Numerical modeling of dissolved mercury dynamics and transformation in sea water in Minamata Bay, Japan

Mercury pollution in Minamata Bay has remained a persistent environmental issue since the discovery of Minamata disease. In this study, hydrodynamic simulations were performed using Delft3D FLOW, and a mercury transformation model was developed with the OPEN PLCT module of the WAQ system. A precipitation-related freshwater inputs module was incorporated to capture the direct impact of precipitation on the water surface. The simulated concentrations of total dissolved mercury (THg) were generally consistent with field observations, supporting the reliability of the model. After intense rainfall, surface water THg levels increased sharply, likely due to the direct influence of precipitation on the water column. Simultaneously, salinity at the bottom decreased, which coincided with elevated THg levels near the sediment. The behavior of methylmercury (MeHg) was more complex. A Decision Tree Regressor analysis revealed that salinity and water temperature were the main drivers of the MeHg to THg ratio (MeHg/THg). The highest MeHg/THg proportion (67.7 %) was observed when salinity dropped below 26.5 ‰, around a temperature of 20 °C. Moreover, under conditions of high dissolved oxygen (DO >6.05 mg/L) and elevated total organic carbon (TOC > 4.408 mg/L), the average MeHg/THg ratio reached 27.9 %, suggesting that oxygen-rich environments with sufficient organic substrates may facilitate mercury methylation. These findings provide new insight into how direct precipitation events and water column properties jointly influence mercury cycling in estuarine systems.