Soil physical–chemical properties and HMs concentrations
The soil physical–chemical properties and HMs concentrations are summarized in Table 1. The soil mean pH value was 6.17 and ranged from 4.16 to 9.04 in different sites. The samples sites of level for pH ≤ 6.0(acidic soil), 6.0 < pH ≤ 7.5(Neutral soil) and pH > 7.5(alkaline soil) were 51.5%, 36.4% and 12.1%, respectively. The average content of TN, TP and TK were 1.33 g kg−1, 1.16 g kg−1 and 23.6 g kg−1, and ranged from 0.7 g kg−1 to 2.4 g kg−1, 0.22 g kg−1 to 20.8 g kg−1 and 10.3 g k g−1 to 28 g kg−1, respectively.
The mean concentration of Cd, Hg, As, Pb, Cr, Cu, Ni and Zn were 0.221, 0.155, 9.76, 32.2, 91.9, 35.2, 37.1 and 108.8 mg kg−1. Except Cd, the average concentration of Hg, As, Pb, Cr, Cu, Ni and Zn exceeded 93.8%, 7.1%, 6.3%, 17.8%, 25.3%, 10.7%, and 32.4% the soil background values for Chengdu, respectively, which indicates that HMs are enriched to a certain extent in soil. The CV of the HMs in the agricultural soils increased in the order Ni(10.1%), Cr(10.9%), Pb(15.4%), As(21.4%), Cd(31.1%), Zn(57.8%), Hg(58.1%) and Cu(59.9%). The exceptionally high variability of Cu, Hg and Zn indicates that these metals differed greatly with respect to different sites, and the existence of abnormally high values is the main reason that the CV was high. It further indicates that Cu, Hg and Zn may be affected by external interference factors. The mean concentration of all HMs in soil were below the risk screening values for soil contamination (GB 15618-2018) (MEEC, 2018), however, the results showed that in 76, 1, 1, 6 and 2 of sample sites the level of Cd, Pb, Cr, Cu and Zn exceeded the risk screening values.
Assessment of heavy metal(loid)s pollution
Index of geo-accumulation
The index of geo-accumulation of HMs in the soil in the study area are shown in Fig. 2. In a descending order of magnitude of Igeo mean value, the eight elements were as follows: Hg(0.18) > Zn(-0.22) > Cu(-0.30) > Cr(-0.36) > Ni(-0.45) > Pb(-0.51) > As(-0.52) > Cd(-0.82), indicating that the soil HMs Zn, Cu, Cr, Ni, Pb, As and Cd in the study area were generally in a no contamination according to the defined classes, while Hg was in uncontaminated to moderately contaminated.
Indexes of geo-accumulation, pollution indexes and potential ecological risk indexes of HMs in study aera. Circles at the top and bottom of box plots correspond to the maximum and minimum values, respectively. The square in the box plot is the average value. Horizontal lines at the top, middle, and bottom of the box plot correspond to 75% percentile, median, and 25% percentile, respectively.
The Igeo values for Zn, Cu, Cr, Ni, Pb, As and Cd in more than 90% of samples were less than zero, only a few outliers in the soil were classified as moderately contaminated or worse. Which meaning point pollution at those sample sites. However, the Igeo values for Hg in 47.34%, 40.61%,10.66% and 1.40% of samples were belonged to < 0, 0–1, 1–2 and 2–3, respectively, 52.66% sample sites were contaminated moderate to strongly. It means that there were non-point source pollution sources of Hg in the study area. Relevant studies have pointed out that the overall distribution trend of soil Hg content in Chengdu Plain is relatively high in the north, which is mainly affected by geological structure, domestic pollution of urban residents and pollutant emission of industrial enterprises17.
Pollution index
The PI and SPI of soil HMs are drawn in Fig. 2. The PI average value of Cd, Hg, As, Pb, Cr, Cu, Ni and Zn were 0.69, 0.09, 0.28, 0.36, 0.56, 0.59, 0.49 and 0.50, and the ranging from 0.17 to 1.60, 0.01 to 0.68, 0.12 to 0.62, 0.12 to 1.29, 0.25 to 1.06, 0.19 to 6.07, 0.14 to 0.72 and 0.18 to 7.28, respectively. The PI value for Cd in 55 sample sites, for Cu in 5 sample sites, and for Pb, Cr and Zn in one site were belong to 1.0–2.0, indicating low pollution according to the defined classes. However PI value for Cu(6.07) and Zn(7.28) in one site were higher 3, belong to the severe pollution. It may be because the sampling point was close to the industrial area. Although the Igeo mean value of Hg was the largest, its PI average value was the lowest. Because of different research perspectives and different reference indicators, the results were inconsistent33.
The SPI is usually applied to evaluate the overall status of HMs contamination. The mean SPI value was 0.62, ranging from 0.31 to 5.22 in Tangchang agricultural soil. In our study, 98.23% of soil samples were not polluted with HMs as 1.52% of samples within low pollution and 0.25% of the samples had a severe pollution level. In general, the soil quality in the study area is generally at a clean level.
Potential ecological risk assessment
The EIi values for each HMs and the RI are shown in Fig. 2. The EIi values of As, Pb, Cr, Ni and Zn in all sample sites were less than 40, meaning a low potential ecological risk with these HMs. For Cu, except that the EIi in one sample was 108, which belongs to considerable potential ecological risk, the other samples were less than 40. For Cd and Hg, 93.4% and 9.5% of soil samples were in low potential ecological risk category, respectively. Significantly, 6.5% of samples for Cd and 55.5% for Hg were in moderate potential ecological risk categories, and one sample for Cd and 29.4% for Hg were in very high potential ecological risk categories. Especially EIi of two soil samples for Hg were high than 320, belongs in dangerous potential ecological risk categories. In this study area the RI mean value was 135.4, and ranging from 62.6 to 514.6. In all samples 76.4% had a low potential ecological risk of HMs, 22.1% had a moderate risk and 1.5% had a considerable risk.
According to the calculation results of Igeo and EIi, it is found that the results of the two methods were consistent and different. Such as the Igeo and EIi of Hg were the largest, however, the Igeo of Cd is the smallest, but its EIi was the second highest. The reason may be that the Igeo focuses on the enrichment of exogenous HMs, on this basis, the EIi focuses more on the potential effects of toxic effects of HMs. In the EIi, the toxicity coefficients of Hg and Cd were the largest, which were 40 and 30 respectively. The reference ratios of the two methods were the same soil background value, so the difference of toxicity coefficients leads to great changes in EIi. Compared with Igeo, the EIi considers not only the content of HMs, but also the biological toxicity of different metals34,35.
Sources of HMs in agricultural soils
Correlation analysis
Correlation tests were used to understand the relationship between different HMs and find their possible sources36. If there is a positive correlation between HMs may be indicative of their common source. As shown in Table 3, the most of HMs exhibited significant correlations(P < 0.05). Especially, Hg-Cd, As-Cd, Cr-Pb, Cu-Cd, Ni-Hg-Pb, Ni-Cu and Zn-Cr-Ni exhibited high significant correlations(P < 0.01), which meaning that these HMs in the study area may have common origin. Moreover, significant correlations were also observed between pH with Cd, Pb, Ni and Cr, TN with Cd, Hg, As and Pb, TP with Cd, As and Cu, TK with Hg, As, Pb, Cr and Ni, OM with Cd, Cr, Hg, As, Pb and Cu, which indicate that the source of HMs in the study area may be closely related to human activities.
Principal component analysis
Principal component analysis (PCA) is used widely for analyzing the sources of soil HMs. Four factors were extracted by maximum variance rotation method and the results are shown in Table 4. The initial eigenvalues of principal component 1 (PC-1), principal component 2 (PC-2), principal component 3 (PC-3) and principal component 4 (PC-4) were greater than 1, and explaining cumulative variance of four PC were 73.416%. PC-1 consists of Pb and Hg, accounted for 20.637% of the total variance. The mean concentrations of Pb and Hg in soil were 32.2 mg kg−1 and 0.155 mg kg−1, significantly higher than the soil background value for Chengdu (Table 1). High value points for Pb and Hg were mainly concentrated in TangChang town government and surrounding residential areas. It seems that anthropogenic activities sources such as traffic and atmospheric subsidence were the major sources of Pb and Hg in agricultural soils. PC-2 consists of Zn, Cr and Ni, accounted for 20.446% of the total variance. These results were consistent with the results of Pearson’s correlation analysis (Table 3). Although many researchers have found that these elements had some degree of homology and may be affected mainly by material and pedogenic processes30,31,37, but their mean concentrations higher than the soil background value for Chengdu. Therefore, the natural sources and industrial activities together constituted the major sources of PC-2. PC-3 consists of Cu and Cd, accounted for 17.063% of the total variance. Very significant correlation was found with Cu-P and Cd-P. The planting of hotbed chives needs to use a lot of phosphate fertilizer. Therefore, it seems that long-term use of chemical fertilizers and pesticides was the major source of PC-3. PC-4 includes only As, accounted for 15.271% of the total variance. The As in the soil of the whole town was generally in a pollution-free state. It seems that the natural sources were the major sources of PC-4 and many studies have reached similar conclusions38,39.
Health risk assessment
Non-carcinogenic risk assessment
Table 5 and Fig. 3 show the results for non-carcinogenic risk. In collected soil samples the HQ value of three exposure pathways (ingestion, dermal contact, and inhalation) for eight HMs were lower than 1. The HI for adults and children were 0.173 and 0.996, respectively. Suggesting no significant non-carcinogenic risk in the study soils. It is noteworthy that the HI of children was higher than the HI of adults, meaning that children were more susceptible to adverse effects. Other studies have shown similar phenomenon1,2,36. The reason may be children’s special behavior and physiological characteristics, such as frequent pica behavior and hand or finger sucking and so on40,41.
Contribution of different exposure pathways and various HMs to hazard index. A stand for HI of adults and B for HI of children by contribution of different exposure pathways, C stand for HI of adults and D for HI of children by contribution of various HMs.
From Fig. 3A,B, It was easy to show that the contribution of different pathways to non-carcinogenic risk was similar between adults and children decreased in the following order: Ingestion > Dermal contact > Inhalation. The HI mean values for adults and children by ingestion, dermal contact and inhalation routes were 0.129 and 0.919, 0.0434 and 0.0761, 0.000757 and 0.0014, respectively. It’s not hard to saw the HI for ingestion route of both adults and children were 1–3 orders of magnitude higher than the other two exposure pathways. Many studies have reached similar conclusions32,36. Thus, the ingestion route may be an important pathway for HMs exposure in study area.
The HI mean value of single HM for children and adults decreased as following order (Fig. 3C,D): Cr > As > Pb > Ni > Cu > Hg > Zn > Cd. Cr, As and Pb were the largest contributors for both adults and children, accounting for 47.33% and 42.37%, 32.68% and 39.64%, 15.95% and 13.26%, respectively. Indicating that attention should be pain to the Cr, As and Pb elements due to their noncarcinogenic risk. Which was basically consistent with the research results of Bo et al.1 and Bao et al.32. Overall, The HMs of Cr, As and Pb were the main non-carcinogenic factors in soil in the study area, and the risk control of these elements should be strengthened.
Carcinogenic risk assessment
The CR of the HMs are shown in Table 5. Three exposure pathways were considered for Cd and Hg in our study. But As and Pb were considered carcinogenic by ingestion and inhalation, and Cr was considered carcinogenic through inhalation. The TCR mean values were 3.68 × 10−5 for adults and 6.27 × 10−5 for children, obviously were in the range 1 × 10−4 from 1 × 10−6, suggesting the TCR caused by HMs in the study area was acceptable on the whole, but it still exceeds the soil treatment threshold value 10−6. The CR average values through the three exposure pathways were CRing 3.65 × 10−5, CRinh 2.53 × 10−7 and CRderm 1.07 × 10−7 for adults, CRing 6.25 × 10−5, CRinh 1.12 × 10−7 and CRderm 4.50 × 10−8 for adults. Clearly the CRing was much larger than CRinh and CRderm for both adults and children, which indicates that oral ingestion is the major exposure pathway for CR. Which was consistent with the research results of Song et al.31 and Bo et al.1. For single HM, the CR value of Pb and Hg for adults were 2.72 × 10−5 and 8.6 × 10−6, and Pb, Hg and Cd for children were 4.63 × 10−5, 1.48 × 10−5 and 1.36 × 10−6, respectively, within acceptable criterion. Overall, the longterm health effects for adults and children are not serious at current single HM level.
In this study, the total contents of HMs in the soils were used to assess health risk, the bioavailability of HMs were not considered, which may have caused the assessment results to be higher than the actual local situation1,31,42. In addition, because the parameters of health risk evaluation for children were set to be more sensitive than those for adults, the non-carcinogenic risk and carcinogenic risk for children were higher than those for adults2,43,44. However, those risks were at acceptable or negligible levels. Therefore, the study area is suitable for safe and clean production of hotbed chives.
Source: Ecology - nature.com
