Introduction

Hazardous metal elements enter in streams and rivers through two main sources, natural sources and anthropogenic activities. Natural source, which represents that the metal concentration originates from parent rocks weathering and erosion. Whereas anthropogenic activities, includes industrial effluents, mining and refining, agricultural drainage, domestic discharges, and atmospheric deposition, etc. [1–5]. However, with rapid industrialization and urbanization development, anthropogenic activities have become a significant contributor to the potential accumulations of hazardous metal in aquatic ecosystems [6, 7]. Therefore, the study of hazardous metal elements in aquatic ecosystems is much important to identify the origin, distribution and contamination level of toxic metals in aquatic environments [8, 9].

In aquatic ecosystems, various kinds of hazardous and toxic substances are easily accumulated in sediments. The sediment, therefore, plays an important role in evaluating pollution level in aquatic environment [9–11]. Recently, the sediments have been widely used as environmental indicators and their ability to trace contamination sources [6, 9, 12, 13].

The Xiangjiang River, the most important river in Hunan province in southern China, provides plentiful fresh water resources for people of this province. With a developing economy and expanding human activity during recent decades, various hazardous metals have discharged into Xiangjiang River, especially in lower reach of this river [14, 15]. However, few data is available for Xiangjiang River. The present study was conducted as a preliminary survey on sediment contamination of Xiangjiang River. The objectives of the study were (1) to determine hazardous metal concentrations in surface sediments from Changsha–Xiangtan–Zhuzhou section of Xiangjiang River; (2) to assess hazardous metals contamination level of this river.

Materials and methods

Sampling and experimental method

The Xiangjiang River, one of the main tributaries of the Changjiang River, originates in the Hai-Yang Mountains in Guangxi Zhuang Autonomous Region, and runs across Hunan Province from south to north and finally enters Dongting Lake (Fig. 1). The Xiangjiang River has a length of approximately 856 km and has a catchment’s area of approximately 94,660 km2 [14]. The Xiangjiang River Basin is exposed to a sub-tropical monsoon climate. The annual temperature averages between 17 and 18 °C. The mean annual precipitation varies from 1,200 to 1700 mm, but 60–70 % of the annual precipitation occurs in the rainy season from April to September, especially from April to June [14, 15]. The vegetable in this basin is dense with relatively intensive biological activities. Red soil is the main type of soil in this drainage basin. This River yields average annual sand content only 0.102–0.173 kg m−3, and is lower than that of the rivers in northern China [15].

Fig. 1
figure 1

Sampling stations of the surface sediments from Xiangjiang River

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In this work, sediment samples of current floodplain from 16 sites were collected along the Changsha–Xiangtan–Zhuzhou section of the lower Xiangjiang River during November 10–16, 2010 using a pre-cleaned and acid washed PVC spade and top 3–5 cm samples were immediately kept in acid washed polythene bags. All the samples were transported to the laboratory for metals analysis of the sediments.

All of the samples for the chemical analysis were powdered in an agate mortar prior to measure. The detailed procedure is following the method, which has been proposed by Jiang [16]. Total 21 metal elements were extracted by high-resolution inductively coupled plasma mass spectroscopy (HR-ICPMS) at the State Key Laboratory for Mineral Deposits Research of Nanjing University. The analytical precision was preliminarily estimated to be <10 % according to duplicate analysis of samples and standards [16].

Pollution assessment methods

Many methods of qualification has been widely used to assess enrichment and pollution level in the sediments, such as enrichment factor (EF), the geo-accumulation index (I geo), pollution index (PI), potential ecological risk index (PERI), and the pollution load index (PLI), etc. [5, 11, 17–26]. In this study, three indexes of the EF, I geo and PI are applied to evaluate anthropogenic influences of heavy metals in sediments of the Xiangjiang River.

The EF is a useful indicator which can reflect the status of environmental pollution. The EF is commonly defined as the observed metal to reference element ratio in the sample of interest divided by the background metal to relatively reference element ratio [17]. Usually, the reference element is relatively a conservative one, such as the commonly used elements Al, Fe, Ti, Mn, Sc, K, etc. [11, 21, 27]. In our case, Zr element has been selected as a reference element, because Zr is a relatively stable element on the earth and relatively hard to transport and transfer during the process of weathering. The EF has been mathematically calculated by the following equation:

$$ {\text{EF = }}\frac{{\left( {C_{X} /C_{\text{Zr}} } \right)_{\text{sample}} }}{{\left( {C_{X} /C_{\text{Zr}} } \right)_{\text{UCC}} }} $$

where (C x /C Zr) Sample is a ratio of concentration of the metal to Zr in the samples of interest, whereas (C x /C Zr)UCC is the ratio of the upper continental crust (UCC) value of metal to Zr. The values of UCC from this study has been utilized as the background values, their concentrations of 21 elements studied in the UCC were in μg kg−1: 11 for Sc, 60 for V, 35 for Cr, 600 for Mn, 10 for Co, 20 for Ni, 25 for Cu, 71 for Zn, 17 for Ga, 112 for Rb, 350 for Sr, 1.5 for Mo, 0.098 for Cd, 5.5 for Sn, 3.7 for Cs, 550 for Ba, 2 for W, 20 for Pb, 0.127 for Bi, 10.7 for Th and 2.8 for U, respectively[28]. There are two different assessment criteria based on EF values [17]. On the basis of the study conducted by Zhang and Liu [17], when EF value ≤1.5, it implies that the metal may be entirely from crustal materials or natural weathering processes, while an EF value >1.5, we can deduce that a significant portion of the metal has originated from anthropogenic activities [17]. Whereas according to criteria studied by Han [29], it can be divided five various contamination classes on the ground of EF value: EF < 2 suggests deficiency to minimal enrichment, EF = 2–5 moderate enrichment, EF = 5–20 significant enrichment, EF = 20–40 very high enrichment and EF > 40 extremely high enrichment [29].

The I geo was originally defined by Müller [30]. To this day, I geo have been commonly used as an indicator to assess the metal pollution in sediments [11, 22, 29]. The I geo enables the evaluation of metal pollution by comparing the current and pre-industrial concentrations [21, 30]. It has been worked out by the following equation:

$$ I_{\text{geo}} {\text{ = log}}_{ 2} \frac{{\left( {C_{n} } \right)}}{{\left( { 1. 5B_{n} } \right)}} $$

where C n is the measured concentration of the examined metal (n) in the sediment, and B n is the geochemical background concentration of the metal (n). Factor 1.5 was used to minimize the effect of possible variations in the background values because of lithogenic effects [30]. Here, B n is the background concentration of element n in the UCC [28]. According to classification of I geo by Müller [30], there are seven classes of I geo from Class 0 (I geo < 0) to Class 6 (I geo > 5): <0 = practically unpolluted, 0–1 = unpolluted to moderately polluted, 1–2 = moderately polluted, 2–3 = moderately to strongly polluted, 3–4 = strongly polluted, 4–5 = strongly to extremely polluted, >5 = extremely polluted and reflects at least 100-fold enrichment above the background values [30].

The PI was originally used by Hakanson [23]. PI has been calculated according to the following equation:

$$ {\text{PI}} = \frac{1}{n}\sum {(C_{i} } /B_{i} ) $$

where C i was measured concentration of metal i and B i was background value of metal i. According to the classification of PI proposed by Hakanson [23], there are four classes of PI from Class 1(PI < 1) to Class 4 (PI > 10): PI < 1 suggests unpolluted, 1 < PI < 2 = moderately polluted, 2 < PI < 10 = strongly polluted and PI > 10 = extremely polluted [6, 23].

Results and discussion

Concentrations of hazardous heavy elements

Concentrations of 21 metals elements and their mean, standard deviation, maximum and minimum values in different sites of the Xiangjiang River are given in Table 1. As the Table 1 shown, the total concentrations are as follows: SC, 1.80–14.88 μg g−1, with an average of 8.91 μg g−1; V, 16.66–144.29 μg g−1, with an average of 73.35 μg g−1; Cr, 13.27–87.99 μg g−1, with an average of 51.99 μg g−1; Mn, 159.50–2413.74 μg g−1, with an average of 1190.05 μg g−1; Co, 2.21–23.14 μg g−1, with an average of 11.55 μg g−1; Ni, 4.71–42.45 μg g−1, with an average of 24.57 μg g−1; Cu, 5.31–188.89 μg g−1, with an average of 43.01 μg g−1; Zn, 38.41–1250.47 μg g−1, with an average of 266.57 μg g−1; Ga, 5.1–22.15 μg g−1, with an average of 14.63 μg g−1; Rb, 91.8–161.57 μg g−1, with an average of 121.92 μg g−1; Sr, 20.4–62.51 μg g−1, with an average of 40.40 μg g−1; Mo, 0.2–2.45 μg g−1, with an average of 1.17 μg g−1; Cd, 0.9–81.79 μg g−1, with an average of 14.97 μg g−1; Sn, 3.8–30.3 μg g−1, with an average of 11.44 μg g−1; Cs, 5.9–18.33 μg g−1, with an average of 13.08 μg g−1; Ba, 208.10–464.74 μg g−1, with an average of 331.88 μg g−1; W, 1.6–22.05 μg g−1, with an average of 9.92 μg g−1; Pb, 18.90–198.01 μg g−1, with an average of 71.10 μg g−1; Bi, 0.3–8.54 μg g−1, with an average of 3.17 μg g−1; Th, 2.6–18.27 μg g−1, with an average of 10.45 μg g−1; U, 0.6–13.62 μg g−1, with an average of 3.44 μg g−1. From these data it is clear that the mean values of the metal concentrations arranged in the following decreasing order: Mn > Ba > Zn > Rb > V > Pb > Cr > Cu > Sr > Ni > Cd > Ga > Co > Sn > Cs > Th > W > Sc > U > Bi > Mo. Therefore, metal elements such as Mn, Ba, Zn and Rb present higher levels in the Xiangjiang River sediments, whereas U, Bi and Mo present the lowest values (Table 1).

Table 1 Concentrations of the surface sediments from Xiangjiang River (μg g−1)
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Compared with the background values of the UCC [28], the metal mean concentrations of Mn, Cu, Zn, Cd, Sn, Cs, W, Pb and Bi in Xiangjiang River sediments are much higher, whereas Sc, V, Cr, Co, Ni, Ga, Rb, Sr, Mo, Ba, Th and U are close or lower than the UCC. The values of the mean concentrations in Xiangjiang River sediments divided by the UCC value decrease in the order of Cd > Bi > W > Zn > Pb > Cs > Sn > Mn > Cu > 1.5 times > Cr > U = Ni > V > Co > Rb > 1.0 time > Th > Ga > Sc > Mo > Ba > Sr. The highest mean concentration among these metal elements in Xiangjiang River sediments is Cd, which is more than 150 times higher than the UCC, the second higher mean concentration is Bi, which is nearly 25 times higher than the UCC. The values of the mean concentrations of W, Zn, Pb, Cs, Sn, Mn and Cu are 4.96, 3.75, 3.55, 3.45, 2.08, 1.98 and 1.78 times UCC values [28], respectively. In contrast, Sc, V, Cr, Co, Ni, Ga, Rb, Sr, Mo, Ba, Th and U mean concentrations are approximately the same as and lower than their UCC values [28], respectively. These imply that W, Zn, Pb, Cs, Sn, Mn and Cu may mainly originate from anthropogenic sources, whereas other metals may mainly have a natural source. However, although metal mean concentrations of Sc, V, Cr, Co, Ni, Ga, Rb, Mo, Th and U in the sediments are close or lower than the UCC values [28], metal concentrations of these metals at some sites are higher than UCC values. As Cu for example, the concentrations at sites XJ03, XJ04, XJ05, XJ06, XJ07, XJ10, XJ11, XJ12, XJ13 and XJ17 are higher than UCC values [28], whereas sites XJ01, XJ08, XJ14, XJ15, XJ16 and XJ18 are lower UCC [28]. This suggests there are point sources along Zhuzhou–Xiangtan–Changsha sections of Xiangjiang River due to discharge of industrial effluents from various sources including untreated sewage, municipal waste and agrochemical runoff from nearby cities and villages directly into the river.

Pollution assessment

The calculated results of EF and I geo of the Xiangjiang River sediments are listed in Tables 2 and 3, respectively. The computed PIs for 16 stations from the Xiangjiang River are described in Fig. 2. As shown in Table 2, the results of the calculated EF values from this study are 0.36–1.06 for Sc, 0.49–1.88 for V, 0.57–2.28 for Cr, 0.24–3.10 for Mn, 0.36–1.81 for Co, 0.46–1.66 for Ni, 0.36–5.9 for Cu, 0.45–13.75 for Zn, 0.38–1.8 for Ga, 0.44–5.35 for Rb, 0.04–0.36 for Sr, 0.23–1.73 for Mo, 7.06–651.47 for Cd, 0.70–4.26 for Sn, 1.44–9.68 for Cs, 0.24–2.21 for Ba; 1.49–8.61 for W, 1.30–7.73 for Pb, 7.57–52.48 for Bi, 0.49–1.44 for Th, 0.48–1.77 for U. On the whole, from these data it is clear that the average EF values of Cd (124.40), Bi (20.58), W (4.28), Cs (3.91), Pb (3.41), Zn (3.33) and Sn (2.03) are higher than 2 (EF > 2), whereas the average EF values of other metal elements are less than 2 (EF < 2), these indicate metal elements of Cd, Bi, W, Cs, Pb, Zn and Sn have an anthropogenic impact on metal concentration in this river, in contrast, all the other metals elements originate mainly from natural source. Based on Han [29] scale, the average EF values of these metals indicate extremely high contamination of Cd, very high accumulation of Bi, moderate pollution for W, Cs, Pb, Zn and Sn, whereas the average EF values of other metals show Xiangjiang River sediments are deficiency to minimal enrichment of these metals (Table 2). However, although the mean EF values of some metals are less than 2 (EF < 2), these metals in some stations present relatively higher EF values (EF > 2). For example, the average EF values of Mn and Cu are 1.71 and 1.49, respectively and correspond with deficiency to minimal enrichment, but stations XJ05, XJ06, XJ07, XJ10 and XJ13 for Mn and stations XJ13 and XJ17 show higher EF values, suggesting the contamination of these metals is attributed to local point source. In fact, we have found some sewage outlets discharge directly into river along the Xiangjiang River during the process of field investigation.

Table 2 Enrichment factor (EF) of hazardous elements in surface sediments from the Xiangjiang River
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Table 3 Geoaccumulation index (I geo) of hazardous elements in surface sediments collected from different sites of the Xiangjiang River
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Fig. 2
figure 2

The PI of 16 stations from lower reach of Xiangjiang River

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It is clear from Table 3 that the calculated I geo values from this study can be divided two clusters: (1) Zn, Cd, Sn, Cs, W, Pb and Bi; (2) Sc, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, Rb, Sr, Mo, Cd, Sn, Cs, Ba, W, Pb, Bi, Th, U. The cluster 1 shows the average I geo values are greater than 0 (I geo > 0), whereas the cluster 2 indicate their average I geo values are less than 0 (I geo < 0). In general, these facts suggest Xiangjiang River has been polluted by metals Zn, Cd, Sn, Cs, W, Pb and Bi, and has not been polluted by metals Sc, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, Rb, Sr, Mo, Cd, Sn, Cs, Ba, W, Pb, Bi, Th, U. According to the Müller scale [30], the EF for Cd ranges from 2.65 to 9.12 with an average of 5.65 and corresponds with Class 6 (extremely polluted and reflects at least 100-fold enrichment above the background values); the EF for Bi varies from 0.67 to 5.49 with an average of 3.55 and corresponds with Class 4 (strongly polluted); the EF for other contamination metal elements shows different variations and mean values: Cs, 0.10–1.72, with an average of 1.16; W, −0.90–2.88, with an average of 1.34; Pb, −0.67–2.72, with an average of 0.99; Zn, −1.47–3.55, with an average of 0.76, Sn, −1.12–1.86, with an average of 0.23, indicating Xiangjiang River are moderately polluted by Cs and W (Class 2), and unpolluted to moderately polluted with respect to Pb, Zn and Sn (Class 1). Whereas other metals fall into Class 0 and can be regarded practically unpolluted. The average EF values for Mn and Cu are less than 0, but stations XJ03, XJ04, XJ05, XJ06, XJ07, XJ10, XJ11, XJ12 and XJ13, and stations XJ07, XJ10, XJ11, XJ12, XJ13 and XJ17 show higher EF values, which fall in Classes 1–6 (from unpolluted to moderately polluted to extremely polluted), suggesting there are some point pollution sources along the lower reaches of Xiangjiang River. The analytical results of I geo of metals in Xiangjiang River sediments are same as the assessment results of EF.

As plotted in Fig. 2, the results of calculated PI in this study range from 0.97 to 46.64. The station XJ013 exhibits the maximum, whereas station XJ018 presents the minimum. On the basis of this scale proposed by Hakanson [23], the PI values of the stations XJ10, XJ11, XJ12, XJ13 are 17.67, 11.08, 22.17 and 46.64, respectively, indicating these four stations have been extremely polluted hazardous metals. The PI values for stations of XJ03-XJ08, XJ14, XJ15 and XJ17 varies from 2 to 10, showing these nine stations have been strongly polluted by metals. The stations of XJ01 and XJ16 are moderately polluted with the PI values of 1.71 and 1.43, respectively (1 < PI < 2), whereas only station XJ18 is not polluted with the PI value of 0.97 (PI < 1). In general, the PIs for various stations from Zhuzhou–Xiangtan–Changsha section of Xiangjiang River present metals pollution.

Conclusions

Total 21 metals in the surface sediments from 16 stations from Xiangjiang River (Zhuzhou–Xiangtan–Changsha section) have been studied in our work. The total metals concentrations show wide variations. The mean concentrations in surface sediments divided by the UCC value decreased in the order of Cd > Bi > W > Zn > Pb > Cs > Sn > Mn > Cu > 1.5 times > Cr > U = Ni > V > Co > Rb > 1.0 time > Th > Ga > Sc > Mo > Ba > Sr.

Based on the calculated EF and I geo of metals, we found metals elements of Cd, Bi, W, Cs, Pb, Zn, Sn had widely been enriched, metals elements of Cu and Mn had locally accumulated, whereas other metals elements Cr, U, Ni, Co, Rb, Th, Ga, Sc, Mo, Ba and Sr had not enriched in the surface sediments from Xiangjiang River. However, the calculated PI indicated almost all the stations from Xiangjiang River (Zhuzhou–Xiangtan–Changsha section) had been polluted by hazardous metals.