The contamination levels of heavy metals (i.e., As, Cd, Cr, Cu, Pb, and Zn) and their transport and accumulation in the soil-plant system were assessed for wetland surface soils of the Suaeda salsa wetland in the Yellow River Estuary, and the influences of water depth and soil physico-chemical properties on heavy metal concentrations were also investigated. Our results showed that both As and Cd contamination was more serious according to the guideline values of various soil and sediment quality standards, whereas the soil in this region was slightly contaminated by other heavy metals. The concentrations of Cd, Cr and Zn were higher in non-flooding wetland than those in flooding wetland, whereas the concentrations of As, Cu and Pb were lower. Moreover, As concentration increased with increasing stand water depth in the flooding wetland, while weak influences of stand water depth on other heavy metals were observed.
The relationships between metal concentrations and soil properties were identified using correlation analysis. Significant positive correlations were observed among the concentrations of Cd, Cr and Zn, indicating these three metals might come from similar sources. Moreover, these three metals were significantly positively correlated with soil salinity but negatively correlated with soil pH values. This was probably because ions in sea water affected the concentrations of Cd, Cr, and Zn. On the other hand, Pb and Cu were differently from these three metals, but similarly to each other. The concentration of Cu was negatively correlated with soil salinity and positively pH, while Pb concentration was significantly correlated with that of organic matter.
Generally, these metals were not accumulated in the roots of Suaeda salsa, but its accumulation ability was still greater compared to another wetland species (Spartina alterniflora). Cu and Zn had the highest enrichment factors compared to other metals. There was medium Cu and Zn accumulation in flooding wetland, and great Cu accumulation in non-flooding wetland. The translocation factor indicated that Suaeda salsa had the strongest translocation ability for As and Cu. In general, the aboveground parts of the plant had higher metal concentrations than underground parts. Flooding could influence metal accumulation in plant organs, and it greatly decreased the enrichment factor of Cu and the translocation factor of Cr, whereas greatly increased the translocation factor of As.