## 4 Discussion
The sources of nickel include nickel plating, non-Ni ore mining, burning of coal and oil, nickel-cadmium batteries, sewage, phosphate fertilizers, and pesticides, which also involve other trace elements [31]. The fact that Ni was only closely related to Cu but not to other trace elements suggests that these are not major contributors to the Ni loading in the Pearl River system. It is interesting to note that Zn was not only closely related to Pb but also to Cu, Cd, and As (Table 6). Zn discharge can be caused by a wide range of economic activities, including mining, smelting, coal burning, manufacturing, and the use of pesticides and fungicides [32; 33; 34], which frequently involve other trace elements. It is also interesting to note that there is a trend showing that the concentration of Cd and Hg decreased from the upper intertidal zone to the lower intertidal zone. This suggests that these two elements tended to be retained in the upper part of the tidal zone and their seaward transport was relatively weak. The extremely high concentration of Cd and Hg in the surface soil layer at some locations was accompanied by a large standard deviation, indicating the significant variation among the three replicate samples collected at the same location. This can be attributed to unusual contamination by local sources at certain isolated spots. Further investigation is needed to understand the causes of these observations. The background concentrations of Cr, Ni, Cu, Zn, As, Cd, Hg, and Pb were 50.5, 14.4, 17, 47.3, 8.9, 0.05, 0.078, and 36 mg/kg, respectively. It can be seen from Figure 2 that the concentrations of the eight investigated soil-borne trace elements were all greater than the respective background concentration of each trace element. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & **Cr** & **Ni** & **Cu** & **Zn** & **As** & **Cd** & **Hg** & **Pb** \\ \hline Cr & 1 & & & & & & & \\ \hline Ni & 0.129 & 1 & & & & & & \\ \hline Cu & \(-\)0.163 & 0.549 ** & 1 & & & & & \\ \hline Zn & \(-\)0.317 & 0.007 & 0.459 * & 1 & & & & \\ \hline As & \(-\)0.382 & \(-\)0.049 & 0.389 & 0.965 ** & 1 & & & \\ \hline Cd & \(-\)0.082 & \(-\)0.163 & 0.396 & 0.550 ** & 0.495 * & 1 & & \\ \hline Hg & 0.316 & \(-\)0.394 & \(-\)0.079 & 0.017 & 0.010 & 0.270 & 1 & \\ \hline Pb & 0.171 & \(-\)0.267 & \(-\)0.089 & 0.450 * & 0.384 & 0.360 & \(-\)0.054 & 1 \\ \hline \hline \end{tabular}  
* significant at \(p<0.01\).
* significant at \(p<0.05\).  
\end{table} 
Table 6: Correlation matrix showing the correlation coefficients between the investigated trace elements. The INPI of the surface soils at the 17 sampling locations ranged from 7.53 to 48.42, values which all fall within the highest pollution category (Category 5), i.e., heavy pollution (Table 7). Using the ecological risk assessment method of [PERSON] [19], among the 17 sampling locations, six locations had an ERI greater than 300 and 12 locations had an ERI greater than 600 (Figure 3), indicating that most of the investigated locations were at high or very high ecological risk. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sampling Location** & **INPI** & **Degree of Pollution** \\ \hline S1 & 12.74 & Heavy pollution \\ S2 & 11.41 & Heavy pollution \\ \hline S3 & 14.52 & Heavy pollution \\ S4 & 16.38 & Heavy pollution \\ \hline S5 & 14.29 & Heavy pollution \\ S6 & 48.42 & Heavy pollution \\ \hline S7 & 16.45 & Heavy pollution \\ S8 & 7.53 & Heavy pollution \\ \hline S9 & 11.72 & Heavy pollution \\ S10 & 9.31 & Heavy pollution \\ \hline S11 & 9.56 & Heavy pollution \\ S12 & 9.90 & Heavy pollution \\ S13 & 17.69 & Heavy pollution \\ \hline S17 & 22.70 & Heavy pollution \\ S18 & 15.18 & Heavy pollution \\ \hline S19 & 8.02 & Heavy pollution \\ S20 & 9.35 & Heavy pollution \\ \hline \hline \end{tabular}
\end{table} 
Table 7: The integrated Nemerow pollution index (INPI) of the surface soils at the 17 sampling locations.