Risk assessment of heavy metals in topsoil along the banks of theYangtze River in Huangshi, China

Abstract: High concentrations of heavy metal in farmland threatens food production and human health. Our study assesses soil quality and the distribution of Cd, Cr, Cu, Zn, Pb and Ni, to identify possible sources of heavy metals along the banks of the Yangtze River in Huangshi. Heavy metal concentrations of 22 topsoil samples were measured using the inductively coupled plasma mass spectrometry instrument and data was analyzed by multivariate statistic approaches. Heavy metal contamination risk assessment was performed using a combination of the Nemerow multi-factor index method, the Hakanson potential ecological risk index method and the Muller index of geo-accumulation method. These methods determined similar results indicating that cadmium (Cd) poses a health risk to residents at the study site while Cr, Cu, Zn, Pb and Ni do not. The Nemerow multi-factor index method demonstrated that 18 samples were heavily polluted, three moderately polluted and one lightly polluted. The Muller index of geo-accumulation method found ten samples were moderately to strongly polluted by Cd, five were moderately polluted, six were lightly to moderately polluted and one was lightly polluted. The Hakanson potential ecological risk index method proved six samples were strongly polluted, seven were moderately polluted and nine were lightly polluted. Since our sampling sites were all in agricultural lands, we recommend the potential ecological risk index method as the most effective given it not only considers the range of pollutants contributing to soil pollution but factors in heavy metals toxicity. We are apt that the source of the high concentrations of Cd found in topsoil is derived from alluvial sediments upstream of the Yangtze River with a high percentage of residual speciation and a low percentage of exchangeable speciation distribution of Cd. This in turns indicates that a high concentration of Cd in soil had little impact on the natural environment. However, 31.9% of the iron-manganese oxides bound speciation indicating that such levels of Cd in soil would be potentially hazardous to the crops, particularly if exposed to a reductive condition.

Keywords: Heavy metals; Risk assessment; Soil chemical analysis; The Yangtze River; Huangshi

Introduction

Soil chemical analysis and research on heavy metal distribution and pollution in China has remained topical and steady. For example, an anomalous heavy metal belt such as Cd was identified along the Yangtze River valley (XI Xiao-huan, 2004). Within this anomalous belt, some investigations had shown levels of total Cd in the local food production (vegetables and crops), exceeding the limitation imposed by the Chinese hygienic standard (YANG Zhong-fanget al. 2005; LI Xiu-juanet al. 2010a; LIU Yanget al. 2016), which could threaten human health. However, other research demonstrated the level of bio-available Cd in the soil remained low (LI Xiu-juanet al. 2010b). Morton-Bermeaet al. (2009) studied pollution and accumulation of heavy metals Cr, Cu, Ni, Pb, Zn and V, in 135 urban topsoil samples in the capital of Mexico. Their results revealed that enriched Pb linked with vehicle emissions, and high levels of Zn and Cu were possibly derived from factory emissions. Through analyzing relationships between Cd and Zn, Cu, Pb, Mn in river sediments in Wuhan, it was found that cadmium sources are rocks and minerals along the Yangtze River (MA Zhen-donget al.2005). Mercury (Hg) and cadmium presented the greatest potential ecological risk of eight heavy metals (Cd, Cr, Cu, Ni, Hg, Pb, Zn and As) on topsoil in Taicang according to the Hakanson potential ecological risk index method (ZHONG Xiao-lanet al.2007). Using the Nemerow multi-factor index method in topsoil, Cd, Cu, Pb and Zn were found accumulating in farmland in Beijing (FU Huaet al. 2006; LI Xiao-xiuet al. 2006). Cadmium (Cd) had more serious impact on the environment in Urumqi than other heavy metals when considering the “toxic-response” factor (WEI Bing-ganet al. 2010). A research of the potential ecological risks of heavy metals was taken on four typical farmlands in Yixing, finding that vegetable and paddy fields were subject to potential ecological risks from Hg, Pb and Zn contamination, while tea and bamboo fields were relative safe (ZHOU Sheng-luet al. 2008). Seven heavy metals (Cu, Cd, Pb, Zn, Cr, As, Ni) were analyzed on cultivated soils along the No. 320 national highway using the Muller index of geo-accumulation method (CHEN J Set al. 2006). Results pointed to an existing soil pollution plume along the highway with pollutants of Cd, Pb, Cr and Cu. Seven soil profiles in three areas along the Elbe River, Germany, were selected to determine geochemical fractions of Cd, Pb, and Ni. Once analyzed, results suggested serious consideration of heavy metals was required in this area, given that releases of these toxic metals in floodplain soils with increased mobilization was more likely and potential environmental risks such as uptake by plants, the transfer of these metals into the grassland and food chain were concerning, as well as transport of heavy metals via water during periods of flooding (Rinklebe J and Shaheen S M, 2014).

The concentration of Cd was high in fluvisol along the Yangtze River in Wuhan, with almost all of the earlier fluvisol along the river having already become farmland (MA Zhen-donget al. 2005). Heavy metals exposure in the environment occurs mainly via migration into the soil- plant-food system (PAN Gen-xinget al. 2002). It is consequently important to investigate and understand heavy metal distribution in soil, and conduct risk assessments to determine potential for heavy metals migration from soil to vegetation. Previous research on heavy metals in soil in Huangshi city has focused predominantly on the Daye area, to the south of Huangshi urban area, where is famous for its copper mine and soil pollution is driven mainly by mining activities. Heavy metal distribution in fluvisol along the banks of the Yangtze River in Huangshi urban area has not been the subject of detailed study to date. Thus, the object of this article is threefold: To learn the distribution of Cd, Cr, Cu, Zn, Pb and Ni in fluvisol of the Yangtze River in Huangshi urban area; to assess soil quality using the Nemerow multi-factor index method, the Hakanson potential ecological risk index method and the Muller index of geo-accumulation method; and to identify possible sources of heavy metals in the study area.

1 Materials and methods

1.1 Study area

Huangshi city is located on the south bank of middle reaches of the Yangtze River in the southeast of Hubei province in China, between longitudes 114°31' and 115°30'E and latitudes 29°30' and 30°15'N. Huangshi has an East Asian subtropical continental climate with the average temperature of 29.2 ℃ in summer and 3.9 ℃ in winter and the average rainfall of 1 382.6 mm/yr. The climate is mild and humid, with short winters and excellent hydrothermal conditions favorable for crop growth. Geologically, Quaternary material is widely distributed in the area. The Pleistocene layer is mainly composed with eluvial, alluvial and fluvial deposits, while Holocene material is mainly alluvial deposits and lake sediments. Fluvial material is the dominant soil type which is derived from Quaternary alluvial deposits and sediments.

The study area is almost low altitude flood land. Groundwater occurs in the unconsolidated sediments with a shallow water table recharged by the Yangtze River. The Quaternary alluvial layers mainly consist of sand, sandy loam and loam (MENG Fan-qiaoet al. 2000).

1.2 Evaluation methodology

Soil quality was assessed according to the three indexes calculated below.

The Nemerow multi-factor index method is based on single factor index method. The single factor index was calculated as follows: wherePiis the index of pollutanti, ciis the measured concentration of pollutanti, andcsis the reference concentration value of pollutanti.The Nemerow multi-factor index was calculated as follows:

where (Pi)maxis the maximum ofPi, and is the mean value ofPi(WANG Yan-xin, 2007). The Nemerow multi-factor index works by first calculating the single factor index of each pollutant. This method considers each pollutant’s contribution to soil pollution and highlights the high-concentration pollutants, and results in a division of samples according to five degrees of contamination (Table 1).

Table 1 The relationship between the Nemerow multi-factor index and the degree of pollution.

The Muller index of geo-accumulation method is widely used in assessing heavy metal pollution in sediments and other materials. This index is calculated as follows:

where Cnis the concentration of elementnin sediment,Bnis the background concentration value of elementnin sediment, andkis a coefficient of the alteration in background value caused by rock differences, generally a value of 1.5 (Muller G,1969). This method not only considers artificial pollution and the geochemical background level,but also recognizes the possible changes of natural diagenesis (ZHOU Xiu-yanet al. 2004). Samples are divided into seven levels according to the contamination degree of a given pollutant (Table 2).

Table 2 Classification of the Muller pollution indexes by degree of pollution on a scale from “Clean” to “Extremely polluted”

The Hakanson potential ecological risk index method considers the toxicity of each heavy metal pollutant and is one of the most popular methods used for the assessment of heavy metal pollution. Chen J Set al. (1989) introduced a calculation method for the “toxic-response” factor for seven heavy metals including Zn, Cr, Cu, Pb, As, Cd and Hg, which was recalculated by XU Zhen-qiet al. (2008) for 10 heavy metals, including Ni. For our purposes, the “toxic-response” factor () was defined using the following parameter: Zn=1 < Cr=2 < Cu=Ni=Pb=5 < Cd=30.reflects the response relationship between heavy metals toxicity and heavy metals in water, sediments and organisms. The contamination factor of pollutanti() was calculated as follows:

The potential ecological risk factor for pollutanti() was calculated as follows:

The requested potential ecological risk index (Hakanson, 1980) for the sample is then defined as:

Table 3 The relationship between the potential ecological risk factor ( ), risk index (RI) and the determined degree of pollution

Table 3 The relationship between the potential ecological risk factor ( ), risk index (RI) and the determined degree of pollution

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1.3 Sampling and analysis

Twenty-two soil samples were collected and their locations were recorded using the Global Position System (GPS) (Fig. 1). The sampling sites were all located in farmland on either side of the Yangtze River. Between the 6-18thAugust, 2008, one kilogram of soil sample was collected at each location from a depth of 0-20 cm using a stainless steel drill and stored in a labelled PVC bag.

Fig. 1 Sample sites for the detection and distribution analysis of heavy metal pollution within the study area on the Yangtze River around Huangshi

Analysis of samples was carried out in September, 2008. Soil samples were air-dried, homogenized, and sieved through 0.25 mm mesh for analysis of total heavy metal. For pH measurements and the speciation analysis of Cd, soil was sieved through 0.15 mm mesh and stored in PVC bags at room temperature. A 10 g sub-sample of the sieved soil sample was added to 25 mL of distilled water, stirred, and centrifuged. The supernatant fluid was used for pH measurement by a multi-parameter instrument (SensION 156, United States, Hach Company). Total heavy metal content was analyzed using the inductively coupled plasma mass spectrometry (ICP-MS) instrument (7 500a, United States, Agilent Technologies) after an ordinary four-acid- digestion. Concurrently to sample testing, standard material (GSS-4), blank samples and parallel samples were set to insure quality assurance of data.

Speciation analysis of Cd was using the Tessier multi sequential extraction method (the exact method is detailed in Table 4) and analyzed using atomic absorption spectrophotometer (AAnalyst 800, United States, PerkinElmer Inc.).

Table 4 Detailed methods used for the Tessier multi sequential extraction of Cadmium; where Exc is the exchangeable speciation, Car is the carbonate bound speciation, Iom is the iron-manganese oxides bound speciation, Org is the organic matter bound speciation, and Res is residual speciation.Samples were centrifuged at a rotational speed of 4 500 r/min for 15 min. The specification of the filter membrane was 0.45 μm

The data was analyzed using statistical product and service solutions software (SPSS 13.0) and Microsoft Excel 2003. Results showed a relative error (RE) of standard samples and parallel samples between 0.584-23.638%, falling within the geological survey technical standard (DD2005-03) made by China Geological Survey (2005) indicating a successful experiment and trustworthy data.

2 Results and discussion

The R-hierarchical clustering of samples (Fig. 2) indicated that elements measured by the ICP-MS were divided into four groups: (1) Cr, Co, Ba and others; (2) Cu, Zn, and Pb; (3) Ni, U, Zr and others; and (4) consisting of Cd, Sn, and Ta. The sedimentary geochemical features and influence factors of elements in one group might be similar. We found that the mean content of Cd, Cu, Zn, Pb, Ni, Sn and Ce had exceeded the environmental background values (NEPA, 1990) in A-layer soil (0-20 cm) in Hubei province. Alluvial soil in the study area is derived from Quaternary sediment (14C dating of 8 100 a) (LI Chang-anet al. 2001), containing minerals including Cu, Pb, Cd, Zn, As, and Hg, amongst others (NEPA, 1995). Cd, Cr, Cu, Zn, Pb, Ni were the most consistently mined heavy metals in the 20thcentury industrial society (WANG Yan-xin, 2007), and these heavy metals were distributed among the four groups we found through the ICP-MS analysis. Thus, these six heavy metals were chosen for further analysis and discussion.

Fig. 2 A dendrogram illustrating the relationships between the elements measured in the ICP-MS analysis

2.1 The distribution features of heavy metals in topsoil

The statistical results of Cd, Cr, Cu, Zn, Pb and Ni in topsoil are shown in Table 5, including maximum, minimum, mean and coefficient of variability (CV). The CV represents the average variance of samples within a population. The content of Zn was the highest with a maximum concentration of 290.599 mg/kg, while the contents of Cd, Pb and Ni were relatively low with a maximum concentration of Cd at 11.927 mg/kg, Pb 48.622 mg/kg and Ni 58.365 mg/kg. The CV of Cr was more than 60% and the maximum was about 20 times the minimum which indicated strong variability. Cd also had a strong variability with a CV of 58.896%. The spatial distributions of Cr and Cd were therefore significant. However, the CVs of Cu, Zn, Pb, Ni were lower than 30%, inferring the variability of spatial distribution in Cu, Zn, Pb, Ni was not detectable. This result could also indicate that the four metals might have the same source (ZHONG Xiao-lanet al. 2007).

Table 5 The statistical characteristics of the level of heavy metals (mg/kg), Cadmium (Cd), (Chromium) Cr, Copper (Cu), Zinc (Zn), Lead (Pb), and Nickel (Ni) and the background values measured in topsoil Unit: mg/kg

The contents of Cr, Cu, Pb, Ni were all lower than the background values (Table 4) (NEPA, 1995). However, the contents of Cd in seven samples (HS08, HS11, HS18, HS24, HS28, HS35, HS50) and the content of Zn in one sample (HS43) exceeded the maximum of background value.

2.2 The risk assessment of topsoil

The metals Cd, Cr, Cu, Zn, Pb, Ni were selected as evaluation factors to assess the quality of topsoil. The reference values for these six heavy metals were 0.6 mg/kg, 250 mg/kg, 100 mg/kg, 300 mg/kg, 350 mg/kg and 60 mg/kg respectively according to the limitation grade II of Environmental quality standard for soilsof the People Republic of China (GB15618-1995).

Soil quality was evaluated using the Nemerow multi-factor index method, the Hakanson potential ecological risk index method and the Muller index of geo-accumulation method. These three methods calculated similarly that Cd posed a health risk to residents in the study area while Cr, Cu, Zn, Pb and Ni posed no health risk.

Single factor indexes of Cd lay between 2.496-19.879 and results demonstrated that 20 samples (HS01, HS02, HS06, HS07, HS08, HS09, HS11, HS12, HS18, HS20, HS22, HS24, HS28, HS35, HS39, HS42, HS43, HS47, HS49, HS50) were heavily polluted, and two (HS03 and HS44) were moderately polluted. However, single factor indexes of Cr (0.022-0.463), Cu (0.182-0.683), Zn (0.346-0.969), Pb (0.035-0.139), Ni (0.471-0.973) were all no more than 1 indicating clean samples. The Nemerow multi-factor indexes of the six metals lay between 1.843–14.290 and 18 samples (HS01, HS02, HS07, HS08, HS09, HS11, HS12, HS18, HS20, HS22, HS24, HS28, HS35, HS39, HS42, HS43, HS49, HS50) were found to be heavily polluted, three (HS06, HS44, HS47) were moderately polluted and one (HS03) was lightly polluted.

The Muller indexes of geo-accumulation of Cd were between -0.002–2.991. Ten samples (HS07, HS08, HS11, HS18, HS24, HS28, HS35, HS39, HS42, HS50) were moderately to strongly polluted, five (HS01, HS02, HS20, HS22, HS43) were moderately polluted, six (HS06, HS09, HS12, HS44, HS47, HS49) were lightly to moderately polluted and one (HS03) was lightly polluted. The indexes of Cr (-6.093– -1.696), Cu (-3.041– -1.136), Zn (-2.117– -0.631), Pb (-5.433– -3.433), Ni (-1.671– -0.625) were all lower than 0, indicating no pollution by these metals.

The potential ecological risk factors () of Cd lay between 44.940-357.810. Five samples (HS11, HS18, HS24, HS28, HS35) were found to embody extreme strongly ecological hazards, seven (HS01, HS07, HS08, HS39, HS42, HS43, HS50) indicated very strongly ecological hazards, four (HS02, HS09, HS20, HS22) were strongly ecological hazards and six (HS03, HS06, HS12, HS44, HS47, HS49) were moderately ecological hazards. Theof Cr (0.044-0.926), Cu (0.912-3.413), Zn (0.346-0.969), Pb (0.174-0.695), Ni (2.355-4.864) categorized these samples as lightly ecological hazard. Potential ecological risk indexes (RI) of the six metals fell between 51.552-364.484, and the resulted in six samples (HS11, HS18, HS24, HS28, HS35, HS50) categorized as strongly ecological hazards, seven (HS01, HS02, HS07, HS08, HS39, HS42, HS43) as moderately ecological hazard and nine (HS03, HS06, HS09, HS12, HS20, HS22, HS44, HS47, HS49) as lightly ecological hazards.

2.3 Comparison of evaluation methodology

The three methods we used each have the capacity to evaluate the levels of a given pollutant in soils. The Muller index of geo-accumulation method particularly emphasizes the certain pollutant of interest. However, the Nemerow multi- factor index and Hakanson potential ecological risk index methods have the capacity to evaluate soil based on multiple pollutants which could reflect the synthesis results. The Nemerow multi-factor index method emphasized high concentration pollutants while the Hakanson potential ecological risk index method considered the heavy metals’ toxicity. Since the sampling sites were all from agricultural lands, the potential ecological risk index method was considered to be more applicable and objective for our purposes, in that it could determine the toxic risk of heavy metal contamination in food crops.

In conclusion, Cd concentrations regularly and widely exceeded the limitation value across the study area, illustrating the extremely high concentrations of heavy metals in the study area are largely caused by this single factor. The correlation analysis (Table 6) showed that there is no strong correlation between Cd and other metals, further strengthening this finding. MA Zhen-donget al. (2005) pointed out that fluvial sediment along the Yangtze River was rich in calcareous substances and José A R Met al. (2006) found that Cd could easily be accumulated in such calcareous soil. The high concentrations of Cd in these soil is therefore likely to be derived from alluvial sediments upstream of the Yangtze River.

Table 6 A Person correlation analysis of heavy metal elements Cadmium (Cd), Chromium (Cr), Copper (Cu), Zinc (Zn), Lead (Pb), and Nickel (Ni) in the Yangtze River study area

2.4 The distribution of different speciation of Cd in topsoil

It is well known that heavy metals availability depends not only on the total heavy metal contents, but also on their mobility and speciation in soil (Brazauskiene D Met al. 2008). The speciation of heavy metals and their relative proportions in soil determine impacts on the surrounding environment and ecosystem (HE Yuanet al. 2007). The variation of heavy metal speciation in soil could be caused by different soil environmental conditions including soil types, land-use, pH, redox potential, inorganic and organic content, among other factors.

The speciation distribution of each sample is illustrated in Fig. 3. The exchangeable speciation and carbonate-bound speciation, which are the most active and poisonous, can easily be absorbed by plants and can convert into other speciations by absorption, leaching loss and reaction. The iron-manganese oxides-bound speciation and organic matter-bound speciation were relatively stable. Residual speciation, which was the most stable, was from soil minerals and hardly absorbed, can remain stable in soil for long time and is harmless to soil and ecosystem health (WU He-xinet al. 2004; CHEN Man-hua, 2005; LIAO Guo-li and WU Chao, 2006).

According to mean values, the percentage of the residual speciation was 52%, iron-manganese oxides bound speciation 30%, carbonate bound speciation 9%, organic matter bound speciation 5% and exchangeable speciation 4%. The proportion of residual speciation was greater than all other speciation, butt was followed by iron-manganese oxides-bound speciation. The iron-manganese oxides bound speciation was balanced among the 23 samples.

According to Fig. 3, residual speciation of 14 samples (HS03, HS06, HS08, HS09, HS11, HS12, HS18, HS20, HS24, HS39, HS43, HS44, HS47, HS50) was more than half the content while the exchangeable speciation of 8 samples (HS08, HS12, HS24, HS28, HS35, HS39, HS42, HS47) were lower than the detection limitation. The exchangeable speciation percentages of 21 samples in 23 samples were lower than 5%, indicating the high concentration of Cd in soil had little impact on the environment. The mean percentage of the iron-manganese oxides bound speciation was 31.9%, demonstrating Cd in soil could be potentially hazardous to crops under a reducing condition.

Fig. 3 A percentage heaped histogram of the Cadmium speciation analysis of 22 samples in topsoil in the Yangtze River study area

3 Conclusions

The average content of Cd in samples lay between 1.498-11.927 mg/kg. For Cr, levels were between 5.494-103.028 mg/kg, for Cu 18.230- 68.261 mg/kg, for Zn 103.723-290.599 mg/kg, for Pb 12.156-48.622 mg/kg and the average content of Ni was between 28.254-58.365 mg/kg. Spatial variability in Cd and Cr distribution was large while spatial variability of Cu, Zn, Pb and Ni was negligible.

The three evaluation methods demonstrated similar results around the toxicity of the heavy metal Cadmium, in that it poses a health risk to residents in the study area. Meanwhile Cr, Cu, Zn, Pb and Ni pose no health risk. The Nemerow multi-factor index method determined that 18 samples were heavily polluted, three were moderately polluted and one was lightly polluted. By contrast, the Muller index of geo-accumulation method found ten samples were moderately to strongly polluted, five were moderately polluted, six were lightly to moderately polluted and one was lightly polluted. The Hakanson potential ecological risk index method categorized six samples as strongly ecological hazards, seven as moderately ecological hazards and nine as lightly ecological hazards. Since the sampling sites were all derived from agricultural lands utilized actively for cropping, the most applicable method, the Hakanson potential ecological risk index, was adopted as it not only considered all of the pollutants contributing to soil pollution but also heavy metals toxicity.

Only single metal of Cd possessing high content, combined with previous research, it suggests that the high concentration Cd is likely to have derived from alluvial sediments upstream of the Yangtze River. The high percentage of residual speciation and the low percentage of exchangeable speciation distribution of Cd in topsoil indicates high concentrations of this metal in soil has had little impact on the natural environment. However, 31.9% of the iron-manganese oxides bound speciation indicated Cd in soil could be potentially hazardous to crops under a reductive condition.

Since heavy metals in urban soils do not remain static in topsoil, further research is required around the mechanisms for Cd migration in the deeper soil profile.Acknowledgements

This study was supported by the National Natural Science Foundation of China (40772155) and the Opening Fund of Institute of Hydrogeology and Environmental Geology, CAGS (KF201505).