*4.3. Cu Concentration Profile in the Sediment after Each Wave Load*

At the end of Stage I, there was little change in the Cu concentrations in the sediment. As presented in Figure 7, slight increase of Cu content was observed in the sediment above the polluted layer, while a relatively larger increase was found below the polluted layer. Since there was a concentration gradient between the polluted slurry and the clean slurry, Cu in the polluted layer diffused into the surrounding layers through pore water [41]. Furthermore, the downward diffusion amount was larger under the effect of gravity.

After wave actions in Stage II, liquefaction occurred in the seafloor sediments. Within the liquefaction range, sediment particles and pore water mixed intensively, leading to a clear migration and diffusion of Cu in sediments. As presented in Figure 7, Cu in the polluted layer diffused into the surrounding sediment at the end of Stage II, and the upward diffusion amount of Cu in the polluted layer was larger than the downward diffusion amount.

At the end of Stage III, the position of high concentration of Cu in the sediment moved downward comparing to that at the end of Stage II. Since fine particles were separated from the soil skeleton during the sediment liquefaction, the particles below the liquefaction interface became coarser [40], which largen the spaces between sediment particles. This intensified downward permeation of Cu into deeper sediments.

The maximum liquefaction depth of the sediment was approximately 17 cm and increased concentration of Cu was also observed near this depth. This was attributed to the vertical diffusion of Cu in sediments on account of the intensive mix of sediment particles and pore water.
