Heavy metal distribution in the wetland sediments of the Liaohe delta: Implications for filter function of wetlands

Heavy metals (As, Cd, Cr, Cu, Hg, Pb, Zn), organic carbon (Corg) and grain size of 223 surface sediment samples in the Liaohe upper delta plain wetland (UDPW) and 150 surface sediment samples in neighboring shallow sea wetland (SSW) were analyzed to evaluate the spatial distribution and assess the risk of metal pollution. Concentrations of heavy metals in sediments from both UDPW and SSW are in the decreasing order of Cr (Zn) > Pb > Cu > As > Cd > Hg. The results of two-sample-t-tests indicate significant higher heavy metal concentrations in UDPW compared with SSW except As and Hg, implying the accumulation or biouptake of particulate metal within the wetland ecosystem and reducing metal input to marine coastal systems. Multiple assessment approaches, namely, the metal enrichment factor (EF), geoaccumulation index (I_geo), contamination factor (CF) and pollution load index (PLI) were jointly used to explore the risks of the anthropogenic contaminations. The results indicate unpolluted nature for Pb and Zn, moderate degree of contamination for the remaining metals both in UDPW and SSW. Furthermore, the distribution of heavy metals is substantially influenced by grain size and the concentration of Corg, and there is significant correlation between the concentrations of heavy metal, especially for the concentration of Corg and heavy mentals in surface sediment samples of shallow sea wetland (r=0.439, p < 0.01), revealing the metal chelation acted by organic matter.