Impact of Reclamation Project on Mercury-Contaminated Sediment Transport from Minamata Bay into the Yatsushiro Sea

Abstract

Minamata Bay which suffered from a major disaster called "Minamata Disease" has already remediated from 1977 to 1990 by carried out highly mercury (Hg) contaminated sediment and reclaimed it in the bay area. Furthermore, many studies reported that the mercury has reached Yatsushiro Sea, the outer part of Minamata Bay. It can be assumed that the sediment transport still occurs from remaining Hg concentration even after the remediation project. In order to investigate it, we try to simulate the transport of sediment contaminated by Hg from Minamata Bay to the Yatsushiro Sea with different coastal morphology on before and after the reclamation projects.Sediment transport and tidal flow with rectangular variable grid both before and after reclamation conditions around the Yatsushiro Sea and Minamata Bay are simulated by DELFT3D. The initial bottom sediment thickness of 1m is set in the only inner side of the bay and six months simulation results for both before and after reclamation area are compared. The change of hydrodynamic condition inside Minamata Bay due to the remediation project gives a slight change in sediment transport pattern. It is estimated that after the reclamation, the sediment transport moves slower than before it due to the change of its magnitude of velocity above the seabed. As an important result, it is clarified that a change of the amount of bottom sediment which can be re-suspended in Minamata Bay caused by the topographical change can also affect the pattern. Also, this result suggests that the southern part of Yatsushiro Sea can be influenced by sediment contaminated by higher Hg included before the reclamation project.

Subtidal benthic communities in Minamata Bay, Japan, approximately 30 years after mercury pollution remediation involving dredging disturbance

“…The effects of coastal topographical changes on benthic community assembly through changes in hydrodynamic and physicochemical regimes have been shown in both the Oiartzun estuary (Borja et al, 2013) and Pearl River estuary (Yang et al, 2018). However, a simulation study in Minamata Bay showed that reclamation slowed bottom current velocity from 6 to 3 cm/s at ebb tide (Fathya et al, 2016), which could help facilitate retention of larvae input from other areas. Marine biological communities are sometimes maintained by self‐retention of larvae from the site rather than by dispersal from other regions (Cowen & Sponaugle, 2009; Giangrande et al, 2017).…”

Vertical Distribution of Mercury in Bottom Sediments and Particle Size Effect on Migration of Mercury-Containing Sediments in the Yatsushiro Sea

“…3). In the horizontal direction, the variable grid established by Fathya et al 14) was utilized, where the grid accuracy of Minamata Bay, and its surrounding regions is up to 62.5 m, whereas the grid accuracy of places further from Minamata Bay is 125 m and 250 m, respectively. The vertical grid adopts the σ coordinate system and is composed of five layers, each having 20% of the total depth.…”

Analysis of Distribution and Migration of Mercury in Sediment in the Yatsushiro Sea Based on the Classification Experimental Method

“…The calculation domain in the model was defined to contain both the Yatsushiro Sea and the Ariake Sea. The horizontal grid is a variable grid established by Fathya et al 14) , and the grid precision is 62.5 m, 125 m, and 250 m, respectively. The vertical grid adopts the σ coordinate system and consists of five layers, each of which is 20% of the total depth.…”