OCPs residues levels in the urban soil collected from Changchun
The concentration of ∑HCHs was higher than that of ∑DDTs in the main urban soil from Changchun, which is contrary to the monitoring results of OCPs in the soil of most research areas in China(Tang et al. 2018; Zhang et al. 2021). However, it is consistent with the previous research results in Changchun (Lu et al. 2020). The HCHs exhibited moderately high vapor pressures and relatively lower octanol-water coefficient (Kow) than DDT pesticides, making them easier to volatilize into the atmosphere and transport for a long distance (Walker et al. 1999), while the application amount is still the decisive factor of OCPs residues in soil. Although the respective concentrations of the rest OCPs have been kept at low levels, the detection frequencies were all much higher than 88% (except for HEPT). This fact indicated that the rest of the OCPs had also been widely used in the different regions of Changchun, while their amounts of application probably were relatively lower than HCHs and DDT. When it comes to compared with those in other urban surface soils from various worldwide regions, the residue level of HCHs and DDTs in the urban soil from Changchun were at a medium level.
Composition and source identification of selected OCPs
HCHs were the most dominant contaminants in the main urban soils from Changchun, which accounted for 41.3% of the total OCPs. HCHs were used as the commercial insecticide in two formulations: (I) industrial technical-grade HCHs, including α-HCH (60–70%), β-HCH (5–12%), γ-HCH (10–15%), and δ-HCH (6–10%), and (II) Lindane, almost entirely composed by γ-HCH (> 99%)(Bajwa et al. 2016). The environmental fates of the four HCH isomers were different in their different configurations and physicochemical properties, such as the stability of them is in the order of β-HCH > δ-HCH > α-HCH > γ-HCH (Wei et al. 2016). The ratio of α-HCH/γ-HCH ranged from 4 to 7 indicating the use of technical HCHs, and nearly 0 indicates the use of lindane (Wang et al. 2017). In the natural environment, γ-HCH and α-HCH can be transformed into β-HCH under microbial decomposition and photosynthesis (Walker et al. 1999). Therefore, the β-HCH/(α-HCH + γ-HCH) ratio was used to determine whether the HCH levels were due the historical pollution. The ratio of β-HCH/(α-HCH + γ-HCH) > 1 can describe the historical use of technology HCHs and a ratio ranging from 0.1 to 0.5 can highlight the lindane as the source (Wang et al. 2016). In the current analysis, γ-HCH was the most abundant isomer, contributing to 46.2% of the total HCHs, followed by β-HCH (31.5%), δ-HCH (13.5%), and α-HCH (8.8%) (Fig. 2). The ratios of α-HCH/γ-HCH ranged from 0.04 to 1.62, with an average mean of 0.22, implying that HCH residues in the main urban area soil of Changchun were most likely from the historical use of lindane. The ratio of β-HCH/(α-HCH + γ-HCH) ranged from 0.06 to 3.25, with a mean value of 0.69, About 58% of the soil samples had a β-HCH/(α-HCH + γ-HCH) ratio between 0.1 and 0.5, and 21% of the soil samples had that > 1. Thus, the historical usage of lindane was the major source of the HCHs in the soil samples from Changchun, and the technology HCHs also have some contributions. This is similar to the residual form of HCHs in the soil of the Anhui industrial zone (Yang et al. 2020).
DDTs were the second dominant contaminant in the soil from Changchun, accounting for 30% of the total OCPs (Fig. 2). The major source of DDTs pollution in China was through the application of technology DDTs and dicofol in industry and agriculture. The technology DDTs are composed of p,p'-DDT(80–85%) and o,p'-DDT(15–20%). Once in the environment, p,p'-DDT can be biodegraded into p,p'-DDE, and p,p'-DDD under anaerobic and aerobic environments, respectively (Gopalan and Chenicherry 2018). Therefore, the ratios of (p,p'-DDD + p,p'-DDE)/p,p'-DDT can be used to judge the “new” or “old” pollution sources of DDTs. If (p,p’-DDE + p,p’-DDD)/p,p’-DDT > 1, suggesting that DDTs had under-gone the long-term degradation and transformation with no new pollutants inputting (Barron et al. 2017). The o,p'-DDT/p,p'-DDT ratio was used to judge the pollution source of DDTs. Research showed that the dicofol-type DDT had a higher o,p'-DDT/p,p'-DDT ratio (1.3–9.3) than the technical DDT (0.2–0.3) (Qiu et al. 2005). In our study, p,p'-DDE was the most abundant isomer, contributing to 37.4% of the total DDTs, followed by p,p'-DDT (35.5%), o,p'-DDT (19.3%), and p,p'-DDD (7.9%) (Fig. 2). The ratios of p,p′-DDD/p,p′-DDE > 1 only found in one samples, indicating the aerobic breakdown condition of p,p'-DDT in the surface soil of the main urban area from Changchun. The ratio of o,p'-DDT/p,p'-DDT ranged from 0.01 to 4.30, with a mean value of 0.61. The ratios of (p,p'-DDD + p,p'-DDE)/p,p'-DDT ranged from 0.07 to 39.56, with the mean value of 3.76, suggesting that the heavy pollution of DDTs in the main urban soil from Changchun was mainly derived from the historical application of technology DDTs or dicofol.
Chlordane was produced in the 1950s in China (Bajwa et al. 2016) and has been widely applied as agricultural insecticides, herbicides, and termiticides against termites in China during the 1960s and 2009(Wei et al. 2016). Technology chlordane contains more than 140 compounds and the most abundant components are cis-chlordane (CC, about 11%) and trans-chlordane (TC, about 13%).Since TC is easier to degrade than CC in the environment, the ratio of TC/CC < 1.0 usually indicates the aging of chlordane (Tang et al. 2018). In this study, the concentration of chlordanes in soils ranged from 0.21 to 12.39, with GM = 2.55ng/g, and AM = 3.03ng/g. These results were higher than those reported in the soils from Ningbo (Tang et al. 2018), Himalayas (Devi et al. 2015), and Beijing (Wang et al. 2008). The ratio of TC/CC < 1.0 in 81% soil samples, suggesting the historical usage of chlordane. HEPT was primarily used in termite, ant, and soil insect control in seed grains. Since HEPT is rapidly converted into HEPX or other metabolites (Qu et al. 2019), the ratio of HEPX/HEPT > 1 and < 1 indicates that HEPT originates from historical and recent use, respectively (Jiang et al. 2009). In our study, the HEPX/HEPT ratio ranged from 0.03 to 6.29, and that was higher than 1 in 67% soil samples, which indicated the historical use of HEPT in the study area.
Technical endosulfan contains 70% α-endosulfan and 30% β-endosulfan (Tesi et al. 2020). Since the α-endosulfan decomposed more easily than β-endosulfan in soil, the ratio of α-/β-endosulfan < 2.33 and > 2.33 indicates historical and recent usage of technical endosulfan, respectively (Devi et al. 2015). The ratios of α-/β-endosulfan were all lower than 2.33 in our study, indicating the historical usage of endosulfan in Changchun.
HCB is an organochlorine fungicide, which is mainly as the seed dressing to eliminate smut in agricultural production(Hu et al. 2022). Industrial HCB is used as an intermediate in the production of pentachlorophenol and sodium pentachlorophenol and can be released into the environment in the process of using sodium pentachlorophenol. In this study, the concentration of HCB in soils ranged from n.d. to 8.67, with GM = 2.09 ng/g, and AM = 2.50 ng/g. HCB usually adsorbed on atmospheric particles as its low vapor pressure and high volatility and can be transported for a long distance driven by the atmosphere. In this study, the detection frequency of HCB was 98%, but the average detection concentration is very low, indicating that HCB in urban soil may come from regional atmospheric deposition.
Aldrin, Dieldrin, and Endrin were never industrially produced and used for insect control in agriculture in China, and also had never been commercially imported from abroad (Tang et al. 2018; Zhang et al. 2017). While these never-used OCPs were also found in the urban soil from Changchun, with high detection frequencies (92% for aldrin, 88% for dieldrin, and 100% for endrin). The residue levels were in the range of nd-6.46 ng/g, with GM = 0.23 ng/g for aldrin and the range of nd-0.9 ng/g, with GM = 0.42ng/g for dieldrin, respectively. The concentration of endrin was in the range of 0.51–2.52, with GM = 1.24ng/g. The residue levels were in general agreement with those in Ningbo (Tang et al. 2018), Shanghai (Jiang et al. 2009), Zhejiang (Jorfi et al. 2019), and those from the peri-urban vegetable soils of Changchun (Zhang et al. 2017). This may be attributed to the long-range atmospheric transport deposition as these chemicals were used in some developing countries around the tropical belt region (Wang et al. 2008).
Effect of land use and geographical distribution of OCPs in Changchun urban soil
Land-use type directly affects the use history and dissipation of OCPs by changing soil conditions, which is considered to be an important factor affecting the residue of individual OCPs in soil. Soil samples were collected from various urban soil sites in Changchun, including six different land types, to reveal which land type had the most polluted soil. Figure 3 presents a bar chart showing the average concentration distribution of individual OCPs in the soils from various land-use types of Changchun. It highlights that the soils from the parking zone, residential zone, and the cultural and educational area sites were the most polluted by the γ-HCH, whereas the industrial zone and the outskirt farmland soils were dominated contaminated by the p,p'-DDE, and the commercial traffic soils only for p,p'-DDT. In general, the most polluted by the total OCPs were the soils from the cultural and educational areas and the park zone sites of Changchun.
To understand the regional variation of OCPs residues in the urban soil of Changchun, the IDW interpolation was conducted to map the geographical distribution based on the measured concentration of selected OCPs. As presented in Fig. 4, the spatial distributions of individual OCPs were roughly similar, with the high levels found at the southwest (PZ5) and northeast (PZ2, PZ4, CE1, RZ3) sites, as well as the sites (CE5) near Jilin Agricultural University in the southeast of the main urban areas of Changchun. This geographical distribution result was similar to the distribution characteristics of OCPs in peri-urban vegetable soils of Changchun (Zhao et al. 2018). OCPs tend to transport with atmospheric particulate or gaseous phase as their semi-volatile properties (Ma et al. 2020). Southwest wind prevails all year round in Changchun, which may contribute to the relatively higher residue of OCPs in the northeast of the study area. Despite the overall trend is similar, there are still some differences in the spatial distribution pattern of DDTs and HCHs on a local scale. This may be due to the sources of DDTs and HCHs varying according to specific human activities in different regions. The highest levels of DDT were detected at sites along the Yitong River, mainly due to the irrigation of the Yitong River, as its water quality is affected by the discharge of domestic and industrial effluents, surface, and groundwater runoff. Meanwhile, the high concentrations of DDTs also appeared in the cultivated land near the East Interchange and outside the northern thermal power plant. A large number of processing and manufacturing industries are distributed in the eastern interchange area which confirms the pollution of soil in Changchun caused by DDTs in the process of industrial production. The variation tendencies of HCHs coincided with the total OCPs, which indicated that HCHs were the dominant OCPs in the main urban soil of Changchun. Chlordane is currently being produced and used to control termites in buildings and dams (Zhang et al. 2017). Hotspots in the southwest and north show that chlordane plays a very important role in the new application of termites. Other hotspots of DDTs, as well as the highest HCHs and chlordane, appear in the southwest, where automobile parts and mold factories are concentrated.
Relationship between OCPs concentration and soil properties
All soil samples were analyzed for total organic carbon (TOC), pH, and moisture content (MC). The detailed analytical methods are presented in our previous study (Zhao et al. 2022) and the results of these soil properties are presented in Table 1. The range of soil pH in the main urban area of Changchun is 5.37–8.36, with an average value of 7.46, which indicated the urban soil pH of Changchun is alkaline. The concentration of TOC in the urban soil ranged from 0.56–4.79%, with an average content of 1.63%. Commonly, OCPs are hydrophobic chemicals and the total organic carbon (TOC) in soil plays an important role in binding OCPs. Once deposited into the soil, most OCPs will be absorbed by the soil organic matter and the strong sorption will inhibit their degradation and leading (Dai et al. 2008). Therefore, the soil TOC is considered to be the very important factor influencing soil OCPs pollution. Previous research had found a positive correlation between TOC and soil concentrations of OCPs (Chakraborty et al. 2015). To further identify the impacts of soil properties to the OCPs pollution in this study, the Pearson's correlation coefficients between the OCP concentrations and soil properties are illustrated in Fig. 5. The results show that a relatively strong correlation existed among individual OCPs monomers except for the endosulfans. TOC had a positive significant influence on the persistence of HCHs in soils, especially α-HCH and HEPTs. Soil pH was significantly negatively correlated with the concentrations of endosulfans. The research shows that endosulfan in alkaline soil is more easily degraded (Qu et al. 2019).
Ecotoxicological and health risks assessment of OCPs in the soil from Changchun
The soil and sediments quality guidelines (SQGs) were adopted to illustrate the potential ecological risk of OCP residues in the main urban soil from Changchun. The guidelines contain two groups of evaluation thresholds, including the threshold effects level (TEL) values, probable effects level (PEL) guidelines proposed by Macdonald et al. (1996) (Pokhrel et al. 2018), and the effects range-low value (ERL), effects range-median value (ERM) guidelines proposed by Long et al. (1995) (Yu et al. 2020). ERL is the threshold of effect concentration with ecological risk probability less than 10%,while ERM means the probability of less than 50%. The occurrence of residues below the TEL value indicates that adverse biological effects may occur rarely, whereas the presence of residues exceed the PEL value suggests that adverse effects may occur frequently (Wang et al. 2017).
According to the guidelines and the statistical results in Table 2, the levels of p,p'-DDT, p,p'-DDD, p,p'-DDE, and ΣDDTs exceeded the TEL values in 90,29,96 and 100% of the urban soil samples, respectively. The concentrations of these compounds were also higher than the ERL values in these samples. Significantly, the level of p,p'-DDT in 17% and 25% of all sampling sites exceeded the ERM and PEL values, whereas the concentrations of other isomers and the total DDTs within the ERM and PEL values, indicated that a rare possibility of adverse ecological effects for DDT metabolites may exposure to the soil organisms. The residue levels of γ-HCH exceeded the TEL and PEL values in 100% of the sampling sites, which suggested that the input and accumulation of lindane corresponding to the urban agricultural activities may cause adverse ecological effects in the urban soil of Changchun. The concentration of dieldrin in 10% of the samples exceeded the TEL value, without exceeding the PEL value, which suggests that the dieldrin can rarely cause toxic biological effects to the urban soil organisms of Changchun. The concentration of endrin all exceeded the ERL value, without exceeding the PEL value, suggesting that endrin in the urban soil from Changchun may cause certain adverse ecological effects, but the probability may lower 10%. As for the heptachlor, there were 44% of the total sampling sites exceeded the ERL value, and 13% of those exceeded the TEL value. While, the concentration of heptachlor in three samples (including CE1, CE5, and PZ1) was over the PEL value, and that in the CE1 sample even exceeded the ERM value, which indicated that the heptachlor residue in some specific sampling sites may exhibit the high possibility of adverse ecological effects.
Table 2
Assessments of potential ecotoxicological risks of selected OCPs in urban soil from Changchun using two sediment quality guidelines (SQGs) (unit: ng/g).
Selected OCPs
|
Range (Mean)
|
ERLa
|
Above ERLe
|
ERMb
|
Above ERMe
|
TELc
|
Above TELe
|
PELd
|
Above PELe
|
p,p'-DDT
|
N.D.-18.46 (4.21)
|
1
|
90%
|
7
|
17%
|
1.19
|
90%
|
4.77
|
25%
|
p,p'-DDD
|
N.D.-2.88 (0.94)
|
2
|
17%
|
20
|
0%
|
1.22
|
29%
|
7.81
|
0%
|
p,p'-DDE
|
N.D.-22.58 (4.44)
|
2.2
|
96%
|
27
|
0%
|
2.07
|
96%
|
374
|
0%
|
DDTs
|
4.40-24.02 (11.87)
|
1.58
|
100%
|
46.1
|
0%
|
3.89
|
100%
|
51.7
|
0%
|
γ-HCH
|
1.54–30.87 (7.24)
|
-
|
-
|
-
|
-
|
0.32
|
100%
|
0.99
|
100%
|
Dieldrin
|
N.D.-0.9 (0.42)
|
-
|
-
|
-
|
-
|
0.71
|
10%
|
4.3
|
0%
|
Endrin
|
0.51–2.52 (1.24)
|
0.02
|
100%
|
45
|
0%
|
-
|
-
|
-
|
-
|
Heptachlor
|
N.D.-7.12 (1.11)
|
0.5
|
44%
|
6
|
2%
|
2.26
|
13%
|
4.79
|
6%
|
-: not available; a: Effects Range Low value; b: Effects Range Median value; c: Threshold Effects Level; d: Probable Effects Level; e: Percentage of samples above the corresponding levels. Guidelines data are from references (Liu et al. 2019; Wang et al. 2017) |
The ILCRs for children, adolescents, and adults in the main urban area of Changchun were calculated to integrate their lifetime risks of exposure to the soil OCPs through the pathways of ingesting, dermal contact, and inhalation, the results are presented in Fig. 6. To be specific, ILCR value ≤ 10− 6 represents very low risk, 10− 6 ≤ value ≤ 10− 4 low risk, 10− 4 ≤ value ≤ 10− 3 moderate, 10− 3 ≤ value ≤ 10− 1 high, and value ≥ 10− 1 very high cancer risk (Niu et al. 2013). The calculated results of the total cancer risk under three exposure pathways for different age groups exposure populations were between the threshold value of 10− 6 and 10− 4, which indicates that the OCPs polluted soils may pose a low cancer risk to children, adolescents, and adults dwelling in Changchun. However, the estimated ILCRs of OCPs residues in individual sampling sites (including CE1 and CE5) exceeds the threshold value of 10− 4 but are lower than 10− 3, which may exhibit a moderate risk to the exposure receptor. Our evaluation results in Changchun urban soil are lower than the exposure cancer risk of OCPs in Ningde agricultural soil (Qu et al. 2015), but higher than that in Iran agricultural soil(Jorfi et al. 2019; Kafaei et al. 2020). For the different age group exposure populations, the total ILCRs under the three exposure pathways and the individual ILCRs under the ingestion route were children > adults > adolescence, while the individual ILCRs under the exposure route of dermal contact and inhalation exhibited adults > adolescence > children. Most of these differences may be owing to children being more likely to ingest contaminated soil by mouth due to their playing with soil (Qu et al. 2017), while their skin surface contact area is relatively small and their respiratory system is underdeveloped compared with adolescents and adults (Qu et al. 2016). Among various exposure routes, the trend of increasing cancer risk all exhibit that ingestion > dermal > inhalation for different age group exposure populations, which also reported for OCPs polluted soils of Xiangfen County, China (Ma et al. 2016). Due to digestion being the major route of exposure in our study, children are more vulnerable in the urban soil from Changchun.