Speciation and migration of heavy metals in sediment cores of urban wetland: bioavailability and risks

To examine the status and risk of heavy metal pollution in an urban wetland in China, the distribution and speciation of chromium (Cr) and other metals (Cd, Cu, Pb, and Zn) were examined. We investigated the impact of three major land uses (residential and industrial (RI), orange plantation (OP), and mixed OP and RI (OPRI)) on the heavy metal characteristics using sediment cores (0-60 cm below water/sediment interface) collected in Sanyang Wetland, China. It was found that all the metals (Cr, Cd, Cu, Pb, and Zn) had lower concentrations in the top layers but higher contents in the bottom layers of sediments. Species of metals in sediments were dominated by their secondary phase (i.e., exchangeable and carbonate bound, Fe-Mn bound and organic bound) with relatively low contents of primary phase (i.e., residual form), except for Cr in RI affected river sediments that had a relatively high content of primary phase (20.97-36.07%). The ratio of secondary phase to primary phase (RSP) and risk assessment code (RAC) methods were applied to assess environmental risk. The results implied that the metal mobility and bioavailability could significantly cause urban wetland environmental quality decline, and thus enhanced strategies should be required to target the capture and removal of metals.