Approximately 0.5 g dried surface sediment samples were digested in 5 mL aqua regia (HNO3:HCl = 1:3) and heated at 140°C until the samples turned grey. Then 3 mL HClO4 was added into the cooled samples (Qiao et al., 2011). The residues were diluted with deionized water to 25 mL for determining concentrations of heavy metals. Sample solutions were stored in the refrigerator at 4°C and then were analyzed for the main heavy metals