3.1. Heavy Metal Contamination
The chemical properties, such as pH, OM, total nitrogen (TN), CEC, and Fe, concentration of these soil samples around smelters are shown in
Table 2. The average pH of the soil samples was 8.55, indicating that the soil in this area is alkaline. The average OM, CEC, and TN were 2.57%, 22.91 cmol/kg, 0.21%, respectively, which means the soil is still relatively fertile.
The mean concentrations of As, Cd, Co, Cr, Cu, Ni, Pb, V, and Zn were 17.34, 2.6, 15.97, 101.07, 18.28, 9.31, 67.26, 45.95, and 72.18 mg/kg, respectively (
Table 3). The mean concentrations of Pb, Cd, As, Zn, and Cr in these soil samples were all higher than that of the background value of Henan Province. However, the mean concentrations of Cu, Ni, and V were not higher than the background value. Among these metals, the mean concentration of Cd was four times higher than the risk screening values for soil contamination of agricultural land in China (0.6 mg/kg, GB15618-2018). Further, the Pb, Cd, and As content in some soil samples also exceeded the risk screening values of China, and the excess rate reached 8.03%, 94.89%, and 10.22%, respectively (
Table 3). This shows that Cd is the most polluting element in soils around the smelter. This result implies that the Pb/Zn smelter has had a great impact on the content of heavy metal(oid)s in the surrounding soil, and the heavy metal(oid)s formed during the production process were distributed via sewage, aerosols, and dust [
26]. The mean content of Pb in soil in the present study was twice as high than that in a study reported by Li et al. in Hunan province [
48], although the As and Cd content were similar.
Table 3 presents the coefficients of variation (CV) for Pb and Cd, which were found to be 167.77% and 165.18%, respectively. These values indicate strong variations, defined by a CV greater than 100%. These significant changes in soil composition are likely the result of smelter discharge and other anthropogenic sources. Conversely, Zn, Cu, and Ni exhibited CVs between 40% and 70%, indicating moderate variations and suggesting little artificial influence by these elements. Furthermore, CVs for Cr, Co, V, and As were less than 40%, indicating weak variations. The findings suggest that these elements are influenced primarily by more uniform local parent materials without anthropogenic influence.
3.2. Spatial Distribution of Heavy Metal(oid)s
Precise spatial distribution maps of heavy metal(oid)s are an essential key to reducing the negative impacts on the ecosystem and residents [
50]. According to
Figure 2, the spatial distribution of heavy metal(oid)s can be classified into three categories based on the influence of the smelter. First, the concentration of Pb and Cd gradually decreases as distance from the center of the smelter increases. However, in the study area, only the west, east, and north areas meet the national soil environmental quality standard of 0.6 mg/kg, indicating that local soil has been severely contaminated by Cd. Furthermore, within a 0.5 km radius of the smelter, the Pb content exceeds the risk screening values of 170 mg/kg. Field surveys reveal that the soil in the study area was not irrigated with sewage and that there was no haphazard piling of smelting slag. Thus, it can be inferred that the presence of Pb and Cd in the soil was due to the emission of smoke and dust from the smelter, which settled near the smelter through atmospheric sedimentation [
24]. The distribution of Pb and Cd takes on an oval shape that extends from northeast to southwest, which is likely linked to the southwesterly wind direction. Zhou et al. [
22] reported that heavy metal(loid)s were mainly deposited within a 2 km distance to the smelters. In this study, the area influenced by Cd is larger than 4 km, Pb is 3 km, while the other elements are less than 2 km. In the process of atmospheric migration, with the increase of distance, the difference of the mean concentrations of the easily diffusible heavy metals becomes smaller [
27].
Second, the concentrations of Zn, Cu, and As in the soil gradually decreased with increasing distance from the smelter, with a less pronounced decrease trend observed for Zn, Cu, and As compared to Pb and Cd. This indicate that Cd diffused more easily into a larger area than Zn, and farther away from the smelter [
27]. The concentration of As in the soil near the smelter exceeded the national standard value of 25 mg/kg, but gradually decreased as the distance from the smelter increased. Regions with higher Cu concentrations were mainly concentrated in the southeast of the study area, and these regions were found to be consistent with the locations of chemical fertilizer plants and animal farms (
Figure 2). It is inferred that the pollution sources of Cu in the study area are the chemical fertilizer plants and animal farms, which contribute to the overall increase of Cu in the soil. Previous research has reported that the average Cu concentration in pig feed sold in China has reached 200–300 mg/kg [
51]. Additionally, aquaculture wastewater from the farms is commonly used for sewage irrigation, and animal manure is used as organic fertilizer in farmland, which is the primary reason for the high concentration of Cu in the soil surrounding the animal farms.
Third, the spatial distribution of Ni, V, Cr, and Co within the study area exhibited noticeable similarity, characterized by a distinct zonal pattern with elevated concentrations in the northwest-southeast along with the presence of 2–3 high-value centers. This distribution pattern is primarily attributable to the presence of the provincial highway (S233) traversing the area from north to south, as shown in
Figure 1. Notably, the highway is surrounded by several industrial facilities such as fertilizer and briquette plants, among others. Soils in close proximity to the highway exhibited slightly higher concentrations of Ni, V, Cr, and Co, with the highest values recorded in the vicinity of the chemical fertilizer plant located in the southwest. The concentrations of these elements in the study area were relatively low, falling below national standards, with the lowest concentrations recorded in the northeastern region of the study area.
In general, the smelter has a significant impact on the concentration of Cd and Pb in the soil, particularly Cd. The distribution of these metals in the vicinity of the smelter displays a clear pattern of point source pollution diffusion model, with concentrations decreasing gradually with distance from the smelter. This phenomenon may be attributed to the deposition of atmospheric particulate matter from the smelter, which then settles in the surrounding area. The dispersion of Zn, Cu, and As is also influenced by the smelter, albeit to a lesser extent, with a limited range of influence. Conversely, the presence of Ni, V, Cr, and Co in the soil is largely independent of the smelter, and is instead influenced by other local industries, such as fertilizer and animal plants.
3.3. Assessment of Potential Ecological Risk (PER)
The potential ecological risk index of heavy metal(oid)s in soils is shown in
Table 3. Notably, the E
r for both Pb and Cd in all soil samples exhibit substantial variance, with Cd displaying the largest range. Relative to the other heavy metal(oid)s, Cd has the most significant range and mean E
r, followed by Pb, As, Co, Cu, Cr, V, and Zn. The mean E
r of Cd exceeds the mean value of the other heavy metal(oid)s by 500–10,000 times, a trend attributed to the higher toxicity response coefficient of Cd. Further, this finding underscores that the Cd levels in soil samples around the smelter are generally higher than the background value. The PER of the nine toxic elements in all the soil samples ranges from 146.86 to 12,549.81, with an average of 1107.18, indicating a high overall ecological risk in the study area (
Table 4).
The percentage of potential ecological risk in different grades for each of the heavy metal(oid)s is shown in
Table 5. For Cd in all sampling sites, 0.73%, 15.33%, and 83.94% of the sites had considerable risk, high risk, and very high potential ecological risk, respectively. For Pb, 91.97% had low risk, 2.19% had moderate risk, 5.11% had considerable risk, and 0.73% of the sites had high risk. For As, 1.46% had moderate risk. Conversely, the potential ecological risks of Zn, Cu, Ni, Cr, Co, and V in all soil samples were low risk (
Table 5). These results indicate that Cd in soil posed the greatest risk to the ecosystem in the study area.
Based on the PER values pertaining to heavy metal(oid)s, it was observed that 40.15%, 51.09%, and 8.03% of the soil samples were associated with high, considerable, and moderate risks, respectively. The total of these three risk levels amounted to 99.27%, thereby suggesting that nearly all the soil within the study area has been exposed to some degree of hazard. Additionally, the contribution rate of Cd to PER in the soil samples ranged from 84.51% to 98.57%, with an average of 92.42%. This underscores the fact that Cd is the primary contaminant present in soils surrounding the smelter.
According to the spatial distribution of PER around the smelter (
Figure 3), the northeast corner of the study area is a low potential ecological risk area, with the PER less than 150, accounting for 0.44% of the total area. This region is situated at a considerable distance from the smelter and has no other industrial facilities in its vicinity. In contrast, moderate, high, and significantly high potential ecological risk areas accounted for 5.71%, 68.78%, and 25.07% of the study area, respectively, as illustrated in
Table 6. The spatial distribution of PER reveals that the areas with significantly high potential ecological risk are primarily concentrated in an oval-shaped region within a 1.5 km radius of the smelter, which closely resembles the spatial distribution of Cd in the study area. Furthermore, the polluted soils with significantly high and high PER covered approximately 93.84% of all the investigated regions, as shown in
Figure 3.