66 - Numerical Modelling of Trace Mercury Dynamics in Minamata Bay Based on In-situ Measurement

"Since Minamata disease was officially discovered more than 60 years ago, mercury pollution has been an important concern in marine environment of Japan. Although the highly total mercury has been dredged, trace mercury regarded as a risk for a mammal and unborn baby due to food chain is remained, which results in much attention for the transport of trace mercury from Minamata Bay to the near-by Yatsushiro Sea. Many experiments have been carried out to measure the sampled sediment cores for investigating the transport dynamics of trace mercury. However, those results are limited by the selected stations. For a long-term prediction in anywhere of the Yatsushiro Sea, some numerical models have been explored while assuming that the critical shear stress and re-suspension data have been calculated as constants for the whole area of the bay. Due to the heterogeneity and variability of terrain, sea water salinity, and climate, a novel model with taking more experimental data into account is required.

In this study, a numerical model was established in Delft3D based on in-situ measurement, where the transport simulation of bottom sediment coupled with the Internal Diffusion Technique was applicated. It was used to predict the transport of trace mercury from Minamata Bay to the Yatsushiro Sea which was assumed to primarily depend on sediment migration. From the modelling result, it could be concluded that the trace mercury deposited near Minamata Bay had insignificant changes, while other areas varied with time. The overall trace mercury migration changed cyclically. Meanwhile, the trace mercury trended to deposit along the seacoast, which was mainly considered due to the terrain effect. The particle size resulting in specific surface area change can affect the adsorption of mercury in sediment. For further understanding of trace mercury dynamics, the influence of particle size will be explored as well."

Authors: Baixin Chi, Shinichiro Yano, Akito Matsuyama,T eppei Abe