Arsenic and heavy metals pollution along a salinity gradient in drained coastal wetland soils: Depth distributions, sources and toxic risks

Salt ions can affect the toxicity, mobility and transfer of metals/metalloids in estuarine wetlands. Soil samples were collected to a depth of 30 cm along a salinity gradient at four sampling sites, including bare land (B), Tamarix Chinensis wetlands (T), Suaeda salsa wetlands (S) and Phragmites australis wetlands (P) in drained coastal wetlands in the Yellow River Estuary, China. Arsenic (As) and heavy metals (i.e., Cd, Cr, Cu, Pb and Zn) were measured to investigate their levels, depth distributions, sources and toxic risks. The results showed that As and heavy metals generally showed a decline with increasing salinities. Arsenic concentrations in all samples exceeded the threshold effects levels (TELs) value and were below the probable effects levels (PELs). The concentrations of Cd, Pb and Zn in all soil samples were below TEL values, while Cr and Cu concentrations were grouped to the range of TELs-PELs in several soil samples. According to the geoaccumulation index (Igeo), Cd exhibited unpolluted to moderate pollution at Sites B, S and P, whereas no pollution for other heavy metals were observed in all soils. Generally, higher Igeo values for Cd were observed at those sites with lower salinities. The average toxic unit (TU) values of As and heavy metals at Sites B and T followed the order Cr > As > Zn > Cu > Pb > Cd, and the followed order was As > Cr > Zn > Cu > Pb > Cd at Sites S and P. As and Cr showed higher contributions to the TUs than other metals. Correlation analysis showed that As and heavy metals were negatively correlated with electrical conductivity (EC), sand content, Cl−, Cl−/SO42- ratio and Mg2+ (P < 0.05), while were positively correlated with soil moisture, clay and silt contents and soil organic carbon (SOC) (P < 0.05). The mutivariate analysis indicated that these heavy metals originated from the same source, while As also had another source. The findings of this work can contribute to pollution control and ecosystem health conservation of coastal wetlands.