Three-dimensional decadal simulation of dissolved tracer dispersion in the Pacific Ocean and its marginal seas

To evaluate the environmental impacts of tritiated wastewater discharge from the Fukushima Daiichi Nuclear Power Plant, we conducted a 10-year three-dimensional simulation of dissolved tracer dispersion in the Pacific Ocean and its marginal seas using the Regional Ocean Modeling System. The model domain was partitioned into 11 regions to quantify transport pathways, concentration evolution and inventory (accumulation) changes. Our results indicate that the dissolved tracer concentration is highest within the surrounding discharge location (SDL) and subsequently disperses to the deep western Pacific (DWP), the deep eastern Pacific, and eventually into western Pacific marginal seas, with subsurface tracer intruding into marginal seas earlier than surface layer. Discharge-induced concentration increments mostly remain below practical detection limits; detectable maxima exceeding natural background levels are primarily confined to the SDL (>70 Bq/m³) and DWP (∼20-30 Bq/m³). Median levels in the SDL and DWP remain on the order 10^-4 or 10^-3 Bq/m³; in other open ocean and some marginal seas (e.g., South China Sea basin) largely stay near or below these values. Regional inventories remain orders of magnitude below natural background levels; and accumulation rates demonstrate a protracted dispersion from the open North Pacific toward the marginal seas under a decadal-scale discharge scenario.