*4.2. Dissolved Cu Concentration Variations in the Overlying Water*

In Stage I (the consolidation stage), the dissolved Cu concentration in the overlying water gradually increased. The dissolved Cu released from the sediments into overlying water through mainly two pathways. One was static diffusion [41]. On account of the dissolved Cu concentration in the sediments were much higher than that in the overlying water, the dissolved Cu was diffused into the overlying water through pore water. The other pathway was consolidation drainage. As the pore spaces between sediment particles were squeezed during consolidation, which induced the pore water seepage in sediment, dissolved Cu in sediment migrated into the overlying water along with the seepage.

In Stage II, sediment liquefaction was observed, and the initial SSCs in the overlying water increased rapidly (Figure 5a) in a short time after the wave load. However, the initial dissolved Cu concentration rapidly declined (Figure 6b). This agreed well with a previous study reported that the heavy metals concentrations in the overlying water and suspended sediments were negatively correlated [42]. The rapid decline in the dissolved Cu concentration at the initial period of this stage may be related to the adsorption process by the increased quality of suspended particles [34,43]. Moreover, as the change of hydrodynamic condition of the overlying water, namely from a static state to a disturbed state, the dissolved Cu above the sediment tank diffused into the surrounding water under wave disturbance. Afterwards, the dissolved Cu concentration then began to increase and reached its peak at approximately 30 min, which was corresponding to the sediment liquefaction process. The sediment and overlying water were intensively mixed during the liquefaction process [34,43], which facilitated the release of Cu from the sediment into the overlying water. The liquefaction interface gradually moved upwards under the subsequent wave actions. Meanwhile, fine particles in the sediment were resuspended into the overlying water during the liquefaction process [22]. These fine particles, which are relatively small in particle size and have larger specific surface area, would strongly adsorb the dissolved Cu and lead to a decrease of dissolved Cu concentration in the overlying water [44].

After 90 min of wave action, the SSCs in the overlying water remained relatively stable. However, the dissolved Cu in the overlying water kept increasing, which might be induced by the following reasons: (1) As the sediment liquefaction interface gradually moved upwards, the sediment structure below the liquefaction layer was strengthened [25]. The sediment particles were compacted and the pore water was squeezed out and caused seepage flow, along with which the dissolved Cu in sediment diffused into the overlying water; (2) The pressure difference between the wave crest and trough also caused the diffusion of Cu in the sediment into the overlying water along with the pore water [21,45].

In Stage III, the liquefied sediment layer was about 2–3 cm thick on the surface. During the initial period of wave action, both the SSCs and the dissolved Cu concentration increased (Figures 5b and 6c). The sediment particles settled on the surface were resuspended as a result of the increased wave height. The strong disturbance in overlying water induced desorption of Cu that previously adsorbed on particles [34]. This led to an increase in the dissolved Cu in the overlying water. After 60 min of wave action, an opposite change tendency of the SSCs and the dissolved Cu concentration was observed, namely the SSCs in overlying water declined (Figure 5b), while the dissolved Cu concentration had an increasing tendency (Figure 6c). The possible reasons for the increase of dissolved Cu concentrations in the overlying water were as follows. First, the dissolved Cu diffused into the overlying water along with the upwards pore water seepages from the sediment under the wave loads [25], leading to a slow increase in the dissolved Cu concentration in water. Second, the sediment particles that adsorbed Cu were resuspended into the overlying water and exposed in water disturbance

arise from wave movements, which could resulted in desorption of the adsorbed Cu on sediment particles [46], and in consequence increased the dissolved Cu concentration in the overlying water. Moreover, re-suspension of the sediment caused the sediments at reduced state to be exposed in an aerobic environment. The organically-bound Cu on the particles were released due to oxidization and degradation of organic matter, and thus increased the concentration of dissolved Cu in the overlying water. In addition, the re-adsorption of dissolved Cu by iron manganese oxide was weakened on account of the combination of dissolved Cu and dissolved organic matter, which also increased the concentration of dissolved Cu in water.
