Table 2 presents the total elemental compositions of the ultramafic soil (0–30 cm depth) and the parent material (90 + cm depth). The bedrock composition is typical of serpentinite, consisting of 40% SiO2, 33.6% MgO, 5.99% Fe, 0.24% Ni, 0.86% CaO, and 0.17% Al2O3. The Ni concentration in the bedrock was 0.27%, showing no significant variation among the soils but a notable difference compared to the serpentine in the bedrock, which had 0.3% Ni, indicating signs of weathering.
The silicon oxide (SiO2) content in the soil ranged from 23.31–51.07%, averaging 40.024%. Typically, SiO2 content in ultramafic soils is less than 45% (Vithanage et al. 2014), aligning with the characteristics observed in the study area. The average concentrations of aluminum oxide (Al2O3), magnesium oxide (MgO), calcium oxide (CaO), phosphorus pentoxide (P2O5), potassium oxide (K2O), and sodium oxide (Na2O) in the ultramafic soil (0–30 cm depth) were 4.869%, 15.320%, 3.969%, 0.078%, 0.574%, and 0.042%, respectively (Table 2). Calcium oxide content was generally low across the soils, with some samples showing a significant deficiency, dropping to less than 0.3%.
Comparing the total chemical content of the parent material (SiO2, Al2O3, MgO, and CaO) with that of the soil, it is clear that the SiO2 content is indicative of the surface soil. The concentrations of Al2O3 and CaO are higher than in the bedrock, indicating some degree of weathering. In contrast, the parent material has a higher MgO concentration. The increased MgO content at a depth of more than 90 cm compared to the upper layer suggests the parent material is rich in magnesium, whereas iron is relatively more abundant in the soil (Bani et al. 2014). Previous research has reported low Al2O3 (Echevarria 2018) and CaO (Tashakor et al. 2014) concentrations and high MgO concentrations (Alexander 2004) in ultramafic soils.
The initial Mg:Si ratio in the bedrock is 2.6, whereas in the soil it is 1.2. This indicates a consistent decrease in the Mg:Si ratio from the bedrock to the soil, corresponding to a loss of magnesium that is more than twice the loss of silicon. This regular decrease in the Mg:Si ratio signifies a substantial loss of magnesium, confirming the degree of soil evolution (Bani et al. 2014).
As far as the Fe:Si ratio is concerned, the trend towards Fe enrichment is present in soils. It is much more pronounced in soils 0.18 in the 0–30 cm horizon of soil I as compared with 0.14 in the bedrock).
The soils are rich in metals typical of serpentines, such as Ni, Co, and Cr, and are relatively infertile in terms of P2O5 and K2O content (Bonifacio and Barberis, 1999). Variations in the percentages of P2O5 and K2O between the lower and upper soil layers may be due to increased mineral decomposition in surface soils. The total phosphorus content of ultramafic soils (0–30 cm) was found to be 0.018% (Xhaferri et al., 2017), aligning with findings from other studies. The total Ni content was high in both the parent material and all soil samples examined (see Table 2).
The average concentrations of minor elements in ultramafic soils (0–30 cm) were as follows: Cr, 2606.57 µg/g; Mn, 1139.18 µg/g; Co, 120.99 µg/g; Ni, 1726.71 µg/g; Cu, 27.76 µg/g; Cd, 0.82 µg/g; and Pb, 5.44 µg/g. These levels generally matched those of the parent material, except for Pb, which was about seven times higher in the topsoil compared to the parent material (+ 90 cm) (Table 2). Numerous studies have highlighted the high concentrations of Ni, Cr, Co, and Mn in ultramafic soils (Reeves et al., 1999; Fantoni et al., 2002; Shanker et al., 2005; Hseu et al., 2007; Susaya et al., 2010; Bani et al., 2009; 2010; 2013; 2014; Butt and Cluzel, 2013; Vithanage et al., 2014; Palm and Volkenburgh, 2014; Kara, 2019).
The World Health Organization (WHO) sets soil standards for Cr, Mn, Co, Ni, Cu, Cd, and Pb at 100, 2000, 50, 50, 100, 3, and 100 µg/g, respectively (Zondo, 2021). Comparing the ultramafic soil results with these standards reveals significantly high levels of Cr and Ni, both exceeding the recommended limits by more than twice. However, the concentrations of Mn, Cu, Cd, and Pb were below the critical thresholds. The elevated levels of Cr, Ni, and Co in the ultramafic soils are attributed to the parent material, which reflects the typical chemical composition of naturally occurring soils, particularly those derived from mafic-ultramafic rocks (ophiolites) (Muhammad et al., 2019; Kara, 2019). Compared to WHO standards, the ultramafic soils exhibit particularly high levels of Cr and Ni, both exceeding the recommended limits by more than twofold, while Mn, Cu, Cd, and Pb remain below critical levels. The high concentrations of Cr, Ni, and Co in these soils are due to the parent material, as the chemical composition of soils generally mirrors that of their parent rocks. Soils derived from mafic-ultramafic rocks, such as ophiolites, typically contain elevated levels of heavy metals like Cr, Ni, and Co (Muhammad et al., 2019; Kara, 2019).
Table 2
Descriptive statistics of the total major and minor element contents of ultramafic soils
Soil Depth
|
Soil Properties
|
Min.
|
Max.
|
Mean
|
|
|
SiO2 (%)
|
23.31
|
51.07
|
40.024
|
|
|
Al2O3 (%)
|
0.02
|
13.96
|
4.869
|
|
|
MgO (%)
|
4.73
|
28.80
|
15.320
|
|
|
CaO (%)
|
0.25
|
22.32
|
3.969
|
|
|
P2O5 (%)
|
0.01
|
0.19
|
0.078
|
|
|
K2O (%)
|
0.11
|
1.23
|
0.574
|
|
|
Na2O (%)
|
0.04
|
0.05
|
0.042
|
|
Soil (0–30 cm)
|
Fe (µɡ/ɡ)
|
42273.90
|
110720.80
|
7.33%
|
|
|
Cr (µɡ/ɡ)
|
450.2
|
5644.1
|
2606.57
|
|
|
Mn (µɡ/ɡ)
|
659.2
|
1680.2
|
1139.18
|
|
|
Co (µɡ/ɡ)
|
38.2
|
187.0
|
120.99
|
|
|
Ni (µɡ/ɡ)
|
254.8
|
2833.0
|
0.17%
|
|
|
Cu (µɡ/ɡ)
|
4.2
|
130.8
|
27.76
|
|
|
Cd (µɡ/ɡ)
|
0.4
|
1.0
|
0.82
|
|
|
Pb (µɡ/ɡ)
|
0.6
|
14.4
|
5.44
|
|
|
SiO2 (%)
|
32.800
|
45.250
|
40.290
|
|
|
Al2O3 (%)
|
0.006
|
1.872
|
0.17
|
|
|
MgO (%)
|
28.390
|
36.610
|
33.617
|
|
|
CaO (%)
|
0.152
|
11.630
|
0.86
|
|
|
P2O5 (%)
|
0.001
|
0.007
|
0.002
|
|
|
K2O (%)
|
0.123
|
0.168
|
0.142
|
|
|
Na2O (%)
|
0.033
|
0.041
|
0.036
|
|
Parent material
|
Fe (µɡ/ɡ)
|
50135.6
|
72042.0
|
5.99%
|
|
|
Cr (µɡ/ɡ)
|
1488.2
|
4175.1
|
2282.48
|
|
|
Mn (µɡ/ɡ)
|
545.4
|
1043.5
|
827.37
|
|
|
Co (µɡ/ɡ)
|
71.0
|
163.0
|
117.15
|
|
|
Ni (µɡ/ɡ)
|
1437.0
|
3127.0
|
0.24%
|
|
|
Cu (µɡ/ɡ)
|
2.0
|
146.7
|
15.28
|
|
|
Cd (µɡ/ɡ)
|
0.6
|
1.3
|
0.79
|
|
|
Pb (µɡ/ɡ)
|
0.4
|
1.2
|
0.71
|
|
The correlation and principal component analyses of the ultramafic soils are outlined in Tables 3 and 4. As indicated in Table 3, Cr shows a negative correlation with CaO and a positive correlation with Ni, Co, and Pb. Similarly, Mn is positively correlated with Fe and Co (Table 3). Additionally, Co has consistent directional relationships with Fe, Cr, Mn, and Ni but shows a negative correlation with CaO (Table 3). These relationships are typical of ultramafic substrates (soils and rocks). Ni content is inversely correlated with CaO, Al2O3, and Cu but positively correlated with MgO, Fe, Cr, and Co (Table 3). This supports findings that ultramafic soils are rich in Ni, MgO, Fe, Cr, and Co but deficient in Ca (Bani et al., 2009; 2013; 2014). Regarding other variables, Cu has a negative correlation with MgO and Ni but a positive correlation with Al2O3. In ultramafic soils, Pb shows similar relationships with Al2O3, SiO2, and Cr and an inverse relationship with MgO. Overall, the total elemental content findings of this study are consistent with previous research (Raisanen et al., 1992; Robinson et al., 1997; Cheng et al., 2009; Duplay et al., 2014; Bani et al., 2014; Tashakor et al., 2017).
Table 3 Correlation analysis of ultramafic parent material and soils
In Table 4, the principal component analysis of ultramafic soils identified four components with eigenvalues of 1 or higher. These components explained 25.97% of the variance in PC-1, 25.95% in PC-2, 17.72% in PC-3, and 12.52% in PC-4, together accounting for 82.16% of the total variance (Table 4). For PC-1, which accounted for 25.97% of the variance in soil chemical element contents, MgO, Ni, Cu, and Al2O3 were grouped together. Among these, Cu and Al2O3 had positive loadings, while MgO and Ni had negative loadings relative to the other parameters (Table 4). PC-2, which explained 25.95% of the variance, showed consistent directional loadings for Fe, Mn, Co, and Ni, with an inverse relationship to CaO. Ni served as a linking variable between PC-1 and PC-2, while CaO linked PC-2 and PC-4. Several soil variables, including MgO, Al2O3, CaO, Fe, Mn, Co, Cd, Cu, and Ni, likely share a common source. In PC-3, SiO2, Cr, and Pb formed a cluster with positive loadings among them. CaO and Cd were grouped in PC-4 (Table 4).
Table 4
Principal component analysis of ultramafic soils
|
PC-1
|
PC-2
|
PC-3
|
PC-4
|
SiO2
|
…..
|
…..
|
0.680
|
…..
|
Al2O3
|
0.882
|
…..
|
…..
|
…..
|
MgO
|
-0.721
|
…..
|
…..
|
…..
|
CaO
|
…..
|
-0.565
|
…..
|
0.607
|
Fe
|
…..
|
0.909
|
…..
|
…..
|
Cr
|
…..
|
…..
|
0.574
|
…..
|
Mn
|
…..
|
0.799
|
…..
|
…..
|
Co
|
…..
|
0.772
|
…..
|
…..
|
Ni
|
-0.726
|
0.588
|
…..
|
…..
|
Cu
|
0.849
|
…..
|
…..
|
…..
|
Cd
|
…..
|
…..
|
…..
|
0.866
|
Pb
|
…..
|
…..
|
0.918
|
…..
|
EV
|
4.19
|
3.12
|
1.38
|
1.16
|
CR (%)
|
25.97
|
25.95
|
17.72
|
12.52
|
CCR (%)
|
25.97
|
51.93
|
69.64
|
82.16
|
**EV: Eigenvalues, CR: Contribution rate (%), CCR: Cumulative Contribution rate (%) |
The pollution index values (EF, Igeo, PLI, CF) of the ultramafic soils are shown in Table 5. Based on the average EF values, Pb had the highest value (6.35), followed by Cu (2.33), Mn (0.98), Cd (0.90), Cr (0.89), Co (0.86), and Ni (0.59). According to the EF classification system, Pb (5 < EF < 20) is categorized as high enrichment, Cu (2 < EF < 5) as medium enrichment, and the other metals as low enrichment since their EF values are below 2 (Sutherland, 2000).
Based on the geoaccumulation index (Igeo) results for the ultramafic soils, Pb (2.131) exhibited the highest value, followed by Cu (0.446), while the other metals had values close to zero. Accordingly, Pb (2 < Igeo < 3) was categorized as moderately severely contaminated, whereas other metals (Mn, Cu, Ni, Co, and Cr) fell into the uncontaminated-moderately contaminated range (0 < Igeo < 1), and Cd (0 < Igeo) was classified as uncontaminated (Müller, 1969).
Based on the pollution load index (PLI) values of the ultramafic soils, the concentrations of Pb and Cu were greater than 1, indicating pollution, while the PLI values of Mn, Cd, Cr, Co, and Ni were less than 1 (Table 5). According to Varol (2011), a PLI greater than 1 signifies pollution caused by the metals present, thus categorizing Pb and Cu as pollutants.
Examining the CF values of the soils, Mn, Cr, Co, and Ni exhibited CF values of 1 or less for heavy metals. Among the other elements, Cu had a CF value of 2.27, while Pb had a CF value of 5.08 (Table 5). According to the evaluation system reported by Hakanson (1980), Pb (3 < CF < 6) was classified as a significant contaminant, Cu (1 < CF < 3) as a moderate contaminant, and Mn, Cr, Co, and Ni as minor contaminants.
Based on the pollution index results (EF, Igeo, PLI, and CF), the index values for Pb and Cu were higher than those for other analyzed elements. Analysis of areas with elevated Pb and Cu levels in the research zone showed their proximity to roads or agricultural lands. The high Pb and Cu pollution indices in soils derived from serpentine parent material in the study area may be associated with their proximity to highways and the use of Cu-containing agricultural chemicals (Fig. 1).
Traffic-related pollution from vehicle emissions and mechanical wear leads to the buildup of heavy metals (Pb and Cu) in agricultural lands (Ndiokwere, 1984; Carlosena et al., 1998; Imperato et al., 2003; Çelik et al., 2005). Significant increases in heavy metal levels, including Pb, Cu, Al, Mo, Hg, and Se, have been observed in agricultural soils near highways (Ho and Tai, 1988; Blake and Goulding, 2002; Lia et al., 2007). Similarly, elevated Cu concentrations have been reported in agricultural soils close to heavily trafficked highways (Zehetner et al., 2009; Zhang et al., 2012).
Additionally, the friction between brakes, moving engine parts, and other vehicle components can contribute to the release of Cu (Davis et al., 2001). While the concentrations of Ni, Cr, and Co in the ultramafic soils of the research area exceeded the maximum critical values recommended by the World Health Organization, their pollution indices were lower than those of Pb and Cu. Therefore, Ni, Cr, and Co mainly accumulate in ultramafic soils from parent material sources. This study highlights the importance of considering both lower-depth and upper-depth soils when assessing the origin of pollutants. The significance of including the parent material (90 + cm depth) in soil heavy metal calculations has been noted (Reimann and Caritat, 2005). Other researchers have also indicated that inaccuracies can arise from the choice of reference material in calculations (Blaser et al., 2000; Githaiga et al., 2001; Aytop et al., 2023).
Table 5
Pollution index values of ultramafic soils
Pollution indices
|
|
Cr
|
Mn
|
Co
|
Ni
|
Cu
|
Cd
|
Pb
|
Enrichment factor (EF)
|
Minimum
|
0.16
|
0.76
|
0.34
|
0.07
|
0.62
|
0.4
|
0.92
|
Maximum
|
1.69
|
1.24
|
1.47
|
0.82
|
8.2
|
1.48
|
15.2
|
Mean
|
0.89
|
0.98
|
0.86
|
0.59
|
2.33
|
0.90
|
6.35
|
Geoaccumulation Index (Igeo)
|
Minimum
|
0.002
|
0.006
|
0.028
|
0.002
|
0.032
|
-1.102
|
0.067
|
Maximum
|
0.005
|
0.013
|
0.069
|
0.005
|
1.360
|
-0.062
|
4.010
|
Mean
|
0.003
|
0.008
|
0.040
|
0.003
|
0.446
|
-0.266
|
2.131
|
Pollution Load Index (PLI)
|
Minimum
|
0.40
|
0.89
|
0.58
|
0.26
|
0.79
|
0.63
|
0.96
|
Maximum
|
1.38
|
1.09
|
1.21
|
0.90
|
2.37
|
1.21
|
2.85
|
Mean
|
0.93
|
0.99
|
0.92
|
0.75
|
1.45
|
0.94
|
2.20
|
Contamination factor (CF)
|
Minimum
|
0.16
|
0.79
|
0.34
|
0.07
|
0.62
|
0.4
|
0.92
|
Maximum
|
1.9
|
1.46
|
1.47
|
0.82
|
5.6
|
1.48
|
8.14
|
Mean
|
0.92
|
1.00
|
0.86
|
0.59
|
2.27
|
0.90
|
5.08
|