Risk Assessments of Heavy Metals to Children Following Non-dietary Exposures and Sugarcane Consumption in a Rural Area in Southern China

  • Xiao-fei Wang
  • Chao-bing Deng
  • Geoffrey Sunahara
  • Juan Yin
  • Gui-ping Xu
  • Kai-xian Zhu
Original Paper
  • 32 Downloads

Abstract

Based on the context of a rural area contaminated by mining waste in Guangxi, China, the health risks of seven heavy metals (HM) to local children were evaluated. Risk assessments of soil HMs to individuals exposed through non-dietary routes and consuming locally grown sugarcane (juice) were conducted. Results indicated that the ranking for risks of HM exposures was juice drinking > non-dietary oral intake > dermal contact > inhalation. For non-carcinogenic risks, Cr and Cd in sugarcane juice posed the most significant risks, whereas As and Pb posed the highest risks through non-dietary exposure routes. A newly established assessment model using a regression equation of hazard index of HMs to ratios of soil HM concentrations/total reference doses, showed that the non-dietary HM risk was closely correlated to the environmental HM levels and human sensitivities, which can be measured by HM total daily reference doses.

Keywords

Heavy metals Non-dietary exposure Health risk Sugarcane Children 

Introduction

Contamination of the environment by heavy metals (HM) has become a worldwide concern to human health (Alloway 1990; Plum et al. 2010; Flora et al. 2012). HM pollution could be generated from industrial disposal and recycling processes of materials containing HMs (Fujimori and Takigami 2014), vehicular emissions (Wei and Yang 2010), metal ore mining and its relevant smelting industries (Bacon and Dinev 2005; Anju and Banerjee 2012; Li et al. 2014b). HM pollution in early developing rural areas can also arise from air-borne particles containing HMs from local rudimentary industries that contaminate surface soil by atmospheric deposition (Liu et al. 2013; Li et al. 2014a). Issues may arise when these soil contaminants are bioavailable to plants. Phytoaccumulation of HMs is one of most important pathways by which HMs can enter and affect different ecological systems (Nagajyoti et al. 2010). Hyper-accumulation by some plants has been used as a phytoremediation technology for sites contaminated with HMs (Saba et al. 2015); however, there are risks. For example, ingestion and absorption of HMs in grains and vegetables from contaminated plants used for phytoaccumulation can pose human health risks (Zhou et al. 2007; Li et al. 2007; Nagajyoti et al. 2010; Khan et al. 2010).

Intensive studies have been conducted on the human health risks posed by exposure to HMs in contaminated soils in mining areas, sewage irrigated areas, and in dust (Wang et al. 2010; Wei and Yang 2010; Liu et al. 2013; Li et al. 2014b). HMs released from mining operations may contaminate water used for irrigation, as reflected by evaluations of HM levels in farmland and contaminated water resources (Liu et al. 2013; Zhang et al. 2015). This type of risk can be more severe in the rural areas of developing countries, where farmland may be located near mining activities due to limited cultivatable land (Li et al. 2007). Examples of hazardous effects of HMs to humans include kidney and bone illness (ItaiItai disease) and respiratory system damage caused by Cd (Järup et al. 1998; Godt et al. 2006), carcinogenic risks of Cr (Naz et al. 2016), and toxicity of Pb to the nervous system and organs (Tong et al. 2000). HMs can induce human health risks through a variety of exposure pathways, including oral, inhalation, and dermal contact (Swarnalatha et al. 2015; Zheng et al. 2015) and dietary (water and food) (Zhang et al. 2015). Children are a vulnerable group harmed by HMs because of their immature physical condition and poor hygiene habit, including hand-to-mouth behaviours or demonstration of pica disorders (e.g., geophagia or eating soil), which increases the risk of HM exposure (Gorini et al. 2014). In the present article, we assess health risks to children following exposure to hazardous HM, using models recommended by the US Environmental Protection Agency (USEPA) (USEPA 1989, 2001, 2002, 2004), and incorporating some physiological data of average individuals (e.g., body weight) within the described local context (MEPC 2014).

The present article describes a typical agricultural activity zone of Karst landform located in a HM-contaminated rural area along the Huangjiang River in Hechi City (Guangxi, China) (Fig. 1). Herein, we evaluated the potential carcinogenic and non-carcinogenic (toxicological) risks to local children through non-dietary exposures and consumption of sugarcane juice, a common drink supplying free sugar to local residents. We focused on the local children group (≤ 6 years, 1 year before legal school age) in this study. Seven elements including Cu, Zn, Pb, Cd, As, Cr and Ni were considered in this study, and will be referred to as ‘heavy metals’, although As is a metalloid. This research aims to examine the health risks of environmental HM to a sensitive sub-population (children), by identifying the characteristics of HM exposure through different non-dietary routes during daily activities, and by drinking local sugarcane juice as an example of a dietary pathway.
Fig. 1

Location of sampling points in the present study

Materials and Methods

Sampling Area

The soil samples were collected from farmland in a humid agricultural zone in the Huangjiang basin (108°14′26″E/25°3′15″N to 108°14′34″E; 25°3′3″N; see Fig. 1). The farmland in this study area was intensively cultivated due to land shortage. In 2001, a catastrophic flood flushed tailings and effluents from the damaged facilities of a Pb–Zn mine (located upstream) into the local water body, causing severe pollution of this area. To date, the major non-edible subsistence crop grown by the local farmers is sugarcane, used as a raw material for emerging bio-fuel industries (ethanol). Due to soil contamination, most staple foods such as rice and other edible plant food, have stopped being produced and purchased by markets outside the region. Reliable tap water facilities were constructed under the direction of the local authorities to replace the traditional drinking water sources, such as wells and river water. However, residents continued to consume fresh sugarcane or its juice. HM in soil can threaten the health of local children especially pre-schoolers (6 years or younger), who spend most of their time playing on the HM-contaminated soil field and would be exposed unintentionally to soil contaminants through dermal contact and inhalation. Children of this locale are also exposed to HM through ingestion, which includes oral intake of dust and drinking homemade juice (crudely squeezed sugarcane juice diluted with fresh water) from local grown sugarcane.

Sample Collection, Pre-treatment and Chemical Analysis

The soil mixture was collected with a wooden spoon using a double diagonal method (sampling depth 0–20 cm); each sample weighed approximately 1 kg. A batch of six soil samples and 2 kg of sugarcane stems were collected on 4 November 2014, and the remaining 31 soil samples were collected on 7 January 2015. Samples were kept in cloth bags and carried back to the laboratory, where they were air dried in a ventilated light-avoiding place. Small pieces of stone and other foreign matter were removed. The air-dried samples were roughly grounded, then finely grounded using an agate mortar. Samples were sequentially passed through a 20-mesh sieve and a 100-mesh sieve, and packed into polyethylene zipper bags prior to analysis.

The concentration of HMs in soil samples was determined by inductively coupled plasma mass spectrometry (ICP-MS), using a 7700e ICP-MS (Agilent Technology Inc. USA) after sample digestion in HNO3–HCl–HF (3:1:1, v/v) using a MARS 6 Microwave Digestion system (CEM Corp. USA) (Su et al. 2015). Certified reference materials of soil samples GSS-14 and GSS-16 (Institute of Geophysical and Geochemical Exploration, China) were used for quality control, with recoveries between 95 and 105%. The sugarcane stems were washed with clean water and flushed with Milli-Q water, wiped dry with a clean cloth, and then squeezed into juice. Sugarcane juice (10 mL) was then mixed with concentrated nitric acid (20–25 mL) and digested, and boiled at 180 ± 5 °C on a hot plate to about 5 mL, prior to cooling to room temperature and mixed with 15 mL aqua regia. This mixture was digested (at 180 ± 5 °C) to obtain a transparent liquid (1–2 mL), which was then diluted with Milli-Q water to 10 mL (final volume). The HM concentration of the extract was determined by 7700e ICP-MS.

Human Exposure Models

HM from the soil could be taken up by local residents following different absorption routes, including oral ingestion (non-dietary or dietary), dermal contact and inhalation. The estimated HM from soil (expressed as the average daily dose, ADD) was assessed using USEPA health risk models (USEPA 1989, 2001, 2002, 2004), which are described by the Eqs. (1)–(4):

$$ {\text{ADD}}_{\text{oral}} = (C_{\text{s}} \times {\text{IngR}} \times {\text{CF}} \times {\text{EF}} \times {\text{ED}})/({\text{BW}} \times {\text{AT}}), $$
(1)
$$ {\text{ADD}}_{\text{inh}} = (C_{\text{s}} \times {\text{InhR}} \times {\text{EF}} \times {\text{ED}})/({\text{BW}} \times {\text{AT}} \times {\text{PEF}}), $$
(2)
$$ {\text{ADD}}_{\text{derm}} = (C_{\text{s}} \times {\text{CF}} \times {\text{SA}} \times {\text{AF}} \times {\text{ABS}} \times {\text{EF}} \times {\text{ED}})/({\text{BW}} \times {\text{AT}}), $$
(3)
$$ {\text{ADD}}_{\text{juice}} = \left( {C_{\text{j}} \times {\text{IngR}} \times {\text{EF}} \times {\text{ED}}} \right)/({\text{BW}} \times {\text{AT}}), $$
(4)
where ADDoral, ADDinh, and ADDderm are average daily doses taken by respectively, oral intake (defined as the unintentional exposure by non-dietary mouth intake of HM, e.g., dust or air-borne particles), inhalation, and dermal contact of soil dust (mg kg−1 day−1); ADDjuice is estimated average daily dose intake through sugarcane juice (mg kg−1 day−1); Cs and Cj are HM concentrations respectively in soil and in squeezed sugarcane juice (mg kg−1); IngR is the ingestion rate of soil dust and juice; InhR is the air inhalation rate; CF is the conversion factor; EF is the exposure frequency; ED is the exposure duration; AT is the average time in days; BW is body weight; PEF is the particular emission factor; SA is the skin area; AF is the adhesive factor, and ABS is the skin absorption factor. Values and units of IngR, InhR, CR, EF, ED, AT, BW, PEF, SA, AF, and ABS are described in Table S1 of Supplementary Material.

Human health risk indicators

Reference doses (RfD) and slope factors (SF) of HM were used to evaluate the non-carcinogenic and cancer risks, respectively, using Eqs. (5) and (6) listed below (USEPA 1989, 2001, 2002, 2004).

$$ {\text{Risk}} = \left( {{\text{ADD}} \times {\text{SF}}} \right), $$
(5)
$$ {\text{HQ}} = ({\text{ADD}}/{\text{RfD}}), $$
(6)
where Risk = cancer risk; SF = slope factor for carcinogenicity; ADD = average daily dose of heavy metal, mg/(kg day); HQ  = hazard quotient, the risk of non-carcinogenic toxic effects; RfD = reference dose.

The human health risks posed by seven HMs (Cu, Zn, Pb, Cd, As, Cr and Ni) were assessed. Four HMs (Pb, Cd, As, and Cr) classified as carcinogens (IARC 2017a, b) were assessed for their carcinogenic risks. The RfDs and SFs values of the evaluated HM could be found in Tables S2 and S3 of Supplementary Materials (IRIS 2017; Liu et al. 2014; Praveena et al. 2015; USEPA 2011; Yang et al. 2013; Zhang et al. 2008).

To further investigate the health risks induced through non-dietary exposures of HM, the hazard index (HI) of three non-dietary pathways for each HM was calculated using Eq. (7) (USEPA 1989). Also, the three non-dietary RfDs values were used to obtain RfDtotal by Eq. (8). RfDtotal was used as an indicator of body tolerance (or sensitivity) to a particular HM through all non-dietary exposures.

$$ {\text{HI}} = {\text{HQ}}_{\text{oral}} + {\text{HQ}}_{\text{inh}} + {\text{HQ}}_{\text{der}} $$
(7)
$$ {\text{RfD}}_{\text{total}} = {\text{RfD}}_{\text{ing}} + {\text{RfD}}_{\text{inh}} + {\text{RfD}}_{\text{der}} $$
(8)

Then the ratio R1 of average soil HM concentration Cs to RfDtotal was described by Eq. (9). R1 was used to co-evaluate the effects of different HM levels in the environment to human health, and the tolerance of individuals to this exposure.

$$ R_{1} = C_{\text{s}} /{\text{RfD}}_{\text{total}} $$
(9)

To assess the potential bioavailability of HMs in sugarcane, we considered the ratio R2 of HMs in sugarcane juice (Cjuice) to soil (Cs), expressed as:

$$ R_{2} = C_{\text{juice}} /C_{\text{s}} $$
(10)

R 2 is used as an indicator of HM bioavailability to humans through sugarcane juice intake in this study.

Statistical Analyses

The statistical analyses of all data were performed by Microsoft Excel 2007.

Results

Concentrations of Selected Heavy Metals in Soil and in Sugarcane Juice

The concentrations of seven HMs determined in soil samples and sugarcane juice are shown in Table 1. According to the data, Zn, Pb and Cd were the major pollutants in the soil samples and exceeded the control limits of the Chinese National Soil Standard (MEPC 1995) for grade II soil, which is defined as ‘the limit values of soil for guaranteeing sustainable agricultural production and human health. However, to assess the HMs in sugarcane juice, and their ratios in relation to soil concentrations, it was determined that Cd possessed a predominately high level compared to its corresponding values in soil (i.e. R2 was 0.14), followed by Zn > Cu > Cr > Ni. In contrast, the R2 values for Pb and As were over 50-fold lower than the others, which suggests that their potential for bioaccumulation in sugarcane juice was very low.
Table 1

Sample concentrations (Cs), soil background levels (STD) and GB II values of study HM

Heavy metals (n = 37)

Cs (mg kg−1)

Soil STD (mg kg−1)

GBIIa

Cjuice (mg kg−1)

R 2

Cu

14.82

3.58

50

0.30

0.020

Zn

410.21

183.36

200

10.31

0.025

Pb

403.39

288.88

250

0.015

0.00004

Cd

1.05

0.55

0.30

0.152

0.14

As

31.69

17.29

40

0.0045

0.00014

Cr

28.43

7.71

150

0.54

0.019

Ni

9.26

2.71

40

0.071

0.008

aGB = GB 15618-1995 (environmental quality standard for soils), maximum limit values of HM concentrations for soil of grade II at pH < 6.5, the values of As and Cr are for dry soil

Exposure Doses

The ADDs of HMs by different routes of exposure were derived using Eqs. (1) to (4). The estimated daily carcinogenic and non-carcinogenic doses are presented in Tables 2 and 3, respectively. For all three non-dietary routes of exposure, the major exposure route was oral, followed by dermal, and then inhalation.
Table 2

Average daily doses (ADDs) and cancer risks of four heavy metals

 

Average daily doses (mg kg−1 day−1)

Cancer risk

 

Oral

Inhalation

Dermal

Juice

Oral

Inhalation

Dermal

Juice

Pb

4.17 × 10−4

1.15 × 10−8

1.17 × 10−6

1.16 × 10−5

3.54 × 10−6

4.83 × 10−10

4.90 × 10−8

9.89 × 10−8

Cd

1.09 × 10−6

2.99 × 10−11

3.04 × 10−9

1.18 × 10−4

6.84 × 10−6

1.89 × 10−10

7.66 × 10−7

7.43 × 10−4

As

3.28 × 10−5

9.03 × 10−10

2.75 × 10−6

3.47 × 10−6

4.91 × 10−5

1.36 × 10−8

1.01 × 10−5

5.20 × 10−6

Cr

2.94 × 10−5

8.10 × 10−10

8.23 × 10−8

4.21 × 10−4

1.47 × 10−5

3.32 × 10−7

1.65 × 10−6

2.11 × 10−4

Total

7.42 × 10−5

3.74 × 10−7

1.25 × 10−5

9.58 × 10−4

AADs and HQs were derived by Eqs. (1)–(6), with heavy metal concentrations of soil (Cs) and sugarcane juice (Cjuice) listed in Table 1

Table 3

AADs (mg kg−1 day−1) (AADs and HQs were derived by Eqs. (1)–(6), with heavy metal concentrations of soil (Cs) and sugarcane (Cjuice) juice listed in Table 1) and hazard quotients (HQ) of seven heavy metals

 

Oral

Inhalation

Dermal

Juice

HQoral

HQinh

HQder

HQjuice

Cu

1.79 × 10−4

4.93 × 10−9

5.01 × 10−7

2.71 × 10−3

4.47 × 10−3

1.23 × 10−7

4.17 × 10−5

6.78 × 10−2

Zn

4.95 × 10−3

1.36 × 10−7

1.39 × 10−5

9.33 × 10−2

1.65 × 10−2

4.55 × 10−7

2.31 × 10−4

3.11 × 10−1

Pb

4.87 × 10−3

1.34 × 10−7

1.36 × 10−5

1.36 × 10−4

1.22

3.81 × 10−5

2.59 × 10−2

3.39 × 10−2

Cd

1.27 × 10−5

3.49 × 10−10

3.55 × 10−8

1.38 × 10−3

1.27 × 10−2

7.40 × 10−5

3.55 × 10−3

1.38

As

3.82 × 10−4

1.05 × 10−8

3.21 × 10−5

4.04 × 10−5

1.27

1.49 × 10−3

2.61 × 10−1

1.35 × 10−1

Cr

3.43 × 10−4

9.46 × 10−9

9.60 × 10−7

4.88 × 10−3

1.14 × 10−1

2.00 × 10−4

1.28 × 10−2

1.63

Ni

1.12 × 10−4

3.08 × 10−9

3.13 × 10−7

6.42 × 10−4

5.58 × 10−3

7.25 × 10−5

3.91 × 10−4

3.21 × 10−2

AADs and HQs were derived by Eqs. (1)–(6), with heavy metal concentrations of soil (Cs) and sugarcane juice (Cjuice) listed in Table 1

Characterisation of Human Cancer Risks

The calculated ADDs for carcinogenic effects and cancer risks of different exposure routes are listed in Table 2. For non-dietary exposure, the ranking of total cancer risks of four HM was As > Cr > Cd > Pb. The most significant exposure route was oral, followed by dermal contact, and then inhalation. For drinking sugarcane juice, the Cd and Cr posed the highest risks, which were much higher than the risk induced by oral intake of corresponding HMs and over the upper limit of the threshold of concern 10−4 recommended by EPA (USEPA 2001), whereas the values for Pb and As were lower than the corresponding oral intake (for both) and dermal contact (for As) values. The low risk values for Pb and As through juice consumption were attributed to their very low bioavailabilities in sugarcane juice. Overall, the total cancer risks induced by four different exposure pathways were juice drinking > oral intake > dermal contact > inhalation (Table 2).

Characterisation of Non-carcinogenic Health Risks

The toxicological (non-carcinogenic) risks of HMs by three non-dietary exposure routes were identified as oral intake > dermal contact > inhalation. Arsenic had the highest non-carcinogenic risk for all three non-dietary routes. For other HMs, the order of non-carcinogenic risks (high to low) for oral intake was Pb > Cr > Zn > Cd > Ni > Cu. For dermal contact, it was Pb > Cr > Cd > Ni > Zn > Cu, and for inhalation, it was Cr > Cd > Ni > Pb > Zn > Cu (Table 3). It is noteworthy that the oral intake risks (HQoral) induced by Pb (1.22) and As (1.27) exceeded the acceptance limit of unity (1.0), indicating an unacceptable non-carcinogenic risk of toxicity (USEPA 2001; MEPC 2014). For non-carcinogenic risks induced by drinking sugarcane juice, the HQJuice values of Cr (1.63) and Cd (1.38) were over the threshold of the accepted limits of 1.0, and much higher than the other HMs; the ranking of HQs by other HMs was Zn > As > Cu > Pb > Ni.

The sum of risks induced by three non-dietary exposures (HI) for each HM was calculated by Eq. (7) (Table 4) and compared with non-carcinogenic risks of drinking sugarcane juice. As shown in Fig. 2, drinking this juice led to higher overall non-carcinogenic risks, but that risk levels of juice drinking for two least bio-accumulative HM (Pb and As), were much lower than their corresponding non-dietary risks.
Table 4

The HIs, RfDtotal and R1 values of seven heavy metals

 

Cu

Zn

Pb

Cd

As

Cr

Ni

HI

4.51 × 10−3

1.67 × 10−2

1.24

1.63 × 10−2

1.54

1.27 × 10−1

6.05 × 10−3

\( RfD_{\text{total}} \)

0.0922

0.6600

0.0080

0.0010

0.0004

0.0031

0.0208

R 1

161

622

50,142

1035

73,685

9106

444

Fig. 2

Non-carcinogenic risks induced by non-dietary/juice drinking for seven heavy metals

Correlation of Exposure to Environmental Heavy Metals and Non-dietary Health Risks

To further assess whether the environmental HM contents are related to human health risk, the RfDtotal derived by Eq. (8), was used as a measure of the threshold of accepted daily intake for that metal through non-dietary routes. The R1 ratios of soil heavy metal (Cs) to RfDtotal were derived by Eq. (9) and are summarised in Table 4. The cancer risks are not included in this analysis, as cancer risk data are unavailable for some HMs. Using Pearson’s correlation analysis, the correlation coefficient r = 0.9939 (degrees of freedom = 5) indicated a very significant linear correlation (p < 0.001, 2 tailed). A plot of R1 against HI for seven HMs is shown in Fig. 3. The graph showed good linearity (r 2  = 0.9879). The linear relationship described by this equation is exclusively applied to risks induced by non-dietary exposure, and indicated that ratio R1 is a useful indicator for co-evaluating the effects of HMs present in the environment and human susceptibilities to this HM.
Fig. 3

The linear regression of R1 [see Eq. (9)] to HI [see Eq. (7)]

Discussion

Results of both cancer and non-carcinogenic health risk assessments show that drinking HM-contaminated sugarcane juice posed higher risks than the overall non-dietary exposures for the investigated HM and highlights the bio-accumulative effects of HM in the soil environments of this region (Table 2, Fig. 3).

When considering non-dietary risks, the dominant factors are the environmental concentrations and toxicity of HM. For example, Pb and As possess high non-dietary risks because of their relatively high concentrations, whereas Zn had the highest concentration in our study, but possessed a low non-dietary risk due to low toxicity. To evaluate how factors such as HM in the environment (soil) can affect children’s health in the studied area through non-dietary pathways, the relationship of soil HM and the non-carcinogenic risks to humans through non-dietary exposure routes were examined. Results demonstrated that the factors affecting their correlation include the soil HM concentrations and human susceptibility to HMs, RfDtotal (sum of Reference doses by three exposure routes) is an indicator of human tolerance (susceptibility) to a particular HM. R1 (the ratio of soil concentrations to RfDtotal) was positively related to the total hazard indexes (HIs) of HM, a regression equation of R1 to HI was derived with significant linearity (Fig. 3).

For dietary exposure (in this case, drinking sugarcane juice), the bioavailability of HM should be considered. For example, Cd had the lowest environmental level but highest R2, which induced the highest cancer risk, and the second highest non-carcinogenic risk in sugarcane juice. The high risk of Cd in juice was likely attributed to its high bioavailability, an observation consistent with other reports that Cd is more bioavailable than other HM in vegetables, rice and other plants (Li et al. 2007; Liu et al. 2014; Zhang et al. 2015). It is noteworthy that even if the concentration of Cr in soil was within agricultural soil restrictions, it possessed an intermediate R2 value. The bioavailability of Cr was also high and posed a very high health risk following consumption of contaminated sugarcane juice (the highest for non-carcinogenic risk and second highest in cancer risk) in the present study. In contrast, exposure to the other two toxic HMs (Pb and As) with lowest R2 values or three HMs with low toxicity (Cu, Zn and Ni) will likely pose a low risk following sugarcane juice drinking (Fig. 2). Our results show that the bioavailability (from dietary exposure) and the toxicities of HMs depending on their environmental concentrations can pose health risks to susceptible individuals (e.g., children) through different exposure pathways.

Conclusions

For the health risk posed by HMs on children through different pathways in the investigated area, the ranking of both cancer and non-carcinogenic risks was drinking juice from contaminated sugarcane > oral intake > dermal contact > inhalation. For the non-dietary exposure, the key factors of risks were environmental level (soil concentration) and toxicity (human tolerance), demonstrated by the proposed linear regression model. For dietary exposure, various bio-accumulative HM can have different bioavailabilities in humans through ingestion and thus influence their final health risk levels. Our results demonstrate that when evaluating health risks of HM, one should consider non-dietary exposures to environmental HM concentrations, and the sensitivities of susceptible individuals or sub-populations. For dietary exposure risk assessment, specific bioavailability issues (e.g. HM uptake in sugarcane) are also important.

Notes

Acknowledgements

This work was supported by the Natural Science Foundation of Guangxi (2013GXNSFEA053001) and the Natural Science Foundation of Guangxi (2015GXNSFEA139001). The authors would like to thank other team members, Yan Tian, Xin Hong, Xiao-Xi Liang and Rong Su, for their hard work and dedication in finishing this study.

Supplementary material

12403_2018_275_MOESM1_ESM.docx (20 kb)
Supplementary material 1 (DOCX 19 kb)

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Copyright information

© Springer Science+Business Media B.V., part of Springer Nature 2018

Authors and Affiliations

  1. 1.Light Industry and Food Engineering CollegeGuangxi UniversityNanningChina
  2. 2.Guangxi Zhuang Autonomous Region Environmental Monitoring CentreNanningChina
  3. 3.Guangxi Zhuang Autonomous Region Environmental Protection BureauNanningChina
  4. 4.Department of Natural Resource SciencesMcGill UniversitySte-Anne-de-BellevueCanada
  5. 5.Scientific Research Academy of Guangxi Environmental ProtectionNanningChina

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