applied as the boundary conditions to the watershed-scale model considering the width of the 2D site/hillslope-scale groundwater model and recharge rates for both models.We demonstrate and validate our framework based on the tritium concentration datasets in surface water and groundwater collected at the Savannah River Site F-Area. Results show that the method can successfully reproduce the contaminant concentration time series in river water.

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.