Introduction

Soil is not only the feculence absorption and purification field of urban space, but also the secondary pollution source of urban water, air and other environmental systems. Being an important part of the urban ecosystem health, heavy metal pollution in urban soil should receive extensive attention.

In European and American countries, research on heavy metals in urban soils started early, and studies on the characteristics, classification and restoration of urban soils have been carried out successively (Carey et al., 1980; Madrid et al., 2002; Manta et al., 2002; Wilcke et al., 1998). In recent years, relevant studies have been carried out in China and rapidly developed (Chen et al., 2005; Cheng et al., 2014; Luo et al., 2012; Pan et al., 2018; Wong et al., 2006). The process of urbanization is closely related to soil heavy metal pollution. It has been reported that metal pollution in urban soils is to a large extent linked to traffic emissions and specific industrial activities in the surrounding area, for both developing (Al-Shayeb, 2003; Loredo et al., 2003; Mller et al., 2005) and developed countries (Cicchella et al., 2008; Mielke et al., 2011). Exposure to urban air is the main factor determining the contents of heavy metals in urban surface soil. Studies on the effects of urbanization on soil heavy metal concentration indicated that up to 59.0% of the anthropogenic heavy metals in urban soils resulted from atmospheric deposition throughout the city. Approximately 15.3% derived from other input loads such as indoor volatilization, waste discharge, fertilization and irrigation, which significantly related to the density of local population and road (Peng et al., 2013). The diffusion patterns of these pollutants and the proximity of urban soils to humans increase the risk of human exposure through ingestion, inhalation or dermal contact (Abrahams, 2002; Ajmone-Marsan & Biasioli, 2010).

Yangtze River Economic Belt (YREB), which stretches across the three major regions of China’s east, middle and west, is a major national strategic development region and a pioneering demonstration region for ecological civilization construction. It covers 11 provinces and cities including Shanghai, Jiangsu, Zhejiang, Anhui, Jiangxi, Hubei, Hunan, Chongqing, Sichuan, Yunnan and Guizhou, with a total area of about 2,052,300 km2, accounting for 21.4% of China, and the population and GDP both exceed 40% of China (National Bureau of Statistics of China, 2020). There are 162 prefecture-level cities in the YREB, and the existing researches on heavy metal pollution of these urban soils are mainly concentrated in the Xiangjiang River Basin, while the researches on cities in other regions are still few and scattered (Deng et al., 2019; Li et al., 2012, 2013; Wang & Lu, 2011; Wang et al., 2008; Wu et al., 2009; Zhang et al., 2006, 2009, 2014). Moreover, most studies indicate that sampling schemes and analytical methods used in these urban soil geochemical surveys are not uniform. These differences hinder meaningful comparisons between studies and highlight the urgent need to harmonize approaches to produce comparable datasets in different locations.

Systematic geochemical mapping is the best method to evaluate the content of chemical elements on the earth’s surface. In 1999, China launched the National multi-purpose regional geochemical survey (NMPRGS) (Li et al., 2014). In NMPRGS, a double-layer grid is applied to collect top and deep soil samples. Generally, topsoil at a depth of 0–20 cm is collected at a density of 1 sample/km2, while the deep soil at a depth of 150–180 cm is collected at a density of 1 sample/4 km2 to reflect the geological background. To reduce the cost of analysis, 4 km2 of topsoil samples and 16 km2 of deep soil samples are synthesized for analysis and testing. All samples are measured by a unified analytical method for 54 elements and indexes, which ensure that the data could be compared (Zhang, 2005).

This study used high-precision data collected by NMPRGS (7354 topsoil samples, 1860 deep soil samples) and conducted research on soil pollution of As, Cd, Cr, Cu, Hg, Ni, Pb, Zn and health risks in 86 urban planning areas above the prefecture level in the YREB (Fig. 1). In addition, research on the distribution pattern and source identification of heavy metals in key cities has been carried out in detail. So far, this is a relatively comprehensive and systematic research result of soil heavy metal pollution and health risk assessment in urban areas of YREB. This research will provide important basic information for urban land use management and pollution prevention in the YREB.

Fig. 1
figure 1

Schematic diagram for urban planning area of cities above the prefectural level in YREB

Full size image

Data processing

The geo-accumulation index (I geo)

An overview of the heavy metal contamination levels in the topsoil of each city was obtained by using the geo-accumulation index (Igeo) values introduced by (Müller, 1969). The Igeo was calculated using the following equation:

$$I_{{{\text{geo}}}} = \;{\text{log}}_{2} \left( {C_{n} /1.5B_{n} } \right)$$

where Cn is the measured concentration of heavy metals in the topsoil and Bn is the geochemical background for the corresponding heavy metals. In this study, the median concentrations of different metals in all 1860 urban deep soil samples were computed as the geochemical background values. A constant 1.5 was introduced to compensate for background value changes that might be caused by petrogenetic variations. Pollution degrees are classified into seven levels based on Igeo values:

Igeo ≤ 0, uncontaminated; 0 < Igeo ≤ 1, uncontaminated to moderately contaminated; 1 < Igeo ≤ 2, moderately contaminated; 2 < Igeo ≤ 3, moderately to heavily contaminated; 3 < Igeo ≤ 4, heavily contaminated; 4 < Igeo ≤ 5 heavily to extremely contaminated; and Igeo ≥ 5, extremely contaminated.

Pollution indexes (PI) and the integrated pollution index (IPI)

The pollution index PI and the integrated pollution index IPI were used to evaluate the pollution levels of heavy metals in urban soil (Lee et al., 2006; Luo et al., 2012). The PI and IPI were calculated using the following equation:

$${\text{PI}}_{i } = C_{i} /T_{i}$$
$${\text{IPI}}_{N} \; = \;\left[ {\left( {{\text{IPI}}^{2}_{{{\text{avg}}}} + {\text{PI}}^{2}_{{{\text{max}}}} } \right)/2} \right]^{1/2}$$

where Ci is the concentration of a given metal in topsoil samples and Ti is the corresponding management target value of the metal. IPIavg is the average value of PIi of all heavy metals involved in statistics, and PImax is the maximum value. Pollution degrees are classified into five levels based on IPIN values:

IPIN ≤ 0.7, safe; 0.7 < IPIN ≤ 1.0, precaution; 1.0 < IPIN ≤ 2.0, slight pollution; 2.0 < IPIN ≤ 3.0, moderate pollution; and IPIN ≥ 3.0, heavy pollution.

China has a vast territory, and the geological background of soil in different cities varies greatly. Some studies have proposed a robust statistical method to describe the variation range of geochemical background values by using median (Me) and median absolute deviation (MAD) (Reimann and Filzmoser, 2000; Thompson, 2011). In this study, Me ± 3MAD of all the urban deep soil samples in the study area is used to represent the variation range of the background value of each element, and the upper limit of the background value is taken as the management target value of the corresponding element.

Human health risk assessment

Human health risk assessment is to estimate the type and probability of health hazards of residents exposed to chemicals in a polluted environment (National Research Council, 1983). Due to behavioral and physiological differences, this study divided urban population into minors and adults to assess and analyze their health risks from exposure to urban soil.

Exposure assessment

The exposure assessment methods used in the research are generally based on the recommendations provided by several US and Canadian publications (HC, 2004; USEPA, 1989; USEPA, 2002; USEPA, 2011; USEPA, 2019). The human body is generally exposed to environmental pollutants through three ways: oral ingestion, inhalation and dermal contact (USEPA, 2011). However, for heavy metals in the soil, oral ingestion and dermal contact are considered to be the main routes of exposure (Ferreira-Baptista & Miguel, 2005; Fryer et al., 2006). Following formulas are used to calculate the average daily intake (ADI) of urban population exposed to soil heavy elements through the above two channels. Related parameter values are shown in Table 1.

$${\text{ADI}}_{{{\text{oral}}}} = \frac{{C \times {\text{IR}}_{{{\text{ing}}}} \times {\text{EF}} \times {\text{ED}}}}{{{\text{BW}} \times {\text{AT}}}} \times 10^{ - 6}$$
$${\text{ADI}}_{{{\text{dermal}}}} = \frac{{C \times {\text{SA}} \times {\text{SAF}} \times {\text{ABS}} \times {\text{EF}} \times {\text{ED}}}}{{{\text{BW}} \times {\text{AT}}}} \times 10^{ - 6}$$
Table 1 Definition and reference value of some parameters for health risk assessment of heavy metals in urban soils
Full size table

Non-carcinogenic risk assessment

Non-carcinogenic risk is usually characterized by the risk quotient (HQ), which is defined as the ratio of the daily ingested dose of a particular chemical substance to the reference dose (RfD). The equation is as follows:

$${\text{HQ }} = \frac{{{\text{ADI}}}}{{{\text{RfD}}}}$$

where, RfD is the reference dose of a specific chemical substance. Hazard index (HI) is used to evaluate the potential non-carcinogenic risk caused by a variety of chemical substances. Hazard index (HI) is calculated by the following formula.

$${\text{HI }} = \, \Sigma HQi \, = \Sigma \frac{{{\text{ADI}}_{{\text{i}}} }}{{{\text{RfD}}_{{\text{i}}} }}$$

If the HI value is less than 1, the exposed individual will not be affected by obvious adverse health effects. If the HI value is greater than 1, non-carcinogenic health hazards may occur, and this possibility tends to increase as the HI value increases. Since there is currently no reference dose value that can be used to evaluate dermal absorption exposure to environmental pollutants, the USEPA (2002) provides a method for assessing dermal risk, multiplying the oral reference dose by a gastrointestinal absorption coefficient. Related reference values are shown in Table 2.

Table 2 Summary of reference dose (RfD) and slope factor (SF) of heavy metals through oral ingestion and dermal contact
Full size table

Carcinogenic risk assessment

The carcinogenic risk is estimated by calculating the incremental probability of an individual suffering from cancer due to exposure to potential carcinogens in his lifetime. The slope factor (SF) converts the average daily intake of a certain toxic substance over a lifetime into the carcinogenic risk (CR) of an individual. If exposed to multiple carcinogenic pollutants, the cancer risks from all pollutants and routes are summed. CR calculation formula is as follows:

$${\text{CR }} = {\text{ ADI }} \times {\text{ SF}}$$

A CR value below 1 × 10−6 has no significant effect on human health. A CR value between 1 × 10−6 and 1 × 10−4 is generally acceptable, while a CR value above 1 × 10−4 indicates a lifetime carcinogenic risk to an individual. The SF of the heavy metals through oral ingestion and dermal contact is also shown in Table 2.

Results and discussion

Heavy metal concentrations in urban soils

The heavy metal concentrations in urban topsoils and deep soils in all of the investigated cities in YREB are presented in Tables 3, 4, 5, 6. Among the cities, the concentrations of the heavy metals varied over a large range. The median concentrations of As, Cd, Cr, Cu, Hg, Ni, Pb and Zn in deep soils are 9.2, 0.110, 82, 27.5, 0.049, 35.0, 26.1 and 78 mg kg−1, respectively. The ratios of the above contents in deep soils and the corresponding background values in Chinese soil (CEMS, 1990) are, respectively, 0.82, 1.13, 1.34, 1.20, 0.75, 1.30, 0.97 and 1.05, indicating the contents of both are relatively consistent, and generally in the order of Cr/Zn > Ni/Cu/Pb > As > Cd/Hg.

Table 3 Statistical summaries of heavy metal concentrations in urban topsoil (0–20 cm) from cities in study area (mg kg−1)
Full size table
Table 4 Statistical summaries of heavy metal concentrations in urban topsoil (0–20 cm) from cities in study area (mg kg−1)
Full size table
Table 5 Statistical summaries of heavy metal concentrations in urban deep soil (150–180 cm) from cities in study area (mg kg−1)
Full size table
Table 6 Statistical summaries of heavy metal concentrations in urban deep soil (150–180 cm) from cities in study area (mg kg−1)
Full size table

The median concentrations of As, Cd, Cr, Cu, Hg, Ni, Pb and Zn in topsoils are 9.3, 0.210, 80, 33.0, 0.130, 32.7, 34.1 and 97 mg kg−1, respectively. The contents of heavy metals in urban topsoils are arranged in the following order Zn > Cr > Ni/Cu/Pb > As > Cd > Hg, which are consistent with the deep/background values, indicating geological background is of great significance in determining heavy metal contents in urban soils.

The contents of above heavy metals in urban topsoils are 1.01, 1.91, 0.98, 1.20, 2.65, 0.93, 1.31 and 1.24 times of urban deep soils. The general increase in the contents of heavy metals in topsoils suggests the influence of human activities in urban areas, and among the eight heavy metals, human activities have the most significant impact on the changes of Cd and Hg contents.

In terms of contents, cities with prominent high contents of Cd, As, Pb and Zn are mainly concentrated in Hunan and Hubei Province, and cities with major high Cr, Cu and Ni contents occur in Yunnan and Guizhou Provinces, while cities in Zhejiang and Jiangsu Province principally display remarkable high contents of soil Hg.

Degree of contamination with heavy metals in urban soils

The geo-accumulation index (Igeo), pollution indexes (PI) and integrated pollution index (IPI) of all of the investigated cities are presented in Table 7.

Table 7 Geo-accumulation indexes (Igeo) and pollution indexes (PIs) of heavy metals in urban soils from cities in study area
Full size table

The overall statistical results show that the Igeo of eight heavy metals in urban soils decreases in order of Hg (0.82) > Cd (0.35) > Pb (−0.20) > Zn (−0.27) > Cu (−0.32) > As (−0.57) > Cr (−0.63) > Ni (−0.68), indicating that on a regional scale, urban soils in YREB are uncontaminated to moderately contaminated by Hg and Cd, while As, Cr, Cu, Ni, Pb and Zn are uncontaminated.

Cd in Zhuzhou has the highest degree of pollution, showing heavily contaminated (Igeo = 3.42, the same below). Soil Cd shows moderate to heavy contamination in 4 cities: Chenzhou (2.91), Huangshi (2.56), Xiangtan (2.41) and Hengyang (2.31). Eighteen cities soil Cd display moderately contaminated level, namely Yuxi (1.77), Deyang (1.66), Changsha (1.60), Zhaotong (1.53), Loudi (1.49), Kunming (1.47), Yiyang (1.38), Leshan (1.35), Tongling (1.34), Chizhou (1.29), Zigong, (1.23), Guiyang (1.20), Shangrao (1.20), Shaoyang (1.18), Bijie (1.18), Changde (1.05), Neijiang (1.03) and Jingzhou (1.02). In general, there are 23 cities with moderate Cd pollution or above, accounting for 26.74% (Fig. 2).

Fig. 2
figure 2

Cd pollution level map of cities in YREB

Full size image

Four cities reveal moderate to heavy contamination of Hg in urban soils, namely Shaoxing (2.82), Ningbo (2.39), Suzhou (2.12) and Wuxi (2.08). Besides, 23 cities are moderately contaminated by Hg, respectively, Zhuzhou (1.75), Hangzhou (1.71), Chengdu (1.71), Changzhou (1.65), Xiangtan (1.63), Jiaxing (1.62), Yangzhou (1.58), Huzhou (1.56), Wenzhou (1.56), Guiyang (1.51), Shaoyang (1.48), Jingzhou (1.47), Yiyang (1.40), Changde (1.40), Shanghai (1.29), Hengyang (1.26), Loudi (1.21), Fuzhou (1.21), Changsha (1.18), Chenzhou (1.15), Huangshi (1.14), Deyang (1.03) and Ji’an (1.03). Totality, there are 27 cities with moderate Hg pollution or above, accounting for 31.40% (Fig. 3).

Fig. 3
figure 3

Hg pollution level map of cities in YREB

Full size image

In addition, soil As displays moderate to heavy contamination in Chenzhou (2.24) and moderate contamination in Huangshi (1.35). Soil Cu is moderately contaminated in Kunming (1.34). Huangshi (1.59), Chenzhou (1.42) and Zhuzhou (1.24) are moderately contaminated by Pb. Soil Zn shows moderate contamination in Huangshi (1.10). Soil Cr and Ni basically show no contamination in all cities.

In YREB, there are complex associations between metal pollutants which vary greatly between different cities. According to the Igeo value, Chenzhou, Zhuzhou, Xiangtan and Huangshi exposed the combined pollution of As, Cd, Cu, Pb, Zn and Hg, while Kunming is polluted by Cd, Cu, Pb, Ni and Hg, Hengyang by Cd, As, Pb, Zn and Hg, Guiyang and Tongling by Cd, As, Pb, Cu and Hg, other cities are, respectively, polluted by 4 or 3 or 2 or 1 element. In total, 12 cities showed no pollution of all eight metals, accounting for only 13.95%.

Integrated pollution index (IPI) reveal that the soil in 7 cities in YREB is heavily polluted, 10 are moderately polluted, 51 are slight polluted, 17 are on precaution, and only one is safe. The 7 heavily polluted cities are Zhuzhou (IPI = 7.51, the same below), Chenzhou (5.44), Huangshi (4.36), Shaoxing (4.28), Xiangtan (3.83), Hengyang (3.58) and Ningbo (3.22), and the 10 moderately polluted cities are Suzhou (2.69), Wuxi (2.61), Yuxi (2.48), Deyang (2.31), Changsha (2.25), Kunming (2.20), Zhaotong (2.18), Loudi (2.11), Chengdu (2.08) and Hangzhou (2.04). Cd and Hg are also decisive pollution indexes in the above cities.

Distribution patterns and source identification in key cities

Zhuzhou City

The Igeo of soil Cd in Zhuzhou ranks first in the cities of YREB and has reached a heavily contaminated level, and the Igeo of soil Pb is in the third place, reaching a moderately contaminated level. The distribution patterns of Cd and Pb in the top and deep soils of Zhuzhou city are recorded in Figs. 4 and 5. The maps demonstrate that the spatial distribution of Cd and Pb show a similar pattern. However, the high value areas of Cd and Pb in topsoils are highly consistent with the Qingshuitang industrial zone, showing different spatial distribution patterns from deep soils.

Fig. 4
figure 4

Distribution of Cd in the topsoil a and deep soil b of Zhuzhou

Full size image
Fig. 5
figure 5

Distribution of Pb in the topsoil a and deep soil b of Zhuzhou

Full size image

Zhuzhou is the second largest city in Hunan Province, belonging to the Chang-Zhu-Tan Metropolitan Region (a group of cities, in which Changsha, Zhuzhou and Xiangtan are the core cities). Zhuzhou is a typical heavy industry base, and its industrial development began in the 1930s. Qingshuitang industrial zone, adjacent to Xiangjiang River, is a well-known smelting and chemical industry base in China. The emission of heavy metal pollutants, such as Cd, Pb and As, had been at a high level for a long time. Since 2007, Zhuzhou became a demonstration area of China's two-oriented society (environmentally friendly and economically efficient), and its environmental quality improved significantly, but heavy industry production over the past decades has caused serious damage to the local environment. Until 2017, the contents of Cd, Pb and As in the sediments of the ponds in Qingshuitang industrial area were still 657, 44 and 12 times of the background values of the soil in Hunan (Yang et al., 2019).

Cd and Pb are the representative elements of non-ferrous metallurgy emission (Wang et al., 2014). The statistical results in this study reflect that the contents of Cd and Pb are 3.58 and 138.94 mg kg−1 in Zhuzhou's overall topsoils corresponding to 0.23 and 34.31 mg kg−1 in the deep soils. The contents of Cd and Pb in topsoil are 15.56 and 4.05 times of that in the deep soil, respectively. Meanwhile, the contents of Cd and Pb in the topsoils are 12.46 and 361.23 mg kg−1 corresponding to 0.13 and 33.25 mg kg−1 in the deep soil of Qingshuitang industrial area. The contents of Cd and Pb in topsoil are as high as 95.85 and 10.86 times of that in the deep soil in this area. As shown in Pearson correlation analysis (Table 8), Cd, As, Pb, Zn, Cu and Hg in Zhuzhou’s topsoils are all significantly positively correlated, indicating that they are of the similar origin, while Cr and Ni are poorly correlated with other metals.

Table 8 Correlation coefficient of heavy metals in topsoil of Zhuzhou
Full size table

According to Jinhe Wang’s and Kai Zhang’s reports, the annual deposition fluxes of Cd, Pb, As, Hg and Cr in Zhuzhou in 2012 were 140.09, 1074.91, 59.69, 0.87 and 9.80 mg/(m2·year). Compared with other cities in China, the concentrations of Cd, Pb, As and Hg in atmospheric dustfall of Zhuzhou were much higher, while the concentration of Cr was close or lower (Wang et al., 2018; Zhang et al., 2014). Moreover, compared with commercial and residential areas, industrial areas have a significantly higher degree of atmospheric pollution (Wang et al., 2017). Combined with the above analysis, it could be concluded that industrial activities are the main source of soil Cd, Pb and other elements pollution in Zhuzhou.

Xiangtan City

The Igeo of soil Cd in Xiangtan ranks the fourth among the cities in YREB and has reached moderately to heavily contaminated level. As an important part of Chang-Zhu-Tan Metropolitan Region, Xiangtan is also a momentous industrial city in Hunan Province. Geographically, Changsha, Xiangtan and Zhuzhou are distributed along the Xiangjiang River from north to south.

The links between Cd contamination in Xiangtan’s urban topsoils and the background in deep soils are exhibited in Fig. 6. The maps exhibit that there is a certain consistency in the spatial distribution patterns of Cd in the top and deep soils. The Cd content of the topsoil in Xiangtan is 1.306 mg kg−1 corresponding to 0.161 mg kg−1 in the deep soil. As 8.11 times that of deep soils, soil Cd content showed obvious accumulation in surface soil. Moreover, it is noteworthy that the distribution of high Cd content in topsoil emerge an obvious trend of migration along the Xiangjiang River.

Fig. 6
figure 6

Distribution of Cd in the topsoil a and deep soil b of Xiangtan

Full size image

Sediment samples along the Xiangjiang River also existed combined metal pollution of Cd, As, Pb and Hg (Liu et al., 2016; Wang et al., 2008), and the concentration of heavy metals in Zhuzhou section was the highest, followed by Xiangtan section and Changsha section (Mao et al., 2013; Zeng et al., 2015), which confirmed the influence of industrial activities in Zhuzhou on downstream cities.

According to Kai Zhang’s report, the concentrations of As, Cd and Pb in atmospheric particles in Zhuzhou and Xiangtan sampling sites were obviously higher than that of Changsha in 2012 (Zhang et al., 2014). Also, research shows that bedrock has little contribution to soil heavy metal pollution in Chang-Zhu-Tan region (Deng et al., 2019; Wu et al., 2007). It can be inferred that, in addition to local industrial activities, soil Cd pollution in Xiangtan is also affected by the industrial wastewater and waste gas discharge from Zhuzhou.

Chenzhou City

The Igeo of soil As and Cd in Chenzhou ranks the first and second in the cities in YREB and has reached a moderately to heavily contaminated level, and the Igeo of soil Pb is in the second place, reaching a moderately contaminated level.

Chenzhou, located in the upper reach of Xiangjiang River in Hunan Province, is a famous hometown of non-ferrous metals in China, with more than 30 large polymetallic deposits such as Shizhuyuan, Huangshaping and Shuikou Mountain. The heavy metal pollution of soil in Chenzhou has been reported in some literatures, among which As is the most concerned (Liao et al., 2005). It has been reported that As content of farmland soil in the Shizhuyuan mining area reached 87.71–1351.9 mg kg−1(Zeng et al., 2006) and widespread cases of excessive heavy metals in crops existed (He et al., 2007; Liu et al., 2005). The mining and smelting of minerals are considered to be the main cause of heavy metal pollution in the soil (Duan et al., 2018; Lei et al., 2008).

In this study, the distribution patterns of As, Cd and Pb in the top and deep soils of Chenzhou are relatively consistent (Fig. 7, Cd and Pb are not shown). The As, Cd and Pb contents of the topsoil in Chenzhou are 120.1, 1.886 and 242.6 mg kg−1 corresponding to 99.1, 1.194 and 143.8 mg kg−1 in the deep soil, and the rates are only 1.21, 1.58 and 1.69. In the box plot of heavy metal content in deep soils of key cities, the contents of As, Cd and Pb in deep soil of Chenzhou are significantly higher than other cities (Fig. 8). The above indicates that the heavy metal pollution in Chenzhou’s urban soil is mainly controlled by geological background and superimposed with man-made pollution factors.

Fig. 7
figure 7

Distribution of As in the topsoil a and deep soil b of Chenzhou

Full size image
Fig. 8
figure 8

Box comparison chart of As, Cd, Pb, Zn and Cu contents in deep soils of key cities

Full size image

Huangshi City

The Igeo of soil Pb and Zn in Huangshi ranks first in the cities of YREB and has reached a moderately contaminated level, the Igeo of soil As is in the second place, reaching a moderately contaminated level, and Igeo of soil Cd is in the third place, reaching a moderately to heavily contaminated level. Huangshi has the most types of heavy metal elements that have reached a moderate pollution level and above, showing a serious situation of polymetallic compound contamination.

Huangshi, located in the southeast of Hubei Province, is in the center of the polymetallic ore deposit belt in the lower reaches of the Yangtze River. With a large number of industrial bases such as steel works, cement factories and smelters, Huangshi is known as the “hometown of bronze,” “cradle of iron and steel” and “hometown of cement” in China.

The spatial distribution patterns of soil Zn, Pb, Cd and As in Huangshi are similar, and there are some differences in the distribution patterns of the four elements in the topsoils from the deep soils (Fig. 9, Pb, Cd and As are not shown). The Zn, Pb, Cd and As contents of the topsoils in Huangshi are 377, 324.8, 5.222 and 77.5 mg kg−1, corresponding to 2.9, 5.0, 15.36 and 4.0 times of that in the deep soils. The contents of Cd, Pb and Zn in the deep soils of Huangshi are slightly higher than that in other cities (Fig. 8), indicating that the geological background has a certain contribution to the accumulation of the above elements in the topsoils. Meanwhile, the differences of heavy metal content distribution between top and deep soils demonstrate that human activities have reshaped the distribution pattern of heavy metals in the topsoils of Huangshi.

Fig. 9
figure 9

Distribution of Zn in the topsoil a and deep soil b of Huangshi

Full size image

In 2012, the contents of Zn, Pb, and Cd in atmospheric dustfall in Huangshi were 2122.53, 949.30 and 41.35 mg kg−1, which were, respectively, 25.4, 35.5 and 240.4 times of the background value of soil elements in Hubei Province and much higher than other cities in China and abroad (Liu et al., 2014). Zn content reached its maximum value in industrial zone and Pb content in commercial zone (Yao et al., 2016). Moreover, Xianjun Xie’s evaluation results point out that the mine wastewater and beneficiation wastewater in this area have been seriously polluted, and Zn, Mn, etc., are the leading factors in water environmental pollution (Xie & Han, 2003). Considering the long-term mining and industrial activities in Huangshi, it could be concluded that the heavy metal pollution in the soil of Huangshi is mainly related to the local human behaviors such as mining and smelting, fossil fuel burning, automobile exhaust emissions and so on.

Kunming City

The Igeo of soil Cu in Kunming ranks first in the cities of YREB and have reached a moderately contaminated level. Located in the middle of the Yun-Gui Plateau, Kunming is the capital city of Yunnan Province and in general a tourist city with less industry. Studies have reported that the average Cu content in street dust in Kunming is 166.6 mg kg−1, with the highest potential ecological risk coefficient (Liang et al., 2011). The contents of heavy metal elements in dust, atmosphere and precipitation in Kunming have a clear accumulation trend in the dry season, and relatively decreases in the rainy season (Liang et al., 2011; Shen et al., 2016; Shi et al., 2018). Cu, Hg, Pb, and Zn in topsoils are highest in vicinity of the smelter and in urban traffic congestion areas (Liao et al., 2015). Research suggests that soil pollution in Kunming is dominated by industrial area pollution, and the pollutants are mainly related to dust reduction, waste residue and wastewater from smelters. At the same time, the impact of traffic pollution on the city's soil quality cannot be underestimated (Miao et al., 2015).

In this study, the Cu content of the topsoil in Kunming is 119.3 mg kg−1 corresponding to 106.2 mg kg−1 and 1.12 times of that in the deep soil, and the distribution patterns of Cu in the top and deep soils of Kunming are extremely consistent (Fig. 10). In addition, background content of Cu in the deep soil of Kunming is significantly higher than other cities (Fig. 8). All these indicates that the Cu pollution in Kunming urban soil is mainly controlled by geological background and anthropogenic pollution has little influence.

Fig. 10
figure 10

Distribution of Hg in the topsoil a and deep soil b of Shaoxing

Full size image

Shaoing City, Ningbo City, Suzhou City, Wuxi City

Shaoxing, Ningbo, Suzhou and Wuxi are the top four cities in the Igeo of soil Hg in YREB, all of which have reached moderately to heavily contaminated levels. Shaoxing, Ningbo, Suzhou and Wuxi have Hg contents of 0.629, 0.572, 0.460 and 0.356 mg kg−1 in the topsoil and 0.224, 0.096, 0.306 and 0.052 mg kg−1 in the deep soil. The Hg content in topsoil corresponds to 2.81, 5.96, 1.50 and 6.85 times of that in deep soil, respectively. The Hg high value zones in the above cities are mainly concentrated in the central or old urban areas, and the trend of Hg content decline from the downtown to the suburbs can be observed (Figs. 10 and 11, take Shaoxing and Ningbo for example). Meanwhile, similar phenomenon can be observed in other cities of YREB. The studies of other scholars also confirmed that among the different heavy metals, Hg changed mostly with the variation of distance to build-up area, indicating that it is greatly affected by human activities (Fang et al., 2011; Han et al., 2006; Li et al., 2013; Tan et al., 2006).

Fig. 11
figure 11

Distribution of Hg in the topsoil a and deep soil b of Ningbo

Full size image

China is the world's largest emitter of Hg (Pacyna et al., 2006, 2010; Pirrone et al., 2010), several studies have shown that Hg emissions from fossil fuels are the largest source of anthropogenic Hg in China (Feng, 2005; Jiang et al., 2006). It has been reported that from 1978 to 2014, China’s anthropogenic Hg emissions totaled 13,294 tons, and the annual Hg emissions increased from 147 to 530 tons (Wu et al., 2016). These Hg emissions may accumulate and deposit in topsoils and enter urban areas through atmospheric deposits. In this study, cities with high levels of soil Hg pollution are mainly concentrated in Jiangsu and Zhejiang Province, which are the two most economically developed provinces in YREB. This confirms that human activities (such as three wastes) in the process of urban expansion and economic development are the main sources and pathways leading to the migration and enrichment of soil Hg.

Non-carcinogenic risk assessment

Non-carcinogenic risk assessment results show that the hazard index (HI) of adults in all cities ranges from 0.06 to 0.48, and the HI values are all less than 1, which would not cause obvious adverse effects on adult health. For minors, the HI values of Chenzhou, Huangshi, Zhuzhou, Hengyang, Xiangtan, Guiyang, Shaoyang, Tongling, Yiyang and Chizhou are greater than 1, and their values are 3.23, 2.22. 1.55, 1.37, 1.28, 1.14, 1.07, 1.04, 1.02 and 1.01, respectively, indicating that minors in the above 10 cities may be exposed to soil environments with potential non-carcinogenic risks. The HI values of minors in all the other cities are less than 1. Due to behavioral and physiological characteristics, minors are more sensitive to environmental pollutants per unit weight than adults (DHAenC, 2012). This is the main reason why minors’ hazard index is greater than that of adults in all cities. Similar conclusion has also been confirmed in other related studies (Liu et al., 2015; Pan et al., 2018).

Intake is the main exposure route of urban population to the 8 heavy metal elements in this study. The median of the risk quotients of the eight elements is ranked as follows: As > Pb > Ni > Hg > Cu > Cr > Zn > Cd. For the two types of exposed people, the sum of the risk quotients of As, Pb, and Ni accounted for more than 90% of the hazard index, indicating that the intake of As, Pb and Ni in soil is the main factor causing the non-carcinogenic risk of the urban public.

Carcinogenic risks assessment

The carcinogenic risks were only estimated for As and Cd here due to lack of a carcinogenic SF for other heavy metal elements. The carcinogenic risk (CR) values of As and Cd for adults range from 4.38E − 06 to 7.42E − 05 and 3.48E − 08 to 6.02E − 07. For minors, the CR values of As and Cd range from 5.72E − 06 to 9.69E − 05 and 4.67E − 08 to 8.08E − 07. This indicates that Cd in urban soil has no carcinogenic risk for the two groups of people, while the impact of soil As on human health is within an acceptable range, and the exposure risk of minors is significantly higher than that of adults no matter what kind of heavy metal elements.

Conclusions

Soil is an important part of the urban ecosystem, and its quality should be recognized. According to geo-accumulation index, on a regional scale, urban soils in YREB are uncontaminated to moderately contaminated by Hg and Cd. Cities with moderate pollution or above of soil Cd and Hg accounted for 26.74 and 31.40% in YREB. Meanwhile, As, Pb, Zn and Cu in a part of cities show different levels of contamination, while Cr and Ni in all urban soils are basically free from contamination. The type of industrialization and the history of urbanization affect the soil heavy metal pollution in majority cities in YREB, and the influence of geological background on the content of heavy metals in urban soils cannot be ignored. The pollution level of heavy metals in the urban soils generally shows the following rule: industrial cities > rapidly developing cities > metropoles > underdeveloped cities. Anthropogenic activities, especially wastewater, waste gas, and waste solid discharged from industrial factories are the main causes of heavy metal soil pollution in cities, although many factories have been closed or moved to other locations in recent years.

Heavy metals in the urban soils investigated generally posed low non-carcinogenic and carcinogenic risks to the adults. However, the non-carcinogenic risk to minors in some cities (e.g., Chenzhou and Huangshi) should be given cautious attention, and the intake of As, Pb and Ni in soil is the main factor causing the non-carcinogenic risk for both adults and minors.

It is suggested to carry out a detailed survey of urban soil pollution in cities with moderate to severe comprehensive pollution, find out the status of soil pollution and the distribution of contaminated land plots, establish contaminated land plot lists and priority control area lists to implement pollution control, restoration and remediation. For cities where soil exposure poses a potential non-carcinogenic risk to minors, the development trend of the risk should be continuously assessed in the future.