**2.5 Quality control**

Every equipment used were firstly calibrated with reference standard. Glassware used for heavy metal analyses were rinsed in dilute HNO3 before usage. All reagents and heavy metal standards used were of analytical grades. Analyses were done in duplicate to ensure precision and accuracy of the obtained data.

### **2.6 Determination of pollution levels in the study area**

To assess the pollution levels in the study area, the soil contamination factor (CF) was used. Therefore, the standard background value which represents the value of the elements, measured relative to the amount of the Upper Continental Crust (UCC) was used as the reference material [65].

*Toxic Heavy Metals in Soil and Plants from a Gold Mining Area, South Africa DOI: http://dx.doi.org/10.5772/intechopen.109639*

$$\text{CF} = \frac{\text{C}\_a}{\text{C}\_{\text{ref}}} \tag{1}$$

Where Ca is the metal concentration in the soil (total), and Cref is the background value of the pristine environment.

The contamination levels were classified based on the following classes: low contamination (CF < 1), moderate contamination (1 ≤ CF < 3), high contamination (3 ≤ CF < 6) and very high contamination (CF ≥ 6).

Pollution load index (PLI) was calculated using Eq. 2 to assess the overall contamination at each site and to distinguish natural origin from anthropogenic sources [66].

$$PLI = \sqrt[n]{CF\_1 \times CF\_2 \times \dots \times CF\_n} \tag{2}$$

Where CF1, CF2 are CF of elements 1, 2, 3, …., n; When the PLI > 1, denotes significant deterioration in the system, 0 < PLI < 1, indicates baseline level of contamination [66].

### **2.7 Determination of heavy metals uptake by plant**

Concentration factor (CF) was introduced to calculate the relationship between the uptake of As, Cd, Pb and Zn from soil by plants. This is a measure of soil-plant transfer that supports the understanding of plant uptake signature [62]. The determined concentration of each metal in the plant (Mplant) was divided by the concentration of each metal determined in the soil (Msoil). A quotient greater than 1 means that the plant has been influenced by the metal (accumulator). However, if the quotient is less than 1, it means that the plant has not been influenced by the metal (excluder).

$$CF = \frac{\mathcal{M}\_{plant}}{\mathcal{M}\_{sol}} \tag{3}$$

Where CF is the concentration factor, Mplant is the metal concentration in the plant and Msoil is the metal concentration in the soil.

### **2.8 Data analysis**

Data obtained from laboratory analysis were subjected to basic descriptive statistics (i.e., mean, and standard deviations) tabulated using SPSS software. In addition, the concentrations of the selected heavy metals were compared with South African agricultural soil standards. Also, SPSS software was used for all the statistical analysis which include Chi-square and one-way ANOVA. The Chi-square evaluated the deviation between the determined concentration of the studied heavy metals from the sample site and the control site. One-way ANOVA was used to determine the significant difference in the determined heavy metal concentration while considering all the sampled sites.

### **3. Results and discussion**

### **3.1 Heavy metal concentration in soil samples**

The results of the described field sampling are summarized in **Tables 1**–**4**. The results depicted the varying concentrations of each heavy metals at each of the


### **Table 1.**

*Concentration of zinc.*

respective sampling points and depths. Results were compared with the upper limit threshold for agricultural soils in South Africa [67].

### *3.1.1 Zinc concentration*

The ICP-MS result of Zn concentration indicated that the concentration of Zn was from 4.6 mg/kg to 9.2 mg/kg at a depth of 0–10 cm (topsoil). At a depth of 10–20 cm (subsurface), it ranged from 6.9 mg/kg to 12,8 mg/kg. These were from 500 m from the gold mine.

At 1 km radius from the mining site, the maximum recorded concentration for Zn was 12.8 mg/kg and the minimum was 6.9 mg/kg at the topsoil. At 10–20 cm, the maximum recorded concentration was 14.7 mg/kg and the lowest was 6.3 mg/kg (**Table 1**). The concentration of Zn at all the sampled site were below the permissible limit of Zn in South African agricultural soil, 200 mg/kg [67].

When the concentrations of Zn from the study area were compared with the Zn concentration from the control site, the mean of Zn concentration at the sampled sites were more than the concentration of Zn from the Background sample at 10–20 cm within the 0.5 km radius of the mine (**Table 1**).

Based on the result from 500 range of the mine, the concentrations of Zn are lower at the topsoil than at the subsurface. Akin to the findings of Ekweu et al. [68] where higher concentration of Zn were reported at a depth of 15–20 cm than at a depth of 0–15 cm. Leaching effect was reported to be responsible. However, in the study of Raulinaitis et al. [69], the concentration of Zn at the topsoil, 36.8 mg/kg was higher than at the subsurface, 18.3 mg/kg.

*Toxic Heavy Metals in Soil and Plants from a Gold Mining Area, South Africa DOI: http://dx.doi.org/10.5772/intechopen.109639*


**Table 2.**

*Concentrations of cadmium.*


**Table 3.** *Concentration of lead.*


*Toxic Heavy Metals in Soil and Plants from a Gold Mining Area, South Africa DOI: http://dx.doi.org/10.5772/intechopen.109639*

### **Table 4.**

*Concentration of arsenic.*

In comparison with the permissible limit of Zn in other countries, the concentration of Zn reported in this study is lower in many folds than the following countries - Austria (111 mg/kg), China (74.2 mg/kg), Germany (225 mg/kg) and USA (60 mg/kg) [70]. As result, we can conclude that the soil within the study area is not polluted.

### *3.1.2 Cadmium concentrations*

The maximum concentration of Cd recorded at the topsoil, 500 m away from the mining site was 0.023 mg/kg and the minimum was 0.008 mg/kg. At the subsurface, the maximum concentration recorded was 0.395 mg/kg and the lowest was 0.010 mg/ kg. Similar concentrations of Cd were also recorded at 1 km radius of the mine at the specific sampling depths (**Table 2**).

The average concentration of Cd is lower at the topsoil than at the subsurface at both distances from the mine. The mean concentration of Cd at both distances showed that the study area is not polluted with Cd because they are lower than the permissible limit of Cd in South African soil used for agriculture, 3.00 mg/kg [67] and the mean concentration of Cd reported in China, 0.1 mg/kg, Japan, 0.41 mg/kg and in the United Kingdom, 0.62 mg/kg [70].

When the concentration of Cd from the study area is compared with the concentration from the Background samples, the mean concentration of Cd is more at the subsurface than the concentration of Cd at the background sample site 500 m away from the mining site (**Table 2**). Based on depths (0 10 cm and 10–20 cm), the mean concentration of Cd at the topsoil is lower is lower than the mean concentration of Cd at the subsurface. This is similar to the findings of Ekwue et al. [68] and Raulinaitis

et al. [69] where they both reported higher concentration of Cd at the subsurface than the concentration of Cd at the topsoil.

### *3.1.3 Lead concentrations*

The highest concentration of Pb recorded was 3.9 mg/kg and the lowest was 2.97 mg/kg at 0.5 km distance away from the mine at the topsoil. At the subsurface, the maximum concentration recorded was 5.1 mg/kg while the lowest recorded concentration of Pb was 0.3 mg/kg.

1 km away from the mine, the concentration of Pb was between 3.2 mg/kg and 5.1 mg/kg at the topsoil and 3.2 mg/kg and 4.9 mg/kg at the subsurface. Lead's average concentration in the topsoil 0.5 km away from the mine is lower than at the subsurface and vice-versa when compared with the mean concentration of Pb at 1 km away from the mine (**Table 3**). This is a result of atmospheric deposition from vehicular activity [71] because most of the sampled sites about 1 km from the mine are closer to roads leading to the farms.

Overall, the study area is not polluted because the mean concentration of Pb is lower than the permissible limit of Pb in South African soil used for agriculture, 100 mg/kg [67]. The mean concentration of Pb is also lower than the mean reported in China, 26 mg/kg, Japan, 20.4 mg/kg and the UK, 29.2 mg/kg at every sampled location [70].

In addition, the average concentration of Pb at all the sampled sites and depths are lower than the concentration of Pb from the control sites. This means human activities are responsible for the elevation of Pb at the study area such as gold mining and farming [72].

### *3.1.4 Arsenic concentrations*

The maximum concentration of As recorded at the topsoil was 0.99 mg/kg and 3.66 mg/kg at the subsurface. The minimum concentration of As recorded was 0.66 mg/kg at the topsoil and 0.15 mg/kg at the subsurface (**Table 4**) at 500 m away from the mining area.

At 1 km away from the mine, the maximum concentration recorded at the topsoil and subsurface are 1.3 mg/kg and 3.5 mg/kg respectively. The lowest recorded concentration of As were approximately 0.6 mg/kg at both the topsoil and subsurface (**Table 4**).

The mean concentration of As were higher at the subsurface than at the topsoil at both 500 m and 1000 m away from the gold mine. This is comparable to the findings of Wahl [73] which reported the same trend in As concentration in soils around a gold mine in Kwa-Zulu-Natal Province, South Africa. The mean concentration of As at both locations and depths are below the permissible limit of As in South African soils used for agriculture, 5.8 mg/kg [67]. This means that the soil is not contaminated.

At the control sites, the higher concentrations of As recorded at a depth of 10–20 cm than at a depth of 0–10 cm. the recorded concentration of As at the control sites were all higher than the mean concentration of As at both depths and distance except the mean concentration of the topsoil at a distance of 500 m from the mine (**Table 4**). Similar to the study of Ekwue et al. [68], the concentration of As increase from the topsoil to the subsurface.

Higher As concentrations have been reported in other countries - Germany (50 mg/kg) [74], Australia (20 mg/kg), [75], China (30 mg/kg), [76]; and Canada (12 mg/kg), [77] at every sampled location.

Generally, the total soil concentrations of As, Cd, Pb and Zn are below the upper limit threshold in comparison to the background samples and to other country's concentration. It suggests that the mining activity has not yet impacted the concentration of heavy metals in the soil because the concentration of the background sample has similar soil concentration as that obtained around the mine.

### **3.2 Pollution load index**

The eq. 1 and 2 were used to calculate the contamination factor, CF, and pollution load index, PLI, of each studied heavy metal, respectively. The results are shown in **Tables 5**–**8**. The CF of Zn ranged from 0 to 2 within the 500 m radius of the mine. It means the area contamination ranged from a low contamination, CF < 1, to moderate contamination, 1 ≤ CF < 6, at all the sampled sites (**Table 5**). The PLI of Zn indicated that all the sampled sites are polluted (**Table 5**).

For Pb, the CF shows that the area is very highly contaminated, CF > 6, at the subsurface (0–10 cm) at both distances (**Table 6**). At 10–20 cm depth, the CF ranged from low contamination to high contamination, 3 ≤ CF < 6 (**Table 6**). The PLI for Pb shows that the study area is polluted.

The CF for As ranged from low contamination to moderate contamination at all the sites (**Table 7**). The PLI result indicated that the studied area is polluted except at the depth of 10–20 cm with PLI < 1.

The CF of Cd results show 82% of the sampled sites are moderately contaminated while 14% are lowly contaminated. However, at a depth of 10–20, the Northeastern site has a very high contamination, CF > 6 (**Table 8**). The PLI result indicated that all the sampled sites are polluted except at a depth of 10–20 cm with a PLI below 1 (**Table 8**).
