Source analysis and ecological risk assessment of heavy metals in farmland soils around heavy metal industry in Anxin County
Descriptive statistical analysis
Basic properties of soil
According to Fig. 3, the soil pH in the study area ranges from 7.50 to 8.18, with a mean of 7.82, regarding the Specification of land quality geochemical assessment (DZ/T 0295-2016)44 for the definition of soil acidity, the Kriging interpolation analysis of soil acidity is conducted by using ArcGIS, and the results are shown in Fig. 4. All the soil samples are alkaline, and the soil in the study area was mainly alkaline. When the soil environment where the heavy metal pollutants are located is alkaline, the heavy metals adsorbed on the soil colloids are easy to combine with the hydroxide ion in the alkaline soil to form stable compounds. Heavy metals’ dissolution rate and solubility decreased, making the heavy metals in the soil not easy to migrate and deposit45. When plants absorb heavy metals, they will cause a decline in crop yield and quality and endanger human health through the food chain.
Box-plot of soil pH distribution.
Spatial distribution of soil pH in sampling area.
Statistical description of heavy metal content
Table 4 is a descriptive statistical analysis of heavy metals in the Anxin County and lists the statistical results of the contents of five heavy metals of Cu, Zn, Pb, Cd, and Ni in the study area. The average contents of Cu, Zn, Pb, Cd, and Ni are 63.73 ± 11.74, 171.66 ± 28.79, 136.69 ± 51.07, 0.73 ± 0.25 and 41.43 ± 2.81 mg·kg−1, respectively. The average values of Cu, Zn, Pb, Cd, and Ni are 2.92 times, 2.19 times, 6.36 times, 7.78 times, and 1.34 times of the background values of soil elements in Hebei, respectively. The contents of Cu, Zn, and Ni in the soil are all less than the risk screening value, and the risk to the quality and safety of agricultural products, crop growth, or soil ecological environment is low and can be ignored in general. The soil Pb and Cd content percentages between the risk screening value and risk intervention value are 16.67% and 64.58%, respectively. The problem of exceeding the standard of Pb and Cd is more serious, and there may be risks to the quality and safety of agricultural products, crop growth, or soil ecological environment, and safe utilization measures should be taken.
The coefficient of variation (CV) is the ratio of standard deviation to the average value, which can characterize the degree of dispersion of data. The CV is classified into a weak variation (CV < 0.1), moderate variation (0.1 < CV < 0.9), or high variation (CV > 0.9)46. By analyzing the characteristics of heavy metal content in the study area, it can be seen that the distribution of the five elements in the local soil is moderately variable, and the CV of Pb and Cd is large. It shows that the distribution of Pb and Cd is uneven, and it is affected to some extent by human activities.
It can be seen from Table 4 that the kurtosis and skewness of Cu and Cd are closer to 0. The K-S test shows that the two-sided significance of Cu, Zn, Pb, and Cd are all greater than 0.05, and the data of the four heavy metals obey the normal distribution. The two-sided significance of Ni is less than 0.05, and there is a significant deviation in the data distribution, which does not obey the normal distribution.
Level distribution characteristics of soil heavy metals
Figure 5 is a spatial analysis of the distribution of different heavy metal elements in the soil of the study area. The results show that Zn shows a high-concentration band distribution in the south of the study area; Cu shows a large-scale block distribution in the study area; Cd and Pb show a similar distribution trend in the study area, with high concentration areas in the southeast of the study area; For Ni, the concentration is higher in the northwest of the study area. According to Figs. 2 and 5, it can be seen that the farmland soils in the study area have generally high levels of heavy metals, and Pb and Cd pollution is severe. It is related to the presence of metal-smelting factories in the surroundings. The extensive industrial model is the primary source of heavy metal pollution in the surrounding soils. At the same time, the large traffic flow in the study area, vehicle exhaust emissions, oil leakage, and rubber tire wear may also be important reasons for the enrichment of heavy metals such as Cd and Pb in the soil in this area47.
Spatial distribution of heavy metal elements in soil of the study area (a) Zn (b) Cd (c) Cu (d) Pb (e) Ni.
Characteristics of Cd and Pb content in soil profile
In the vertical section of the soil, the distribution of elements is not only restricted by its geochemical properties, soil-forming parent material, and soil-forming processes under natural conditions48 but also closely related to human activities49. The same element may have multiple distribution types in different sections of soil profiles, but the probability of a specific type is relatively high. In the four soil profiles of D0, D1, D2, and D3 studied in this work, the vertical distribution characteristics of heavy metals Cd and Pb on the profiles have both commonalities and differences. According to Figs. 6 and 7. The heavy metal cadmium and lead in the soil is mainly distributed in the 0–20 cm of the soil surface. As the depth increases, the content of elements that migrate to a depth below 20 cm is significantly reduced, and the content of cadmium and lead generally appears to become smaller from shallow to deep. Trends and changes are relatively large. The enrichment of elements in the surface soil may be related to the fact that the farmland has been affected by the surrounding human activities50, including perennial agricultural farming, road transportation, and the production activities of the surrounding heavy metal processing industry. These factors have significant effects on the vertical migration and enrichment of heavy metals in the soil. The vertical distribution trends of heavy metal Cd and Pb elements in the four soil profiles are the same, but the contents in the 0–20 cm layer are different. The main reason is that the four soil profiles are located in different blocks. D1, D2, and D3 are heavy metal-contaminated farmland soils in the study area.
Cd concentrations at different depth in soil profiles. (a) D3 soil profile (b) D2 soil profile (c) D1 soil profile (d) D0 soil profile.
Pb concentrations at different depth in soil profiles. (a) D3 soil profile (b) D2 soil profile (c) D1 soil profile (d) D0 soil profile.
Availability characteristics of heavy metals in farmland soil
The available content of heavy metals in soil
Table 5 lists the statistical results of the practical content of the two primary polluting heavy metals, Cd and Pb, in the study area. The practical content of Cd and Pb ranges from 0.22 to 1.15 mg kg−1 and 10.00 to 75.10 mg kg−1, respectively, and the average effective content of Cd and Pb is 0.47 ± 0.18 mg kg−1 and 28.69 ± 13.75 mg kg−1, respectively. By analyzing the dispersion degree of the available content of Cd and Pb in the study area, it can be seen that the distribution of Cd and Pb in the local soil is moderately variable. At the same time, the coefficient of variation of Pb is more significant, indicating that Pb is affected by human activities to a certain extent. The effect is more vital than Cd.
Heavy metal element availability index
The heavy metal element availability index is the ratio of the applicable state content of an element to the total amount of this element in the sample. The activity index can reflect soil’s current and potential supply levels51. The study area is mainly dominated by Cd and Pb pollution, so the practical content of Cd and Pb at 48 sites in the study area is analyzed (Fig. 8). The results show that the availability index of the two heavy metal elements is quite different. The mean values of Cd and Pb availability indices are 58.42% and 20.26%, respectively, and the soil Cd availability index is higher than other elements. Relevant studies have shown that applying calcium fertilizer in weak alkaline soil can reduce the availability index of soil Cd52.
Analysis results of activation rate of lead and cadmium in soil.
Characteristics of heavy metal content in wheat
Table 6 lists the statistical results of Cd and Pb content in wheat grains in the study area. The contents of Cd and Pb varied in the range of 0.02–0.09 mg kg−1 and 0.02–1.03 mg kg−1, respectively, and the average contents of Cd and Pb were 0.04 ± 0.02 mg kg−1 and 0.28 ± 0.22 mg kg−1, respectively. By analyzing the dispersion degree of Cd and Pb content in wheat grains in the study area, it can be seen that the distribution of Cd and Pb in wheat grains is moderately variable, while the coefficient of variation of Pb is closer to 1, indicating that Pb is affected by soil to a certain extent. The change of Pb content has a significant influence. National Food Safety Standard—Limits of Contaminants in Foods (GB2762-2017)53 stipulates that the limit values of Cd and Pb in wheat grains are 0.1 mg kg−1 and 0.2 mg kg−1, respectively. Among the 48 wheat samples investigated, the Cd content meets the standard, but 4.17% of the samples are close to 0.1 mg kg−1 (more than 0.09 mg kg−1). The content of Pb in 50% of the samples exceeds the standard, and the Pb in the wheat grains exceed the standard seriously.
Evaluation results of heavy metal pollution in soil
Evaluation results of single pollution index method
The evaluation of five heavy metal elements by the single pollution index method is shown in Fig. 9, and it can be concluded that the soil heavy metal pollution in the study area is Cd, Pb, Cu, Zn, and Ni in order. The pollution degree of Cd is relatively severe; the average value of the pollution index is 1.22, the variation range is 0.58 ~ 2.62, and the CV is 0.35. Followed by Pb pollution, the average value of the pollution index is 0.80, the range of change the variation range is 0.38 ~ 1.98, and the CV is 0.37. The maximum single pollution index of Cu, Zn, and Ni did not exceed 1, indicating that Cu, Zn, and Ni slightly polluted the sampling area. Field investigation found many small-scale non-ferrous metal recovery and smelting plants around the study area, and many trucks carrying scrap cables and metals pass by the eastern side of the research area. Many Cd and Pb-containing substances from factory production and automobile exhaust entered the soil through atmospheric deposition and waste residue infiltration54.
Evaluation results of single factor pollution index method.
Evaluation results of potential ecological risk index method
Figures 10 and 11 are the evaluation results of heavy soil metals’ potential ecological risk index. Except for Cd, the average value of the single factor ecological risk index of the other four heavy metal elements is less than 40, which belongs to low ecological risk. The order of the five heavy metals in the study area from the largest to the smallest is Cd > Pb > Cu > Zn > Ni, Cd and Pb are seriously polluted in this area. The Cd and Pb single-factor ecological risk indexes CV are 0.35 and 0.37, respectively, higher than those of the other three heavy metals, indicating that Cd and Pb are unevenly distributed in the soil in the study area are greatly affected by human activities. The average value of Cd’s single factor ecological risk index is 233.51, which belongs to high ecological risk. The proportions of the single factor ecological risk index reaching consultable, high, and very high are 31.25%, 60.42%, and 8.33%, respectively.
Statistical analysis of single factor ecological risk index. (a) The single factor ecological risk index of five heavy metals. (b) Evaluation results of the single factor ecological risk index of five heavy metals.
Evaluation results of potential ecological risk index method.
The potential ecological risk index method (Fig. 11) shows that the higher ecological risk is distributed in the east and central-southern study area. 56.25% of the samples are of moderate hazard level, and the samples with considerable hazards accounted for 43.75%. Pollution may be related to the “three wastes” produced by non-ferrous metal smelting in the study area and the exhaust gas produced by vehicles on traffic roads. Heavy metals such as Cd and Pb enter the surrounding farmland soil in different ways and accumulate continuously55.
Evaluation results of geoaccumulation index method
According to Fig. 12, it can be seen that the geoaccumulation index of Ni in the study area is less than 0, which is a pollution-free level; The average values of the Cu and Zn geoaccumulation indexes are 0.94 and 0.53, which are at a light pollution level; The geoaccumulation index of Pb is 2.00, which is at a middling pollution level, and the geoaccumulation index of Cd is 2.29, which is at a moderate pollution level.
Statistical analysis of heavy metal geoaccumulation index in soil. (a) The geoaccumulation index of five heavy metals. (b) Evaluation results of the geoaccumulation index of five heavy metals.
The proportion of soil sample pollution classification shows no Ni pollution in the study area, and the pollution rates of the four elements of Cu, Zn, Pb, and Cd all reach 100%. Among them, for Pb, 52.08% of the soil is at a middling pollution level, 43.75% of the soil is at a moderate pollution level, and 4.17% of the soil is at a biased pollution level; For Cd, 29.17% of the soil is at a middling pollution level, 66.67% of the soil is at a moderate pollution level, and 4.17% of the soil is at a biased pollution level; For Cu, 58.33% of the soil is at a light pollution level, and 41.67% of the soil is at a middling pollution level; For Zn, 97.92% of the soil is at a light pollution level, and only 2.08% of the soil is at a middling pollution level. Cu and Zn contribute less to soil pollution, while Pb and Cd contribute the most. The significant standard deviation of Pb and Cd indicates that the Pb and Cd have a high degree of dispersion in the geoaccumulation index and a high variation coefficient. The two elements in the soil are significantly interfered with by the production activities of heavy metal processing enterprises around the farmland, which leads to severe pollution of Pb and Cd in the local soil. According to the evaluation results of the geoaccumulation index (Fig. 13), it can be concluded that the areas where the soil is at Pb moderate-biased pollution level are mainly distributed in the southeast and southwest of the study area; the farmland near the road in the southeast of the study area have Cd moderate-biased pollution levels in the soil. It is affected by the waste non-ferrous metal recycling, electrolysis, and other related production activities of surrounding enterprises and the exhaust gas generated by the driving of vehicles.
Evaluation results of geoaccumulation index method (a) Pb (b) Cd.
Principal component analysis
The principal component analysis is a multivariate statistical analysis method that converts multiple indicators into a few unrelated comprehensive indicators and classifies the comprehensive indicators according to specific rules. It is often used to identify the source of elements in the medium and obtain the contribution rate of different sources to the same element56,57,58. Using SPSS software to perform KMO test on the data, the obtained statistic value is 0.742, and the accompanying probability of the Bartlett sphericity test is 0.000. Therefore, principal component analysis and discriminant classification techniques can be used to study pollution types of farmland around the heavy metals industry in Anxin County, Hebei Province of China, and the analysis results are shown in Table 7 and Fig. 14. Carrying out Varimax orthogonal rotation on the Kaiser normalized factors, two principal components with eigenvalues greater than one are obtained, with variance contribution rates of 54.033% and 20.678%, respectively, and the cumulative contribution rate of 74.711%. It can explain most of the information of heavy metal elements in the soil of the researched area.
Principal component analysis on factor loading.
Cd, Pb, Zn, and Cu show higher loads on the first principal component, respectively 0.859, 0.845, 0.840, and 0.738. The four elements are significant within the 95% confidence interval, indicating that the four elements have the same pollution source. The spatial distribution of the four heavy metal elements is relatively similar. The areas with higher content are concentrated in the southeast of the study area, the distribution area is close to the road, and there is a heavy metal processing plant area around. The average value of the four heavy metal elements is higher than the soil background value, indicating that it is significantly affected by human activities. Related research shows that metal smelting and other production activities will increase the content of Cd in the atmosphere, Cd sedimentation causes pollution to the soil59, and the accumulation of industrial waste is also an essential source of Cd pollution60,61. Most cars use leaded gasoline as fuel, and long-term use has caused the accumulation of soil Pb62,63. Because of its high corrosion resistance and high thermal conductivity, Cu is an essential component of vehicle braking systems and automotive radiators64. The wear of automobile parts will cause copper to enter the surrounding environment, so It is also often used as the identification element of the source of traffic65,66. Maximilian et al.67 found that Zn can also be a marker element for mobile sources. Cu, Pb, and Zn will enter the surrounding environment as the car parts wear out during the driving process68,69. PC1 is closely related to Cd, Zn, Cu, and Pb and has the characteristics of a traffic source and an industrial source. Therefore, it is speculated that PC1 may be a traffic source and an industrial source, mainly affected by vehicle traffic, exhaust emissions, and industrial production.
Ni exhibits a higher load on the second principal component, which is 0.990. The Ni in the soil mainly comes from rock weathering, atmospheric dust reduction, irrigation water (including nickel-containing wastewater), farmland fertilization, and the decay of plant and animal residues58,70. The evaluation result of the soil accumulation index of Ni is pollution-free, and the contribution of human activities to Ni is relatively low, mainly natural sources71. The content is mainly affected by the soil-forming parent material. Therefore, it is speculated that the pollution sources represented by PC2 are natural sources and agricultural sources.
Comparison of heavy metal pollution in farmland soil in different regions
In order to better analyze the pollution characteristics of heavy metals in different types of farmland soils, the results of this study were compared with the average values of heavy metal contents obtained by other studies. The comparison results are shown in Table 8. Compared with foreign studies, the concentration of Cd and Pb in this study area is higher, which may be related to the different industrial structures. For example, fertilizers containing Cd are not used in Kerman. Shanxi Province has a long history of gold mining and smelting, and the management system is not standardized. The random stacking of tailings slag from the gold mining and smelting process is the source of heavy metal pollution in the soil of nearby farmland78. Compared with the study by Zhou et al.73 in Hebei Province, the average content of Cd and Pb is slightly lower, but the difference is not significant because the selected study area is different, and there are differences in human interference. Due to the different soil properties in different plots, the results obtained are also different, but overall, there is serious Cd and Pb pollution. The results of this study are consistent with those of previous studies.