**5. Conclusions**

Sediment in the subaqueous Yellow River Delta, classified as sandy silt, is prone to liquefaction under wave actions. Experimental investigations of the release of dissolved metals from the interior of sediment due to wave-induced sediment liquefaction has shown that variation trends of SSCs and dissolved Cu concentration in the overlying water were different. Moreover, the mechanisms of migration and diffusion of dissolved Cu in static diffusion stage and liquefaction stage were analyzed.

In the static diffusion stage, the dissolved Cu concentration increased slightly due to the static diffusion and the consolidation and drainage of seafloor sediments. In the liquefaction stage, the arc shaped liquefaction interface moved downward during the initial period of wave loads and reached the maximum depth of about 17 cm after 30 min of 7 cm height wave actions. During the rest period of wave actions, the liquefaction interface gradually moved upward and then remained at a relatively stable depth. The dissolved Cu concentration declined at the initial period of liquefaction due to the adsorption by the increased quality of suspended particles, which are fine sandy silts with large specific surface area. On account of the intensively mix of sediment and overlying water during the liquefaction process, the dissolved Cu concentration increased to a peak value as the liquefaction interface reached its maximum depth. Sediment liquefaction greatly facilitated Cu release from interior of sediments to the overlying water. The concentrations of dissolved Cu in the overlying water during the liquefaction phase were much higher than that in the consolidation phase. Moreover, the dissolved Cu concentrations kept increasing as the wave height increased under the comprehensive function of many factors, including the diffusion of dissolved Cu with pore water seepage, desorption of the

adsorbed Cu on sediment particles, and weakened re-adsorption of dissolved Cu due to its combination with dissolved organic matter.

The migration and diffusion of Cu in the sediment were also intensified during the liquefaction phase. In the initial process of sediment liquefaction, the upward diffusion quantity of Cu was significantly higher than the downward diffusion quantity. As the liquefaction interface gradually became stable, the downward diffusion quantity increased due to the change of skeleton of sediment below the liquefaction interface. Overall, the diffusion range of Cu in sediment was generally consistent with the liquefaction range, indicating that the sediment liquefaction expanded the range of heavy metal pollution in sediment.

Heavy metal release amount due to sediment resuspension or liquefaction was found to be significant and should be considered in the long-term management of contaminated sediments in the study area. Control measures, such as pollutant discharge management and site remediation, may be performed to reduce the release quantity of contaminants. The understanding of heavy metal release mechanisms from liquefied sediments and its impact on the coastal environment could be improved by further laboratory and filed studies.

**Author Contributions:** Conceptualization, F.L. and Y.J.; methodology, F.L.; formal analysis, H.Z., W.L., and H.W.; investigation, F.L., H.Z., and H.W.; data curation, F.L.; writing—original draft preparation, F.L. and W.L.; writing—review and editing, F.L.; visualization, F.L., H.Z., and W.L.; supervision, F.L.; project administration, F.L.; funding acquisition, F.L.

**Funding:** This research was supported by the National Natural Science Foundation of China (grant number 41807247, and grant number 41807229), and the Special Fund for Shandong Post-doctoral Innovation Project.

**Acknowledgments:** The authors appreciate the assistant of our group members in the experiments.

**Conflicts of Interest:** The authors declare no conflict of interest.
