*2.5. Statistical Analysis*

Statistical Package for Social Sciences (SPSS) version 20 (IBM, Armonk, NY, USA) and Microsoft Offices Excel 2013 (Microsoft Corporation, Albuquerque, NM, United States) were used to analyze the data. Pearson's correlation coefficient was used to analyze the relationships between elements, soil pH and TOC. The potential sources of trace elements were identified using factor analysis (PCA and cluster analyses).

### *2.6. Quality Analysis and Quality Control*

To ensure the quality of the experiment all reagents and chemicals used were analytical-reagent grades. All plastic containers were soaked with 10% HNO3 (63%) for 12 h and washed three times with double distilled water to remove other contaminants, dried in oven at 60–65 ◦C for 24 h. The relative standard deviations (RSD) of three consecutive measurements of the standard solutions were used to determine the precision of ICP-MS. The obtained percentage of relative standard deviation (% RSD) was less than 10% indicating a good precision of the instrument. The recovery value of elements ranged between (90–104%).

### *2.7. Methods of Evaluating Contamination Level and Ecological Risk of Trace Elements*

### 2.7.1. Potential Ecological Risk Index (RI)

Potential ecological risk index helps to evaluate the pollution of trace elements in the soil [40]. It is the sum of the ecological risk factor of a single element in the sample [41]. The following formulas given by [41] were used to calculate potential ecological risk indices of the elements.

$$\mathbf{C}\_f^i = \mathbf{C}\_S^i / \mathbf{C}\_R^i \tag{1}$$

$$\mathbb{C}d = \Sigma^i\_f \mathbb{C}^i\_f \tag{2}$$

$$E\_f^i = \mathbb{C}\_f^i \ast T\_f^i \tag{3}$$

$$RI = \Sigma\_f^i / E\_f^i \tag{4}$$

where *Cif* is the contamination factor for trace elements; *CiS* is the measured concentrations value of elements in the soil; *C*iR is the background reference values of trace elements in the soil; *Cd* is a degree of contamination; *Eif* is the ecological risk index of a single element; *Tif* is the toxicity coefficient of an element and *RI* represents the total potential risk index of elements [40] (Table 1).

Due to the absence of soil background reference values of trace elements in Huangpi district, Wuhan soil background values were used as a reference: Cr (90), Ni (40), Cu (35), Zn (100), As (15), Cd (0.20), Hg (0.15), Pb (35) mg/kg [42] and Hubei Province soil background values were used for Co (15.4) [32], Sb (1.65 mg/kg) [43] and Fe (29,400 mg/kg) [40]. *Tif* values given by [41] were Cr = 2, As = 10, Cd = 30, Ni = Cu = Pb =5, Zn =1, Hg = 40 and Co = 2, Fe = 0 and Sb = 15.


**Table 1.** The standard given for the contamination factor, degree of contamination, ecological risk factor and risk index of elements.

Source: [41].
