Salinity Front and its Effects on the Trace Metals Accumulation in Surface Sediment from Hangzhou Bay, China

Estuarine fronts accumulate pollutants, such as trace metals, causing the deterioration of the aquatic environment. However, few studies have focused on the accumulation of trace metals in surface sediments and its underlying mechanism. This study synchronously investigated the salinity front and distribution of 36 trace metals in the surface sediment in Hangzhou Bay, China, a typical macrotidal, highly polluted estuarine bay. The results showed that most (28 of 36) trace metals exhibited similar enrichment in the frontal zone. The enriched metal contents were significantly correlated with iron and clay content. Therefore, the salinity front controlled the transport of clay particles known for their high metal adsorption capacity, leading to their deposition in the frontal zone, finally resulting in the accumulation of trace metals in that region. A conceptual diagram was developed to explain how the estuarine salinity front sorts the grain sizes of surface sediment. The primary processes are the “valve” and “wall” actions of the front boundary. The valve action controls the vertical exchange of sediment along the stratified side of the front, resulting in clay accumulation above the frontal halocline boundary, while the wall action moves the clay along the frontal boundary downward to the bed. Under the combined actions of the frontal valve and wall, a high-quality zone of clay accumulates in the bottom sediment near the frontal zone. This hypothesis was supported by a field investigation of the grain-size distribution of surface sediment, salinity profile distribution, and turbidity distribution in the surface and bottom layers. Our findings provide further insight into the processes of sediment-associated heavy metal accumulation of macrotidal and microtidal sediment-rich estuaries.