Samples were digested by a mixture of HNO3 and HClO4 (4:1, w/ w), as described by Li, Liu, and Wan (2008). Minerals were determinedn using inductively coupled argon plasma atomic emission spectroscopy (Optima 5300DV, PerkinElmer, USA), except for Hg, Pb and Se, which were measured by atomic fluorescence spectrometry (AFS-230E, China). The content of mineral was quantified by a calibration curve obtained with known concentrations of standards.