3.1.1. Concentrations of Soil Heavy Metals
As shown in
Table 2, the average concentrations of eight heavy metals (Cu, Zn, Hg, Ni, Pb, Cr, Cd and As) in the farmland soils of the Xiaohe River irrigation area were 43.22 ± 16.44, 146.47 ± 42.44, 0.26 ± 0.12, 38.07 ± 6.47, 37.21 ± 14.06, 57.16 ± 12.81, 0.41 ± 0.25 and 13.08 ± 4.27 mg·kg
−1, respectively. The concentrations of all the soil heavy metals detected in this study exceeded the background values of soil elements of Shanxi Province (
Table 2) [
19], while they were lower than the environmental risk control standard for soil contamination of agricultural land (GB 15618−2018, grade II, China) [
25]. Compared to the average values of soil heavy metals in China [
26], only the concentration of Cr (57.16 ± 12.81 mg·kg
−1) in the study area was lower than the standard (61.0 mg·kg
−1), which was about 93.7% of its average value. The average values of Cu, Zn, Hg, Ni, Pb, Cd and As were 187.9%, 197.9%, 400%, 141%, 137.8%, 422.7% and 118.9% more than the average values of soil heavy metals in China, respectively. Notably, the concentrations of Hg and Cd seriously exceeded the standard, followed by Cu and Zn, indicating a certain risk of heavy metal pollution in the study area.
Relevant studies show that in the Loess Plateau and North China Plain where human activities are intensive and soil erosion is serious, the weathering of surface rocks under human action will accelerate, which will lead to the increase of heavy metals in shallow soil [
13,
16]. At the same time, the solid waste and wastewater produced by human production and living activities will also lead to a concentration of heavy metals in shallow soil higher than the environmental background value. The Xiaohe River irrigation area in this study is located in the ecological transition zone between the Loess Plateau and the North China Plain. The environmental gradient changes are obvious and show vulnerability to external interference. Human activities have a great impact on the landforms of this area. The concentration characteristics of soil heavy metals are consistent with the previous research results [
8,
14,
26,
27].
The variation coefficients (CV) of eight soil heavy metal concentrations in the study area generally decreased in the following sequences: Cd (0.61) > Hg (0.46) > Pb (0.39) > Cu (0.38) > As (0.33) > Zn (0.29) > Cr (0.22) > Ni (0.17), respectively. The CV of Ni, Cr, Zn and As were between 0.16 and 0.35, belonging to medium variation, and the variation coefficients of Cu, Pb, Hg and Cd were higher than 0.36 with a high variation. Generally, the concentration of trace elements in the natural background of a specific region less affected by human activities is low and the spatial variation is small [
3,
18], but the CV of soil heavy metals in this study were relatively high. This is mainly due to the influence of intensive human activities on sampling sites in the agricultural irrigation area. Therefore, the higher CV of soil heavy metals in the study area were mainly affected by the changes in local natural environmental factors and agricultural activities.
The skewness of Ni and As was lower than 0 with a negative deviation, while the skewness of the other six soil heavy metals was greater than 0, showing a positive deviation. However, the kurtosis of all the soil heavy metals was greater than 0, and the data distribution was relatively concentrated in this study (
Table 2). The kurtosis and skewness of Cu, Zn, Hg and Cd were all high, indicating that a higher accumulation state of Cu, Zn, Hg and Cd in the soil of the Xiaohe River irrigation area [
28], which resulted in the continuous enrichment of heavy metals in the soil of the study area. In general, compared to similar studies in other regions [
1,
2,
9,
11,
20], the concentrations of soil heavy metals in the Xiaohe River irrigation area were generally at a lower level, but the pollution of Hg and Cd were serious, together with a risk of Cu and Zn pollution.
3.1.2. Spatial Distribution of Soil Heavy Metals
As a whole, due to the fragmentation of land use types and the diversification of human activities, the concentration and changes of soil heavy metals were affected by many factors [
2,
26]. Relevant studies show that the physical and chemical properties of the soil will affect the migration and adsorption capacity of heavy metals. The adsorption capacity of soil for cadmium is very strong, and the migration capacity of cadmium in soil is weak and almost immovable. At the same time, cadmium rarely migrates to deeper layers, so it accumulates in the surface soil [
29,
30]. The migration capacity of mercury in soil is similar to that of cadmium in soil. Due to the influence of a limited biological cycle and humus in the upper layer of soil, mercury in soil accumulates in the surface soil [
30,
31]. The toxicity of arsenic accumulated in the surface soil decreases with the increase in the number of bound organic groups [
32]. Lead mainly exists as Pb
2+ insoluble compounds in the surface soil, while the concentration of soluble lead is generally high in acidic soil, et al. [
33]. In order to analyse the spatial distribution characteristics of soil heavy metals more scientifically and comprehensively in the study area, the sampling sites were classified based on soil types and land use types in this study.
The soil types of the seven sampling sites in the study area were cinnamon soil and fluvo aquic soil [
13]. The seven sampling sites were classified into outer suburbs farmland (WW and CJ), suburban farmland (ZJ and SD), and urban areas (XH, ZQ and XW) according to the land use types, respectively. According to our field investigation, cultivated land, forest land, unused land and grassland are the main land use types in the outer suburbs farmland. Human farming activities and the living activities of a small number of residents are the main human activities in this area. Construction land, urban residential areas and a small amount of basic transportation land are the main land use types in the suburban farmland. Production and living of urban residents and a small amount of transportation are the main human activities in this area. Industrial and mining enterprises and tertiary industries are the main land use types in the urban areas. A large number of commercial activities, agricultural and sideline product processing, transportation and other activities are the main human activities in this area. The spatial distribution of soil heavy metal concentrations in the study generated a sequence of urban area > suburban farmland > outer suburbs farmland (
Figure 2). The average concentrations of Hg, Cd, Cu and Zn in soils of urban areas were significantly higher than those of suburban farmland and outer suburbs farmland, and the average content of Ni in soils of outer suburbs farmland was significantly lower than that in urban areas and suburban farmland. The average concentrations of Ni, As and Pb in XH, ZQ, ZJ and XW were significantly higher than those in WW, CJ and SD, while the average concentration of Cr showed no significant change in all sampling sites (
Figure 2).
Previous studies showed that Ni, As and Cr in soil mainly originated from natural factors, such as hydrological erosion, rock weathering, etc. [
26,
28]. On the other hand, Hg, Cd, Cu and Zn originated mainly from human factors, such as agricultural sewage irrigation, mining activities, fossil fuels, garbage infiltration, etc. [
34]. Consequently, the sampling sites in the study area are mostly distributed in the river valley, and most of the surface tidal soil and cinnamon soil have been reclaimed as farmland finally leading to serious hydrological erosion and obvious rock weathering. The long−term agricultural activities and urbanization construction in recent decades have led to serious hydrological erosion and obvious rock weathering in the study area. At the same time, the study area is located in the lower reaches of the Xiaohe River with flat terrain, and the relatively slow water flow velocity will also lead to the accumulation of heavy metals. Therefore, the average concentrations of Ni, As and Cr in soil exceeded the soil background value of Shanxi Province.
China has the largest Cd reserves in the world, and the output of Hg, Cd, Cu and Zn has more than doubled in recent 10 years [
35]. Therefore, mining activities have great hidden dangers to environmental safety and soil heavy metal pollution. However, there was no cadmium ore distribution in Shanxi Province, and field investigation showed that the sampling sites with higher than average concentrations of Hg, Cd, Cu and Zn were mostly distributed along national roads, cities, towns and other areas with intensive human activities (
Figure 3a–c,g). Based on the above analysis, it was speculated that human activities were the main factors affecting the spatial distribution of soil heavy metal concentrations in the study area, and mining activities have little impact, while transportation and the service industry were also the main sources leading to local soil heavy metal pollution.
When the study area is dominated by point source pollution and the potential pollution sources are complex, more analysis methods and field investigation are particularly important for the traceability of soil heavy metals [
7,
11,
18,
20]. In order to reveal the spatial distribution characteristics and their relationship to potential pollution sources of soil heavy metals in the study area more accurately, kriging interpolation and correlation analysis were carried out in this study. The current distribution of the study area and spatial distribution of soil heavy metals in the Xiaohe River irrigation area (
Figure 3) were presented by ArcGIS kriging interpolation.
The spatial distribution change of Cd was the most significant; the sampling site with the highest concentration (XW) was 8.3 times that of the sampling site with the lowest concentration (CJ). Followed by Hg, the sampling site with the highest concentration (ZQ) was 8 times greater than that of the sampling site with the lowest concentration (WW). In the meantime, the concentration of Cd of all the sampling sites exceeds the background value of Shanxi Province [
19] and the average value of soil heavy metals in China [
26]. Furthermore, compared with the average value of soil heavy metals in China, the over standard rates of Cd and Hg were 423% and 400%, respectively. Therefore, the heavy metal pollution of Cd and Hg should be paid attention to by the local government. The spatial distribution of Pb also changed significantly. The maximum value was found at sampling site XW (66.03 mg·kg
−1) and the minimum value was found at sampling site WW (9.16 mg·kg
−1). The more significant spatial distribution characteristics also showed that point source pollution was the main factor affecting the distribution of Pb in the study area [
24,
28]. Moreover, concentrations of Cu, Zn and As in the study area have exceeded 188%, 198% and 189% of the average value of soil heavy metals in China, respectively, and the spatial variation was obvious, indicating that the point source pollution at each sampling site has a great impact on the concentrations of Cu, Zn and As in the soil (
Figure 3). It is worth noting that the concentrations of Cr and Ni in the study area were low and the spatial distribution change was not obvious. In particular, the concentration of Cr was 93.7% of the average value of soil heavy metals in China, indicating that non−point source pollution or natural sources were the main factors affecting the concentrations of Cr and Ni in the study area [
14,
21].
The correlation analysis results support the conclusions of the spatial distribution characteristics of soil heavy metals and the field investigation of potential pollution sources (
Table 3). There was a significant positive correlation between Zn and Cu, Pb, Cd, Ni and Hg in the study area (
p < 0.01).
Relevant studies show a significant positive correlation between heavy metals indicating that the concentrations of heavy metals show a synergistic change trend in nature. In particular, it shows similarity or convergence in potential pollution sources and environmental geochemical behaviours [
1,
4,
10]. In recent decades, secondary and tertiary industries in China and other developing countries have developed rapidly, and their proportion in the national economic pattern has gradually increased. A large number of handicraft, manufacturing, construction and coal chemical enterprises have appeared in the study area. In the early 20th century, the environmental detection means in the study area were relatively limited and the local government’s awareness of environmental protection was relatively weak. A large amount of wastewater, waste residue and waste gas containing heavy metal pollutants was directly discharged, which seriously polluted the soil in the study area and was difficult to remove from the natural substrate [
15,
16]. Although the local government has invested a lot of manpower and material resources to control the local environment in recent years, especially regarding heavy metal pollution, the concentrations of heavy metals in the soil of the study area were still at a high level due to the characteristics of difficult biodegradation, obvious accumulation effects and high repair difficulty of heavy metals.