Introduction

Polycyclic aromatic hydrocarbons (PAHs) refer to a type of persistent organic pollutants composed of two or more benzene rings. They have attracted much attention in the field of environmental protection owing to their "three causes" effects1. Among them, 16 types of PAHs have been included in the priority list of pollutants by the U.S. Environmental Protection Agency (EPA)2. All human production activities such as fossil fuel combustion, motor vehicle exhaust emissions, coke and asphalt production, generate PAHs3. PAHs can enter plants through plant leaves or settle into the soil from the atmosphere, and then migrate, metabolize, and accumulate in plants through plant roots, thereby threatening human health through the food chain4. In addition, soil, as an important environmental medium, is a storage and transfer station for PAHs in the natural environment, which bears more than 90% of the environmental load of PAHs. The amount of PAHs entering the human body from soil is higher than that from other environmental media, such as air and water5.

In recent years, research on PAHs has mostly focused on the analysis and determination of PAH content in crops, farmland soils, and soils around cities, or on the risk assessment of PAHs in the urban atmosphere in different seasons to humans6,7,8. However, there are few studies on PAHs in urban park soils. In addition, studies have reported that plants can promote the degradation and transformation of PAHs in the soil through root exudates, and the leaves of plants can effectively accumulate PAHs in the atmosphere through the effects of gas diffusion and dry–wet sedimentation9,10. The study of PAHs in common plants in parks can provide a more detailed understanding of the source and spatial distribution of PAHs in parks.

Urumqi is an important node city of the "the Belt and Road" strategy, an important industrial, cultural, political, economic, scientific, technological and transportation center in ** of garbage18. The reason may be that Urumqi is affected by the new coronavirus (COVID-19), and the amount of artificially produced PAHs decreased. The ILCRs in adults and children in both summer and winter are as follows: ingestion > dermal > inhalation. Compared with other exposure routes, the lifetime carcinogenic risk of inhalation is nearly negligible. This is because the lifetime carcinogenic risk caused by children's frequent hand-to-mouth contact while playing in the park is much higher than that of inhalation. The results show that the ILCRs of the PAH exposure pathway of soil samples from Urumqi parks in winter were all lower than the minimum acceptable risk level. The lifetime carcinogenic risk of adults and children is negligible, but high ILCRs in summer require particular attention.

Figure 5
figure 5

ILCRs value of PAHs in park soil.

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The NIPI characterizes the impact or effect of soil pollutants. NIPI can be divided into five levels: NIPI is less than or equal to 0.7, where soil pollution is at a safe level; NIPI is equal to 1.0, where soil pollution is at the warning line; NIPI between 1.0 and 2.0, where soil pollution is at a weak level; NIPI between 2.0 and 3.0, where soil pollution is at a moderate level; and NIPI greater than 3.0, where soil pollution is severe37.

As shown in Fig. 6, in summer, the NIPI range of park soil in Urumqi is 0.025–3.532, with an average value of 0.715. The NIPI value of 75% of the sampling points was less than 1.0, and the ecological risk of the soil was safe. The NIPI value of 20.8% of the sampling points is between 1.0 and 2.0, mainly located in South Park, Zhiwu Park, Shihua Park and Toutunhe Park. The soils of these parks have more PAH accumulation factors than the soils of other parks. At the same time, the maximum value of NIPI is 3.532, which is a severe pollution level. This may be related to the pile of garbage in the park. The NIPI range of soil in Urumqi city parks in winter was 0.004–0.564, with an average value of 0.095. The NIPI value of all sampling points was less than 0.7, indicating that the ecological risk of soil in all parks in winter is at a safe level. The ecological risk of park soil in Urumqi in winter is much lower than that in summer.

Figure 6
figure 6

NIPI value of PAHs in park soil.

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In general, the ecological risks of soil in Urumqi parks can be ignored. Among them, South Park, Zhiwu Park, Shihua Park and Toutunhe Park should adopt a variety of control measures to reduce the ecological risk of soil PAHs through strict environmental management.

Conclusion

In eight typical parks in Urumqi, the average concentration of PAHs in soil in the summer was 3.304 mg/kg. The average concentration of PAHs in soil in the winter was 0.644 mg/kg. The single PAHs in some soil sampling points in summer were higher than the national "Construction Land Standard" (GB36600-2018) soil pollution risk screening value. In summer, 91.6% of the soil samples were moderately or severely polluted, while 20.8% of the soil samples in winter were moderately or severely polluted. The average value of PAHs in soil in the park area in summer was 5–6 times that in winter. PAHs are mainly composed of high-ring PAHs from traffic sources, but the proportion of low-ring PAHs 2–3 increases significantly in winter. Compared with other regions, pollution is more serious, especially in the summer.

In summer, the plants with high PAHs content in leaves are short herbs, while in winter, they are tall arbors. The proportion of seven carcinogenic PAHs was higher in the roots than in the leaves. PAHs in 70% of plant leaf samples were higher in winter than in summer. PAHs are mainly composed of five rings in summer, from traffic sources, and three rings in winter, from the combustion of coal biomass. Compared with plants of other areas, pollution is lighter.

The TEQ values of soil PAHs in South Park, Zhiwu Park, Shihua Park and Toutunhe Park were higher. The summer soil of these four parks is at a low pollution level, compared with other parks. The mean ILCRs in adults and children in all parks reached a low-risk level in summer. The cancer risk of children in different seasons is much higher than that in adults. The sequence of ILCRs for different exposure routes is ingestion > dermal > inhalation, and the lifetime cancer risk of inhalation can be almost ignored.

Materials and methods

Overview of the study area and sample collection

In this study, eight typical parks in Urumqi City were selected as the soil sampling sites. As shown in Fig. 7, these are: South Park, Shuimogou Park, **aolügu Park, Renming Park, Zhiwu Park, Shihua Park, Hongguangshan Park and Toutunhe Park. The sample collection periods were from November 20 to November 22, 2020 (winter), and from June 13 to June 14, 2021 (summer). According to the area of the park, nine sampling points were selected for each park, and every third soil samples were mixed into a mixed sample. The depth of sampling was 0–10 cm. In total, 48 surface soil samples were collected. Meanwhile, South Park was selected for the sampling of plants because it has a large number of plant species and higher passenger flow. Ten kinds of common plants in the park were selected, including five kinds of arbor plants, namely, longclaw willow, juniper, small leaf poplar, white elm and Platycladus orientalis, and five kinds of herbaceous plants, namely clover, silver edge grass, golden chrysanthemum, wild Artemisia and peony. Three plants with the same growth status were selected for each plant, and their leaves and roots were collected. The leaves and roots of each plant collected at different sampling points were mixed separately. There were 30 plant samples in total.

Figure 7
figure 7

Map of sampling sites in parks in Urumqiy (The ArcGIS 10.8 was used to create the geographic map of the study area, URL: http://36.112.130.153:7777/DSSPlatform/index.html).

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Sample handling

We accurately weighed 15 g (accurate to 0.01 g) of soil samples and 1 g of plant samples after freeze-drying, grinding, and screening, put them into 250 ml and 50 ml centrifuge tubes, added 30 ml and 20 ml (v:v = 1/1) dichloromethane acetone solution, respectively, and allowed to stand for 2 h. Homogenates were extracted on the Beater (ULTRA-TURRAX) for 1 min, and then extracted in a water bath constant temperature (30 °C) oscillator (SHA-C) for 30 min. Subsequently, the mixture was centrifuged using a centrifuge (CT18RT) at a speed of 10,000 r/min for 5 min, and then 5 ml of upper organic phase clear liquid was taken. The mixture was concentrated at 40 °C with a parallel evaporator (BUCHI- Syncore), and the volume was combined with 2.5 ml of n-hexane, swirled for 15 s, and then passed through a 0.22 μm membrane injection bottle for testing.

Test analysis

GC–MS analysis test conditions: gas chromatography conditions; injection temperature: 280 °C; no shunt; injection volume: 1.0 μL; column flow rate: 1.0 ml/min (constant flow rate); column box temperature: 60 °C. Mass spectrometry reference conditions electron bombardment source (EI); ion source temperature: 230 °C; ionization energy: 70 eV; interface temperature: 280 °C. Mass scanning range: 33–555 amu; solvent delay time: 4 min. Scanning mode: full scan mode (qualitative analysis) and selective ion SIM mode (quantitative analysis).

Quality control

The quality of the sample was controlled by a blank test, recovery of the blank sample, and parallel sample. One blank sample was used in each batch of samples for the control. No soil or plants were added to the blank sample. Other reagents and treatment conditions were the same as those of normal samples, to ensure cleanliness and pollution-free instruments, reagents, and experimental containers. Each sample was analyzed using a double horizontal sample. The detection limits of 16 PAH monomers ranged from 1.3 × 10–6 to 8.5 × 10–5 mg/kg. Standards (200 μg/kg) of 16 PAHs at known concentrations were added to the uncontaminated matrix samples, and the actual concentrations were measured by the same pre-treatment and quantification method as the samples after 2 h of standing. The value of recovery rate was obtained from (Measured concentration of spiked matrix / spiked concentration)*100%. The experimental analysis shows that the recovery rate of the blank addition was in the range of 75.64–115.89%; there by meeting the requirements of Soil and sediment–Determination of polycyclic aromatic hydrocarbon by Gas chromatography–Mass Spectrometry Method (HJ 805–2016).

Toxicity equivalent concentration method (TEQ)

The carcinogenicity and toxicity of benzo(a)pyrene (BaP) were the strongest among the seven carcinogenic PAHs. Usually, BaP is used to evaluate carcinogenic PAHs, and the toxic effect factor (TEF) is a parameter describing the carcinogenic ability of each monomeric PAH corresponding to BaP38,39,40. The formula for calculating the equivalent concentration of PAHs (TEQ) based on the toxic equivalent factor is as follows40,41:

$${\text{TEQ}} = \sum ({\text{PAH}}_{{\text{i}}} \times {\text{TEF}}_{{\text{i}}} )$$
(1)

where PAHi is the content of PAH monomer i, TEFi is the toxicity equivalent factor of PAH monomer i, TEQ is the toxicity equivalent of the equivalent concentration of the compound, and the TEF value of BaP is 1, which is the highest among all PAHs42. According to the toxicity equivalent of BaP, we used the toxicity equivalence factor (TEF) shown in Table 4 to convert the concentration of PAHs into the toxicity equivalent concentration of PAHs.

Table 4 Parameters used in lifetime carcinogenic risk assessment.
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Incremental Lifetime Carcinogenic Risks (ILCRs)

The incremental lifetime carcinogenic risk (ILCRs) of soil PAH exposure was evaluated according to USEPA standards43. Children and adults are exposed through three main exposure routes: ingestion, dermal and inhalation44. The ILCR equation for soil is as follows:

$${\text{ILCR}}_{{{\text{Ingestion}}}} = \frac{{{\text{CS}} \times \left( {{\text{CSF}}_{{{\text{Ingestion}}}} \times \sqrt[3]{{\frac{BW}{{70}}}}} \right) \times {\text{IR}}_{{{\text{soil}}}} \times {\text{EF}} \times {\text{ED}}}}{{{\text{BW}} \times {\text{AT}} \times 10^{6} }}$$
(2)
$${\text{ILCR}}_{{{\text{Dermal}}}} = \frac{{{\text{CS}} \times \left( {{\text{CSF}}_{{{\text{Dermal}}}} \times \sqrt[{3}]{{\frac{{{\text{BW}}}}{{{70}}}}}} \right) \times {\text{SA}} \times {\text{AF}} \times {\text{ABS}} \times {\text{EF}} \times {\text{ED}}}}{{{\text{BW}} \times {\text{AT}} \times {10}^{{6}} }}$$
(3)
$${\text{ILCR}}_{{{\text{Inhalation}}}} = \frac{{{\text{CS}} \times {\text{(CSF}}_{{{\text{Inhalation}}}} \times \sqrt[{3}]{{{\raise0.7ex\hbox{${{\text{BW}}}$} \!\mathord{\left/ {\vphantom {{{\text{BW}}} {{70}}}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{${{70}}$}}}}{)} \times {\text{IR}}_{{{\text{air}}}} \times {\text{EF}} \times {\text{ED}}}}{{{\text{BW}} \times {\text{AT}} \times {\text{PEF}}}}$$
(4)
$${\text{ILCR}} = {\text{ILCR}}_{{{\text{Ingestion}}}} + {\text{ILCR}}_{{{\text{Dermal}}}} + {\text{ILCR}}_{{{\text{Inhalation}}}}$$
(5)

where CS is the toxic equivalent concentration of monomeric PAHs in the sample (mg/kg), 106 is the conversion coefficient of PAH concentration, and CSF is the carcinogenic slope factor (mg/(kg·d)), based on the carcinogenic ability of BaP. The CSFDermal, CSFIngestion, and CSFInhalation are 25, 7.3, and 3.85 (mg/(kg·d)), respectively44. Other parameter values were obtained from the Guidelines for Site Environmental Assessment (DB11/T 656-2009), as shown in Table 4.

Nemerow multifactor pollution index (NIPI)

The Nemerow multifactor pollution index (NIPI) is one of the most widely used indicators to reflect the ecological risk of soil pollutant exposure37. It can characterize the role of soil pollutants in a seriously polluted environment. The calculation method for the NIPI is as follows:

$${\text{P}}_{{\text{i}}} = \frac{{{\text{C}}_{{\text{i}}} }}{{{\text{C}}_{{\text{s}}} }}$$
(6)
$${\text{NIPI}} = \sqrt {\frac{{{\text{P}}_{{{\text{iavg}}}}^{{2}} + {\text{P}}_{{{\text{imax}}}}^{{2}} }}{{2}}}$$
(7)

where Ci is the measured concentration of monomer i, Cs is the soil environmental quality standard value of monomer i, and Pi is the pollution index of PAH monomer i in the soil. Piavg is the arithmetic mean of PAH pollution index of each monomer, Pimax is the maximum value of the PAH single factor pollution index at the ith sampling point, and NIPI is the multi-factor pollution index at the sampling point. This study used the class II standard value in the soil environmental quality standard (GB15618-2008).

The use of plants in the present study complies with international, national, and institutional guidelines, the collection of plant samples was done with the consent of local relevant departments and institutions.