## 4 Discussion
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Sample** & **Depth** & **Cr** & **Ni** & **Cu** & **Zn** & **As** & **Cd** & **Hg** & **Pb** \\ \hline \multirow{4}{*}{S14} & 0–10 cm & 79.60 \(\pm\) & 24.20 \(\pm\) & 34.20 \(\pm\) & 109.30 \(\pm\) & 39.79 \(\pm\) & 4.02 \(\pm\) & 0.07 \(\pm\) & 33.68 \(\pm\) \\  & 0.74 & 0.56 & 4.12 & 9.33 & 22.99 & 3.70 & 0.00 & 3.22 \\ \cline{2-10}  & 10–20 cm & 82.70 \(\pm\) & 27.90 \(\pm\) & 36.20 \(\pm\) & 101.99 \(\pm\) & 18.51 \(\pm\) & 0.42 \(\pm\) & 0.07 \(\pm\) & 33.69 \(\pm\) \\  & cm & 7.25 & 0.37 & 3.35 & 7.16 & 0.97 & 0.03 & 0.01 & 2.40 \\ \cline{2-10}  & 20–30 cm & 107.00 \(\pm\) & 32.80 \(\pm\) & 45.20 \(\pm\) & 138.67 \(\pm\) & 40.15 \(\pm\) & 1.54 \(\pm\) & 0.10 \(\pm\) & 51.64 \(\pm\) \\  & cm & 18.10 & 3.68 & 6.78 & 21.62 & 15.17 & 0.88 & 0.02 & 7.93 \\ \hline \multirow{4}{*}{S15} & 0–10 cm & 81.60 \(\pm\) & 27.40 \(\pm\) & 38.90 \(\pm\) & 65.74 \(\pm\) & 12.66 \(\pm\) & 2.26 \(\pm\) & 0.08 \(\pm\) & 34.33 \(\pm\) \\  & 6.28 & 0.23 & 0.93 & 47.90 & 7.04 & 1.90 & 0.00 & 3.96 \\ \cline{2-10}  & 10–20 cm & 106.00 \(\pm\) & 34.60 \(\pm\) & 39.60 \(\pm\) & 112.05 \(\pm\) & 20.55 \(\pm\) & 0.47 \(\pm\) & 0.08 \(\pm\) & 32.75 \(\pm\) \\  & cm & 24.5 & 6.24 & 2.98 & 4.54 & 1.71 & 0.05 & 0.01 & 2.77 \\ \cline{2-10}  & 20–30 cm & 99.30 \(\pm\) & 31.90 \(\pm\) & 53.00 \(\pm\) & 312.17 \(\pm\) & 17.74 \(\pm\) & 1.08 \(\pm\) & 0.10 \(\pm\) & 45.59 \(\pm\) \\  & cm & 9.77 & 1.81 & 9.39 & 173.62 & 5.84 & 0.43 & 0.02 & 4.72 \\ \hline \multirow{4}{*}{S16} & 0–10 cm & 83.50 \(\pm\) & 30.60 \(\pm\) & 43.50 \(\pm\) & 119.37 \(\pm\) & 21.71 \(\pm\) & 0.51 \(\pm\) & 0.10 \(\pm\) & 34.98 \(\pm\) \\  & 13.20 & 0.09 & 2.27 & 7.94 & 0.90 & 0.10 & 0.00 & 4.71 \\ \cline{2-10}  & 10–20 cm & 151.00 \(\pm\) & 48.30 \(\pm\) & 44.70 \(\pm\) & 126.34 \(\pm\) & 21.39 \(\pm\) & 0.59 \(\pm\) & 0.08 \(\pm\) & 38.28 \(\pm\) \\  & cm & 68.10 & 17.20 & 5.16 & 8.33 & 0.72 & 0.04 & 0.01 & 2.74 \\ \cline{2-10}  & 20–30 cm & 91.30 \(\pm\) & 30.90 \(\pm\) & 60.80 \(\pm\) & 190.22 \(\pm\) & 21.79 \(\pm\) & 0.62 \(\pm\) & 0.10 \(\pm\) & 39.54 \(\pm\) \\  & cm & 1.68 & 0.74 & 12.00 & 44.09 & 1.81 & 0.04 & 0.02 & 1.65 \\ \hline \multirow{4}{*}{S17} & 0–10 cm & 91.30 \(\pm\) & 32.00 \(\pm\) & 39.20 \(\pm\) & 120.63 \(\pm\) & 40.38 \(\pm\) & 0.87 \(\pm\) & 0.09 \(\pm\) & 45.68 \(\pm\) \\  & 2.97 & 1.31 & 2.07 & 15.90 & 19.91 & 0.36 & 0.03 & 9.21 \\ \cline{2-10}  & 10–20 cm & 95.30 \(\pm\) & 30.00 \(\pm\) & 42.30 \(\pm\) & 113.64 \(\pm\) & 21.69 \(\pm\) & 0.54 \(\pm\) & 0.08 \(\pm\) & 41.54 \(\pm\) \\  & cm & 3.03 & 0.42 & 1.64 & 5.59 & 0.85 & 0.04 & 0.02 & 1.49 \\ \cline{2-10}  & 20–30 cm & 95.80 \(\pm\) & 32.70 \(\pm\) & 48.70 \(\pm\) & 131.08 \(\pm\) & 22.61 \(\pm\) & 0.65 \(\pm\) & 0.08 \(\pm\) & 42.34 \(\pm\) \\  & cm & 5.41 & 3.11 & 1.76 & 2.42 & 1.31 & 0.06 & 0.02 & 3.19 \\ \hline \multirow{4}{*}{Average} & 101.55 \(\pm\) & 30.37 \(\pm\) & 40.76 \(\pm\) & 124.84 \(\pm\) & 26.26 \(\pm\) & 0.94 \(\pm\) & 0.28 \(\pm\) & 43.35 \(\pm\) \\  & 2.74 & 0.75 & 1.08 & 8.93 & 2.16 & 0.13 & 0.08 & 1.40 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Mangrove soil-borne trace elements (mg/kg) in the lower intertidal zone. Mercury was also not related to any other trace elements (Table 6). Mining is a major source of mercury in many places [26]. However, Hg discharge related to mining activities is always accompanied with discharges of heavy metals and metalloids [27]. Therefore, Hg discharged into Pearl River was likely to be due to other sources, such as pulp and paper industries [28]. There was a close relationship between Pb and Zn in the investigated mangrove soils (Table 6). This can be explained by the close association between lead sulfide and zinc sulfides in lead-zinc deposits [29]. Mining and smelting of lead-zinc ones result in simultaneous discharge of both Pb and Zn. The close relationship between Ni and Cu (Table 6) could be attributed to their concurrent presence in nickel-copper sulfide ores [30].