Introduction

The eco-environment in the Three Gorges Reservoir Area (TGRA) in China has received considerable attention after June 12, 2003, when its water level reached 135.4 m and the Three Gorges Hydropower Station began to generate electricity [1, 2]. However, after the construction of the Three Gorges dam, serious pollution problems have arisen in Wanzhou, a city in the centre of the TGRA, because of the more than 2,000 sewage drains flowing out to the upper Yangtze River every year [3, 4].

Heavy metals are persistent environmental pollutants [5] because they are non-biodegradable and can readily accumulate to toxic levels in water, soil, and biota [6, 7]. After their release into the environment, heavy metals are first rendered water soluble through oxidation processes and are then adsorbed and accumulate on the water-level fluctuating zone (WLFZ) soil until they eventually reach the bottom sediments [3, 8]. In addition, environmental pollution by heavy metals of anthropogenic origin can be caused by improper farming activities, such as irrigation using contaminated wastewater or the excessive use of fertilisers and insecticides on WLFZ soil [9, 10].

In the present study, the contents of typical pollutants (Cd, Cr, Cu, and Ni) in soil were analysed, and the ecological risk of the metal contaminants was assessed using the Hakanson potential ecological risk index. The objective of the present study is to evaluate the soil quality in the WLFZ of the Yangtze River within the Wanzhou Section (YRWS) for the environmental protection of the Three Gorges Area.

Materials and Methods

Sampling and Preparation

Samples were collected from dry beds during the dry season. A total of 36 soil samples (depth <20 cm) were collected during the summer season (August 2010) in the WLFZ of the Yangtze River (Wanzhou Section) and in its two branches, the Zhuxi River and Mixi valley, according to a systematic sampling scheme (Fig. 1). At present, both are seriously polluted. Each soil sample, consisting of 5–10 subsamples, was randomly collected from the surroundings of each site and homogenised to form a representative sample [11, 12]. The composite soils were stored in sealed polyethylene packages at 4°C after being air-dried at room temperature and sieved through a 1-mm mesh. The soil samples were digested in a 3:1 HNO3/HClO4 (v/v) solution using a MARS Xpress microwave digestion instrument (CEM, USA). The sample digests were filtered, and the volume was adjusted to 50 ml with ultrapure water. The samples were then stored in polyethylene containers at +4°C prior to analysis.

Fig. 1
figure 1

Map of the sampling sites

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The Cd, Cu, Cr, and Ni concentrations in the digested solution were determined using a flame atomic absorption spectrophotometer (Analyst 200, PerkinElmer, Shelton, USA). All laboratory glassware were soaked in 1:4 HNO3 for 24 h and then rinsed prior to use. The reagents for the trace metal element analysis were of superior grade or higher. The solutions were prepared using ultrapure water.

The standard reference materials obtained from the Centre of National Standard Reference Material of China were analysed as part of the quality assurance and quality control procedures. Satisfactory recoveries were obtained for Cd (93–98%), Cr (98–103%), Cu (96–101%), and Ni (96–103%).

Data Analysis and Evaluation

The potential ecological risk index method, which is based on the characteristics of a heavy metal and its environmental behaviour, was advanced by the Swedish scholar Lars Hakanson [13, 14], according to the characteristics of heavy metal and its environmental behaviour. The potential ecological risk index method is an approach to evaluate the heavy metal contamination from the perspective of sedimentology. It not only considers the heavy metal level in the soil, but also associates the ecological and environmental effects with toxicology and evaluates the pollution level using the comparable and equivalent property index grading method. The potential ecological risk index is related to the individual pollution coefficient, the heavy metal toxicity response coefficient, and the potential ecological risk individual coefficient, and is expressed as follows[13]:

$$ {\text{RI}} = { }\sum {E_{\text{i}}} $$
(1)
$$ {E_{\text{i}}} = {T_{\text{i}}}{f_{\text{i}}} $$
(2)
$$ {f_{\text{i}}} = { }{C_{\text{i}}}/{B_{\text{i}}} $$
(3)

where E i is the potential ecological risk individual coefficient and T i is the toxicity response coefficient of a certain kind of metal toxicity using the standard heavy metal toxicity coefficient developed by Hakanson as a reference. The heavy metal toxicity level is ranked in the order, Cd > Cu > Ni > Cr, and the toxicity coefficients of Cd, Cu, Ni, and Cr are 30, 5, 3, and 2, respectively. f i is the individual pollution coefficient, C i is the actual observed value of heavy metals in surface soil, and B i is a relative ratio, which is obtained from the background values of soil heavy metals in China [15]. The detailed grading standard of heavy metal individual pollution coefficients and pollution ecological risk coefficients are listed in Table 1.

Table 1 E i and RI grading standards
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Results and Analysis

Total Heavy Metal Contents

The descriptive statistics of the metal levels in the WLFZ of the YRWS and its branches (Mixi valley and Zhuxi River) are summarised in Table 2, which show a distinct change in the heavy metal contents among the soil samples. The Cd, Cr, Cu, and Ni contents varied between 1.26 and 3.60, 23.71 and 46.90, 11.23 and 62.03, and 19.00 and 52.66 mg/kg, respectively, with an average content of 2.14, 35.43, 18.47, and 34.52 mg/kg, respectively. The components in the soil samples were arranged in the following decreasing order based on the mean content: Cr > Ni > Cu > Cd. The soil background values of Sichuan and China [15] were used as the reference values. The Cd and Ni contents in the soil samples exceeded the reference values in China by about 28.6- and 1.1-fold, respectively, and were 26.8- and 1.4-fold higher than the soil background values of Sichuan, respectively. These results clearly demonstrate an anthropogenic contribution.

Table 2 Statistical values of heavy metal contents in the soils (per milligram per kilogram)
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The heavy metal spatial distributions in the mainstream and in the two major tributaries of the Yangtze River within the Wanzhou Section are shown in Table 2. The total metal contents were ranked in the following decreasing order: for Cd and Cr, Zhuxi River > mainstream > Mixi valley; for Cu, mainstream > Zhuxi River > Mixi valley; and for Ni, Mixi valley > Zhuxi River > mainstream. The maximum Cd and Cr values in the soil, which reached 3.6 and 46.9 mg/kg, respectively, were found in the WLFZ of the Zhuxi River. The average Cd, Cr, Cu, and Ni contents in the WLFZ of the Zhuxi River were 3.04, 41.81, 18.65, and 34.30 mg/kg, respectively, with the Cd and Cr contents higher than those in other regions. The Zhuxi River has a basin area of 228.8 km2 and is an important tributary of the Yangtze River. Approximately 1.7 million people from the area of the Three Gorges Project live around this river. A large amount of wastewater was discharged into the Zhuxi River, and the land in its WLFZ was irrationally utilised for farming using significant amounts of fertilisers and insecticides. Furthermore, a check dam was built in 2008, which slowed the water flow and inhibited the diffusion of contaminants. The distribution of these elements indicates the major role of human activities as the pollutant source [16, 17].

Correlations between Heavy Metals

Heavy metals in soils are mainly the original contents of rocks and parent materials and may also result from various soil formation processes and anthropogenic factors, such as contamination due to human activities. Correlations between homologous heavy metals exist, and these can be used to determine whether the sources of these heavy metals are the same [18, 19].

If there is a significant correlation between the elements, the probability that they came from the same pollution source is high, and such source may result from the combined pollution imposed by human activities or by natural factors (geochemical sources) [20, 21].

Table 3 shows the correlation coefficient matrix and lists the Pearson’s product moment correlation coefficient. Weak direct correlations were found between Cd and Cu (r = 0.230), Cd and Ni (r = 0.002), Cr and Cu (r = 0.155), Cr and Ni (r = −0.156), and Cu and Ni (r = 0.117), which were all at the 0.01 level. However, the Cd values show a very significant direct correlation with Cr (r = 0.739), suggesting that both metals came from the same sources (pesticides, fertilisers, and so on) and followed a similar accumulation pattern.

Table 3 Correlation analysis between different heavy metals (n = 36)
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Potential Ecological Risk Assessment

The potential risk of heavy metals in the WLFZ soil of the Yangtze River was assessed using the potential ecological risk index method of Hakanson. The risk index (RI) represents the sensitivity of various organisms to toxic substances and illustrates the potential ecological risk caused by heavy metal contamination in soil. In the YRWS soil, the Cd content reached the strong pollution level, whereas the other elements had lower potential ecological risk coefficients (Table 4).

Table 4 Potential ecological risk indices of the heavy metals
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The potential ecological risk levels (hazard indices) of the four kinds of heavy metals in the YRWS were ranked in the following order: Cd (802.61) > Cu (4.43) > Ni (4.15) > Cr (1.23). The potential ecological risk levels in the different regions were ranked in the following order: Zhuxi River > mainstream > Mixi valley. The WLFZ within the Zhuxi River had the highest potential ecological risk and was judged a health risk to the population living in the area. Evidently, actions are needed to alleviate the environmental pressures and to prevent further heavy metal accumulation. Some measures recommended include the reduction of urban wastewater discharge into the tributaries and the pre-purification of wastewater to meet the national standard prior to discharge. Farming should also be regulated in the WLFZ within the TGRA because it can cause additional amounts of fertilisers, pesticides, heavy metals, oil residues, garbage, sewage, and so on to be introduced into the soil and water.

Conclusions

  1. 1.

    The investigation of 36 soil samples from the mainstream and tributaries (Zhuxi River and Mixi valley) of the YRWS reveals a certain accumulation of Cd, Cr, Cu, and Ni. The Cd, Cr, Cu, and Ni contents varied between 1.40 and 3.60, 23.71 and 46.90, 11.23 and 62.03, and 19.00 and 52.66 mg/kg, respectively, with an average content of 2.14, 35.43, 18.47, and 34.52 mg/kg, respectively. The Cd and Ni contents in the WLFZ soils in the YRWS exceed the soil background values in China by 28.6- and 1.1-fold, respectively, clearly demonstrating an anthropogenic contribution.

  2. 2.

    The heavy metal distributions in the YRWS are in the following decreasing order: for Cd and Cr, Zhuxi River > mainstream > Mixi valley; for Cu, Zhuxi River > Mixi valley > mainstream; and for Ni, Mixi valley > Zhuxi River > mainstream.

  3. 3.

    The observed statistically significant (P = 0.01) direct correlation between Cd and Cr suggests that the heavy metal pollution in the Yangtze River WLFZ within the Wanzhou Section originates from human activities

  4. 4.

    The potential ecological risk levels in the different regions were ranked in the following order: Zhuxi River > mainstream > Mixi valley. The potential ecological risk in the WLFZ within the Zhuxi River was the highest and may pose certain risks to the health of the people in that area.