Hazardous Substance Transport to a Yodo River Drinking-Water Intake During Nankai Trough Tsunami Run-Up

Tsunami run-up can transport released substances toward river drinking-water intakes. This study evaluates transport to the Kunijima intake in the Yodo River using a coupled two-dimensional horizontal–three-dimensional hydrodynamic model with tank-release, dissolved-substance transport, and density calculations. Twenty-three cases examine salinity, substance density, release location, release volume, and river-flow and operating conditions under one Nankai Trough tsunami scenario. Previously published density and release-location simulations are extended through intake-concentration analysis. A formaldehyde-based reference threshold of 0.08 mg/L provides a common comparison benchmark rather than a generic safety criterion. The substance did not reach the intake in the freshwater-like downstream-salinity case during the 36 h simulation, whereas the highest-salinity case exceeded the threshold for approximately 30 h. The density response was non-monotonic, with 1100 kg/m3 producing the highest peak among tested densities. Increasing release volume mainly increased concentration magnitude; the higher-flow condition delayed arrival and reduced the peak. Density diagnostics support salinity-related stratification, while analytical advection tests demonstrate resolution-dependent numerical spreading. The results support scenario-based interpretation of intake concentration responses, but absolute concentrations and exceedance durations remain conditional estimates without direct validation of the complete transport application.