**1. Introduction**

Heavy metal pollution in soils or sediments is a serious problem for biota and has been extensively studied worldwide because of its excellent ecological transference potential and manifest adverse effects on ecosystems and human health [1,2]. Heavy metals in soils or sediments can be derived through natural sources, such as weathering of rocks, or by pollution generated by human activities. Many efforts have been done to distinguish between the natural background values and anthropogenic

inputs, and to evaluate the possible enrichment of heavy metals due to human activities [3–8]. Sediments in rivers, estuaries, bays, and in the seabed of some offshore oil fields are often contaminated by heavy metals on account of sewage discharge, surface runoff, and the industrial activities and oil spillage of offshore oil fields [9–13]. It is widely recognized that sediments are an important sink for most pollutants because of their strong adsorption capacity. The heavy metal pollutants discharged into the marine water are easily adsorbed on the fine particles and organic matter, migrating and settling into seafloor sediments [14,15]. These settled heavy metals might migrate into marine water during sediment resuspension and cause harm to marine ecosystems.

Sediment resuspension can be caused by physical processes such as tidal, wind-driven currents, and wind waves [16]. In a shallow water environment, waves have been found to dominate the sediment resuspension process due to wave orbital shear stresses [17–20] or wave pumping of sediments [21]. Moreover, wave-induced pore pressure build-ups significantly promote the resuspension of sediment particles [22] and sediment liquefaction under extreme wave loads will lead to massive resuspension of particles, which is remarkably more than that under non-liquefaction conditions in quantity [23]. Meanwhile, a large quantity of fine particles, which have complex physicochemical properties, resuspended from the interior of sediment into the overlying water along with pore water seepage in the liquefaction process [24,25]. These factors would have a combined influence on the release of heavy metals accumulated in sediments.

The Chengdao area of the Yellow River Delta has a complex sedimentary environment that formed by the historical course variation of the Yellow River. The surface sediment in this area contains a high silt content, which is prone to resuspension, even liquefaction, under wave loads [26]. Moreover, elevated concentration of heavy metals, especially Cu, in sediment were reported in this area [11,13]. Under hydrodynamic disturbance, such as wind waves, the accumulated heavy metals in sediment could release into the overlying water body along with the sediment resuspension process.

Many efforts have been done to investigate the release of heavy metals during sediment resuspension processes. Numerical models coupling laboratory flume experiments were applied to study the remobilization during resuspension events of contaminated sediments. Wang et al. [27] explored the Cd release from sediment in Jinshan Lake during the post-dredging period by combining field surveys, laboratory experiments, and numerical simulations, the magnitude of Cd release was observed to be larger in high-water year and under higher bottom shear stress induced by the tidal flow. The exchange kinetics of lead between water and contaminated sediments in reservoirs were explored by a chemical speciation model, which was calibrated with laboratory resuspension experiments [28].

In-situ monitoring techniques are also extensively used in analyzing sediment resuspension and pollutants release process. A scientific instrument platform carrying sensors that measured suspended solids, current, wind velocity, and pressure was deployed to support the analysis of the mechanisms of sediment resuspension [18]. A 49-day field observation data set including temperature, suspended sediment concentration, chlorophyll concentration, and currents were collected by an in-situ monitoring platform to analyze the impact of typhoon on the sediment dynamics [29]. High-frequency on-line monitoring were performed to identify the daily variations of resuspension of heavy metals [30].

Laboratory flume experiments, which have advantages that the variables are controllable, were also extensively applied in studies about heavy metals remobilization during sediment resuspension, including particle entrainment simulator (PES) [31], annular flume [27,32,33], wave flume experiments [34], etc.. Nevertheless, the simulation with PES or annular flume only considered the hydrodynamic disturbance of surface sediment, and few studies focused on the heavy metal release during wave-induced sediment liquefaction process [34].

In view of this, the objective of the present study was to investigate the remobilization of heavy metal Cu under wave actions, especially during sediment liquefaction, through a series of controlled wave flume experiments using the sandy silts collected from the Yellow River Delta. Two sequential phases, i.e., consolidation phase and liquefaction phase, were included in the study to identify the variation of the concentration of resuspended particles, and the concentration of Cu in the overlying

water and within the sediment. The study provides a reference for understanding changes of the marine ecological environment in the subaqueous Yellow River Delta. marine ecological environment in the subaqueous Yellow River Delta.

water and within the sediment. The study provides a reference for understanding changes of the

*J. Mar. Sci. Eng.* **2019**, *7*, x 3 of 14
