Heavy Metals in Surface Sediments from Mangrove Zone in Zhangjiang River Estuary, South China

Concentrations of selected heavy metals (Cr, Cu, Zn, As, Cd, Hg and Pb) in surface sediments from 18 sites were examined in Zhangjiang Estuary Mangrove National Natural Reserve (ZEMNNR), which is the largest natural mangrove forest north of the Tropic of Cancer in China. The ranges of heavy metals expressed in mug/g sediment dry weight were as follows: Cr (48.14 to 110.01), Cu (18.15 to 43.18), Zn (119.53 to 136.13), As (12.05 to 16.16), Cd (0.25 to 0.58), Hg (ND to 0.10) and Pb (55.61 to 67.71). Slight distribution changes implicate inflow from Zhangjiang River offered most Zn, As, Cd, Hg and Pb; while for Cr and Cu, anthropogenic activities might be another important source. The levels of Pb exceed sediment quality criteria (I Class) and may show a potential risk to ecosystems. The values of I_geo indicate that Zhangjiang estuary mangrove zone is moderately polluted by Pb and slightly polluted by Cd, As and Cr.