Source apportionment and source-specific risk evaluation of potential toxic elements in oasis agricultural soils of Tarim River Basin

Concentrations of PTEs in agricultural soil of oases

The descriptive statistical results of 11 PTE concentrations in agricultural soils of the four oases are shown in Table 1. The concentration order of different PTEs in the four oases was different. In terms of mean values, the concentrations of Co, Cu, Ni, Pb, Sb, V and Zn in Kashgar Oasis agricultural soil were higher than those in the other three oases, and the concentrations of Sn and Tl in Hotan Oasis agricultural soil were the highest. The Cv value of As in the Hotan Oasis was the highest (0.31), indicating that compared with that in other oases and other PTEs, the As in Hotan Oasis soil was more likely to have external inputs other than natural sources⁵⁹. According to the “Soil Environmental Quality · Agricultural Land Soil Pollution Risk Control Standard (Trial)” (GB15618-2018)⁶⁰, the corresponding elements of oasis agricultural soil in the source region of the Tarim River did not exceed the pollution risk control standard. Compared with the soil environmental quality provisions in the “Environmental Quality Assessment Standard for Producing Areas of Edible Agricultural Products” (HJ332-2006)⁶¹, the corresponding elements in the soil of the four oases were also below the limits.

Compared with the PTE concentrations in the agricultural soil of oases in other arid areas, the concentrations of Cd, Cu and V in the soil of the study area were lower than those of the Bortala River Basin and Ili River Basin³ in the northern Tianshan Mountains, while the concentrations of Co, Ni, Pb and Zn were similar to those of corresponding elements in agricultural soil of the Bortala River Basin and Ili River Basin³. Compared with the concentrations of PTEs in agricultural soil of Wuwei and Jiuquan cities in the Hexi Corridor^62,63, the concentrations of Cu, Ni, Pb and V elements in agricultural soil of the four oases in the study area were lower. Similarly, the concentrations of Cu, V and Zn in the study area were much lower than the corresponding concentrations in the agricultural soil of the Bahariya Oasis, Egypt⁶⁴.

Geochemical baseline values of PTEs in oasis agricultural soils

The PTE concentration of the subsoil (60–80 cm) after logarithmic and reciprocal conversion passed the KS normal distribution test (p > 0.05). The GBVs of PTEs in agricultural soils of the four oases in the source region of the Tarim River were calculated according to the cumulative frequency method, and the results are shown in Supplementary Figs. S1–4 and Table 2. The GBVs of PTEs in agricultural soils were very different among the four oases. For example, except for those of Ni, Sn, Tl and V, the GBVs of PTEs in agricultural soils of the Hotan Oasis were much lower than those of the other three oases (Table 2). There was also a large gap between the GBVs of PTEs in agricultural soil and the soil background values in China⁶⁵, Xinjiang^66,67 and Xinjiang agricultural land⁶⁶. In particular, the GBV of Zn obtained in this study was 3.6–5 times the soil background value of Xinjiang agricultural land. Under the same land use type, there are differences in the abundance of environmental PTEs due to heterogeneity in the disturbance degree of human activities and the parent material and soil formation processes⁶⁸. Therefore, there were some differences in the GBVs of each PTE among the four oases in this study when the land use types were all agricultural soils.

Pollution characteristics of PTEs in agricultural soil of oases

According to the GBVs of the above elements, the PF, PLI and P_N of PTEs in agricultural soil of the four oases were calculated, and the results are shown in Fig. 2. The results of PF showed that (Fig. 2a), except for As and Cd in the Hotan Oasis, the PTEs of agricultural soils in the four oases were at uncontaminated and slightly contaminated levels, indicating that the oasis agricultural soils in the source region of the Tarim River were affected by human activities to a certain extent. From the perspective of individual elements, the PF of Cd in agricultural soils of the four oases was higher than that of other PTEs. Among them, Cd in most soil samples from the Hotan Oasis (81.25%) showed mild contamination, while Cd in the remaining samples (18.75%) showed moderate contamination. Since Cd in soil is generally regarded as an indicator of agricultural activities involving fertilizer use, the results suggest that soil in oases (especially the Hotan Oasis) in the source region of the Tarim River may be seriously affected by agricultural activities.

The results of the pollution load index of multiple PTEs showed that the agricultural soils in the Yarkant River Oasis, Kashgar Oasis and Aksu Oasis were between uncontaminated and moderate contamination levels, and most soil samples in the Hotan Oasis were moderately contaminated. P_N showed similar results. The reason for this result was the high PF values of As and Cd in the agricultural soil of the Hotan Oasis (Fig. 2a), which caused the high comprehensive pollution index in this area. It is worth noting that compared with those in the other three areas, most PTEs (As, Cd, Pb, Sb, V and Zn) in the agricultural soil of the Hotan Oasis had lower mean values, but their PLI and P_N values were higher. This was closely related to the lower GBVs of PTEs in the Hotan area.

Source apportionment and source specific risk assessment of PTEs in oasis agricultural soil

Source apportionment of soil PTEs

Factor analysis and correlation analysis were used to identify the main sources of PTEs in the main oasis agricultural soil in the source region of the Tarim River. The results are shown in Table 3 and Supplementary Fig. S5. Table 3 shows that PTEs in the agricultural soil of the Yarkant River Oasis had two principal factors. The first principal component F1, which explained 61.6% of the total variance, mainly described Co, V, Cu and Sb and moderately described Cd, Ni and Tl. The second principal component, F2, accounted for 21.8% of the variance and mainly described As, Pb and Sn. There were three main factors for PTEs in Kashgar Oasis agricultural soil. Factor 1, which explained 58.9% of the total variance, was mainly composed of As, Co, Cu, Ni, Pb, Sb, V and Zn; factor 2, which accounted for 24.8% of the variance, was mainly composed of Sn and Tl; and factor 3, which accounted for 7.9% of the variance, was composed of Cd. There were three main factors for the PTEs of Aksu Oasis agricultural soil. Factor 1 was mainly composed of Cd, Co, Cu, Ni, Pb, V and Zn; factor 2 was mainly composed of Sn and Tl; and factor 3 was composed of As and Sb. PTEs in agricultural soil of the Hotan Oasis had only one main factor, which was composed of all (11) elements. To verify the above results of PTE extraction from oasis agricultural soil by PCA, Spearman correlation analysis was performed on PTEs from the soils of the four oases. The results showed that the correlations between PTEs in agricultural soils of the four oases were consistent with the results of PCA, indicating the reliability of the PCA results.

To further determine the specific sources and contribution rates of PTEs in oasis agricultural soil in the source region of the Tarim River, the PMF model was adopted for analysis, and the results are shown in Fig. 3. Overall, the classification results of PTEs in agricultural soils of the four oases by the PMF model were consistent with the results of PCA. According to the above results, eight environmental factors related to the source of soil PTEs were selected: distance from factory (DF), distance from road (DR), pH, soil type (ST), total nitrogen (TN), soil fine silty particle size percentage (Fine silty), soil silty particle size percentage (Silty), and soil coarse silty particle size percentage (Coarse silty). A Geodetector model of PTEs and environmental factors in soil was constructed, and the results are shown in Supplementary Table S5. Among them, the TN content in soil was considered to be an index related to the intensity of agricultural activities⁶⁹. Since the particle composition of atmospheric dust in the study area is mainly silt⁷⁰, the silty particle size content of soil was selected as the atmospheric dust index in this study.

In the Yarkant River Oasis, Co, Cu and Zn showed no pollution at most of the sampling points for the first source factor of PTEs in agricultural soil (Fig. 2), indicating that the levels of these elements in soil were less affected by human activities. Meanwhile, the results of the factor detector showed that ST was one of the factors explaining the spatial distribution of Co, Cu, Sb, V and Zn (Supplementary Table S5). The Geodetector results revealed TN as one of the strong drivers of Cd and Tl, the former of which is found in pesticides and fertilizers and the latter of which may also enter agricultural soil through sewage irrigation and fertilizers contaminated by industrial wastewater^71,72. Therefore, it is inferred that F1 in the agricultural soil of the Yarkant River Oasis had a mixed source of natural resources and agricultural activities. The three elements in F2 (As, Pb and Sn) were closely related to fuel combustion and traffic factors, among which Pb is usually selected as an identifying element for traffic sources (including leaded exhaust gas, vehicle tires and brake pads)⁷³. In the results of the Geodetector model, the best explanatory factors for Pb and Sn were Silt and DR, while the best explanatory factor for As was Slit. Therefore, F2 was inferred to be the source of road dust/atmospheric dust.

The weights of F1 in Kashgar Oasis agricultural soil PTEs were mainly included As, Co, Cu, Ni, Pb, Sb, V and Zn; the weights of F2 were mainly included Sn and Tl; and F3 were mainly included Cd (Fig. 3b). Similar to the results observed for the Yarkant River Oasis, the factor detector results showed that F1 might have a mixed anthropogenic source, including transportation, industrial and other sources. Both Sn and Tl in F2 are widely used in industrial manufacturing, and Sn can be used as an additive to enhance the properties of steel or alloys⁷⁴. Tl is used in many different industrial manufacturing and medical fields, and metal smelting, sulfuric acid production, coal burning, cement manufacturing and other industrial activities involving the use of Tl minerals are the main pathways by which this element enters the environment⁷¹. At the same time, the strongest explanatory factor for Sn and Tl in the results of the geographic detector was DF, so F2 was inferred to be an industrial source. F3 contained only Cd (related to agricultural activities), so F3 was inferred to be the source of agricultural activities (Supplementary Table S5).

In the PMF results, the weights of F1 for the PTEs of Aksu Oasis agricultural soil were mainly included Cu, Ni, Cd, Zn, Co, V and Pb; the weights of F2 were mainly included Sn and Tl; and the weights of F3 were mainly included As and Sb (Fig. 3c). Similarly, from the factor detector calculation results, it was inferred that F1 in the Aksu Oasis was a mixed source. According to the results of Geodetector analysis, the best explanatory factors for Sn and Tl in the agricultural soil of the Aksu Oasis were TN and ST. Previous studies have also shown that Sn in soil may also come from agricultural practices (pesticides)⁷⁵, so F2 was inferred to represent agricultural activities and natural sources (rock mineralization). Similarly, according to Supplementary Table S5, F3 was inferred to be the source of agricultural activities. Previous studies have also concluded that agricultural activities such as the application of phosphate fertilizer are also the main source of As and Sb in soil^76,77, which was consistent with the results of the Geodetector model.

The explanatory factors for the PTEs of agricultural soil in the Hotan Oasis were the total nitrogen content and the silty particle size percentage, indicating that the PTEs in agricultural soil in the Hotan Oasis mainly came from agricultural activities and atmospheric dust fall. The economy in the Hotan Oasis is dominated by irrigated agriculture, and since most oases border large deserts, wind and sand disasters are extremely serious (annual average dust days exceeding 220 days)⁷⁸. The sources of PTEs inferred by the Geodetector model in this study were consistent with the reality.

Source-specific ecological risk assessment

Based on the results of source analysis, the source-specific ecological risks posed by agricultural soil PTEs were evaluated in this study, and the results are shown in Fig. 4. The total ecological risks caused by PTEs in the agricultural soils in the Yarkant River Oasis, Kashgar Oasis and Aksu Oasis were null, while those at of all sampling sites in the Hotan Oasis were moderate risk. According to the results of the source-specific ecological risk of PTEs, there was no direct relationship between the contribution degree of source-specific risks to the total ecological risk and its contribution to the existence of PTEs in soil. In particular, the agricultural activity source (F3), which accounted for only 7.9% of the PTEs in Kashgar Oasis agricultural soil, contributed the most to the total risk, the mixed source (F1), which contributed the most to the existence of PTEs in soil, contributed the second most to the total risk, and the industrial source (F2) contributed the least. The reason for this result was that the toxicity factor of Cd (30) introduced by agricultural activities was significantly higher than that of PTEs released from other sources, which was consistent with the conclusion of other studies^16,19 that the main source of highly toxic elements was more likely to cause ecological risks than that of low-toxicity elements.

To explore the spatial distribution of ecological risks generated by PTEs from different sources, a distribution map of ecological risks posed by specific sources at each sampling point was generated (Fig. 5). The total ecological risk of the sampling sites near Awat County in the Aksu Oasis was higher than 40, indicating moderate risk. In addition, the total ecological risk at the other sampling sites was at a low level. However, the ecological risks caused by mixed sources (F1) and agricultural activities and natural sources (F2) were high in the southwestern Aksu Oasis and the sampling sites near Aksu city, resulting in high total ecological risks. The ecological risks at all the sampling sites in the two oases in Kashgar (Kashgar Oasis and Yarkant River Oasis) were low. Notably, the sampling sites near Kashgar city had high total ecological risks due to the high ecological risks caused by PTEs from mixed sources (F1) and agricultural activity sources (F3). The total ecological risk at all sampling sites in the Hotan Oasis was moderate. In particular, the sampling sites near Hotan city had the highest total ecological risk generated by PTEs from agricultural activities and atmospheric dust sources (73.71).

Source-specific human health risk assessment

According to the proportions of different sources of each PTE obtained from the PMF model, the source-specific health risks of PTEs in agricultural soil in the four oases to the human body were calculated, and the results are shown in Tables 4 and 5 and Fig. 6. Although the results of noncarcinogenic risk caused by different sources of PTEs in agricultural soil in each oasis showed that adults and children were not at risk, the total THI values of PTEs for children in the four regions were all greater than 1, indicating that soil PTEs in the study area posed significant noncarcinogenic risks for children. In terms of carcinogenic risk, the total TCR values of PTEs for adults and children were on the order of 1E−05, within the range of 1E−06 and 1E−04, indicating that the carcinogenic risk of soil PTEs for the human body is acceptable in the study area. In general, the noncarcinogenic and carcinogenic risks of PTEs from different sources in children were higher than those in adults, which can be explained by children having more opportunities PTE contamination through hand-to-mouth ingestion and dermal contact than adults due to the areas where they play and unhealthy eating habits (e.g., children are more likely to suck their fingers)⁷⁹. Therefore, different parameters were set when employing the health risk assessment model.

In terms of the proportion of human health risks caused by PTEs from different sources (Fig. 6), those from mixed sources, which were considered to have the largest contribution to PTEs in the Aksu Oasis, Kashgar Oasis and Yarkant River Oasis, were not the largest. This might be explained by the presence of more toxic elements, such as As and Tl, in other sources of the three oases contributing the most (Fig. 3), while their low R_fD may explain their greater noncarcinogenic risk. PTEs from agricultural activities and natural sources accounted for the largest proportion of carcinogenic risk in the Aksu Oasis, while mixed sources accounted for the largest proportion of carcinogenic risk in the Kashgar Oasis and Yarkant River Oasis. This was mainly because the presence of As, Cd, Co, Ni and Pb elements with carcinogenic risk in the above two sources accounted for a large proportion (Fig. 3), thus posing a large carcinogenic risk. The results of the proportion of health risks attributed to different sources of PTEs to the human body showed that the control of human health risks of PTEs cannot be determined based on their concentration alone. Therefore, in view of the obvious noncarcinogenic risk of soil PTEs in the four oases to children, from the results of the contribution of different sources of PTEs to the noncarcinogenic risk to children, agricultural activities and natural sources, industrial sources and atmospheric dust fall were the priority control sources in the Aksu Oasis, Kashgar Oasis and Yarkant River Oasis, respectively.

Source: Ecology - nature.com