Distribution and behavior of trace metals in the sediment and porewater of a tropical coastal wetland

Vertical profiles (0–30 cm below surface) of four trace metals—Cadmium (Cd), Chromium (Cr), Lead (Pb) and Zinc (Zn)—in the sediment and sediment porewater of an ecologically important intertidal mudflat in the Mai Po and Inner Deep Bay Ramsar Site were thoroughly studied over a period of 10 months (from March 1999 to January 2000). Two surveys, one in summer and another in winter, involving a total of eight sampling stations were conducted to study the seasonal variation of the remobilization characteristics of these trace metals in the mudflat sediment. The range of depth averaged concentration of these trace metals in the mudflat sediment was: 0.3–0.8 μg/g (Cd); 9.8–91.0 μg/g (Cr); 7.3–69.1 μg/g (Pb); and 39.5–192.0 μg/g (Zn), while that in the sediment porewater was: 0.3–121.1 μg/l (Cd); 3.0–2704.1 μg/l (Cr); 2.6–105.6 μg/l (Pb); and 32.6–4238.3 μg/l (Zn). In general, levels of dissolved trace metals in the sediment porewater were much higher in the summer than in the winter while their concentrations in the sediment were more or less the same throughout the year. Enrichment of Cd, Pb and Zn in the sediment porewater of the upper oxic layer and that of Cr in the oxic–sub-oxic boundary was generally observed. Regions in the vicinity of the Mai Po mangroves and the river mouths of Shenzhen River and Shan Pui River were found to be hotspots of trace metal pollution. Benthic diffusive fluxes of trace metals from the mudflat sediment were also estimated. Of the four trace metals, cadmium showed the greatest tendency toward remobilization from the sediment phase to the more bio-available porewater phase.