J. Cent. South Univ. (2012) 19: 2634-2642
DOI: 10.1007/s11771-012-1321-x
Assessment of heavy metals in sediment cores from Xiangjiang River, Chang-Zhu-Tan region, Hunan Province, China
LONG Yong-zhen(龙永珍)1, DAI Tai-gen(戴塔根)1, CHI Guo-xiang(池国祥)2, YANG Liu(杨柳)1
1. Key Laboratory of Metallogenic Prediction of Nonferrous Metals of Ministry of Education
(Central South University), Changsha 410083, China;
2. Department of Geology, University of Regina, Regina S4S OA2, Canada
? Central South University Press and Springer-Verlag Berlin Heidelberg 2012
Abstract:
Samples were collected from two core sediments (C1 and C2) of Xiangjiang River, Chang-Zhu-Tan region, Hunan Province, China. The heavy metal contents are relatively higher, especially for the surface or near the surface layers. The calculated anthropogenic factor values indicate that all the heavy metals except for Cr in the core samples are enriched, especially for Cd, with the maximum enriching coefficients of 119.44, and 84.67 in C1 and C2, respectively. The correlation of heavy metals with sulphur indicates that they are precipitated as metal sulphides. Correlation matrix shows significant association between heavy metals and mud. Factor analysis identifies that signified anthropogenic activities affect the region of Xiangjiang River.
Key words:
heavy metals; core sediment; pollution; Xiangjiang River, Chang-Zhu-Tan region；
1 Introduction
With the exponential growing of population and developing of industries along the river and estuarine areas, the pollution of urban rivers has become more and more serious around the world [1]. Various kinds of hazardous and toxic substances into rivers via several pathways, including disposal of liquid effluents, terrestrial runoff and leachate carrying chemicals originating from numerous urban, mining, industrial and agricultural activities, as well as atmospheric deposition [2], especially those in industrial and population centers, have led to a significant increase in metal contamination [1, 3]. Sediments are important carriers of heavy metals in the hydrological cycle. The sediment history broadly reflects the contamination history of an area [4]. Metals accumulated in this way may be subsequently released to the water as a result of either physical disturbance or diagenesis, which could affect the water quality and the bioaccumulation and bioassimilation of metals in aquatic organisms, resulting in potential long-term implications on human health and ecosystem [5]. Core sediments provide useful information on the changes in the quality of the river from a past period. Vertical profiles of pollutant species in sediment cores have been commonly used as “pollution records” over the last decade [6]. The assessment of heavy metal pollution in sediments is of prime importance to improve management strategies [7]. Over the last few decades, the study of sediment cores has shown to be an excellent tool for establishing the effects of anthropogenic and natural processes on depositional environments. A number of recent works have used sediment profiles to study the contamination history of different environments [1-2, 6, 8-9].
Xiangjiang River, the biggest river in Hunan Province, China, is regarded as the mother river of Hunan Province. In the meantime, Hunan is known for its richness of mineral resources, to which the province owes the name of “the Country of Nonferrous Metals” [10]. Many mines and industrial areas have been developed near or beside Xiangjiang Rive especially in City of Zhuzhou. Detection of heavy metal pollution in Xiangjiang River dated back to 1966, and in 1978, Xiangjiang River became one of the most severely polluted rivers in China [11]. In 2007, over 5.67×108 t of industrial waste water and 1.19×109 t of domestic sewage were discharged in Xiangjiang River, accounting for 37%, 6.0% and 14.1% of the total amount of discharged Cd, Pb and As, respectively, in China [10]. The heavy metal pollution seriously threatens the lives of the local people [12]. A series of poisoning cases such as "blood lead" and "cadmium rice" have been reported along Xiangjiang River [13-14]. In 2009, Xiangjiang River was listed by the Ministry of Environmental Protection Department of China as the first target in the “Remediation Projects of Major Rivers”, as well as included in the “12.5 Development Program of Hunan Province” in 2010 [15].
The main goal of this work is to determine the heavy metal distribution, possible anthropogenic influence and their interrelation in the sediment profiles of Xiangjiang River sediment cores in the Changsha and Zhuzhou (Chang-Zhu-Tan region), finding the degree of contamination in order to obtain a global view of the historic sediment quality of the river sediment system as a contribution to the knowledge and rational management of these regions in future.
2 Materials and methods
2.1 Sample collection
Sediment core samples were collected at two selected locations (C1 and C2, Fig. 1) during 2006 with the help of a fiber boat across Xiangjiang River and samples were limited only to the riverine area due to sampling restrictions in industrial town zones. Core 1 (C1) was collected in the Yueliang Channel-bar, downstream of Xiangjiang River in Changsha City, whose geographical coordinate is 28°18′02.20″ N and 112°56′09.33″ E, with an elevation of 25 m. Core 2 (C2) was collected in the Gusang Channel-bar, downstream of Xiangjiang River in Zhuzhou City, 52.28 km apart from C1, whose geographical coordinate is 27°49′15.85″ N and 113°00′46″ E with an elevation of 36 m (Fig. 1). A PVC coring tube (2.5 inch in diameter and 2.5 m in length) pre-cleaned with acid was used for the collection of core samples. The PVC tube was driven into the riverbed until about 50 cm of the pipe remained above ground and the rest was filled with ambient water on top. The PVC tube was sealed using a Plumber’s dummy and it was pulled out from the sediment. The water on top was then decanted, the pipe was just cut off above the top of the cored sediment and plastic bags were taped over both the ends of the PVC tube. Geochemical data presented in this work have not been corrected for compaction, as it is likely to be uniform down the length of the core [16]. The core sediments were sub-sampled at 3 cm interval and a thin film zone of sediment next to the PVC tube was left in the core itself to avoid contamination.
Fig. 1 Map of Chang-Zhu-Tan region showing Xiangjiang River and locations of sampling stations (Insert map showing location of study area in China)
2.2 Analytical methods
Textural studies were performed in all the core samples to estimate sand, silt and clay contents [17]. The results are presented as sand and mud (silt and clay) due to the low level of finer particles. Determination of total sulphur was done following the standard procedure of Ref. [18]. Sub-samples of bulk sediment at definite core intervals were dried at 45 ℃ until no further mass loss occurred, then ground to fine powder in a mortar and pestle. About 0.20 g dry sediment was totally digested using a closed vessel microwave assisted system (Anton Paar GmbH, Multiwave 3000) according to the USEPT 3052 method [19]. Contents of As, Cd, Pb, Zn, Cr and Cu in sediment were determined by inductively coupled plasma mass spectroscopy (VG PQ ExCell ICP-MS). The recovery of the elements As, Cd, Pb, Zn, Cr, Cu and S were within 84%-106% of the certified or information values given by NIST and National Standard Research Center. The limits of detection for As, Cd, Pb, Zn, Cr and Cu were 0.1, 0.002, 0.04, 0.8, 0.06 and 0.03 ng/g, respectively.
In order to characterize the studied river basin according to the types of heavy metal contaminants in sediments, correlation analysis [20] and multivariate analysis (PCA) [21] were applied for the data set of SS and heavy metal concentrations in sediment samples of the cores. The significance level and KMO and Bartlett’s test of sphericity were performed to test the adaptability of PCA. The statistical treatments were performed using PASW statistics 18 for Microsoft Excel.
3 Results and discussion
3.1 Sediment texture
Textural studies reveal that the mud contents of sediment cores range from 12.45% to 95.43% with mean value of 57.69% in C1 and from 11.25% to 92.02% with mean value of 49.54% in C2, and the standard deviations are 27.52% and 25.25% in C1 and C2, respectively (Table 1). The highest sand contents at 51-54 cm in C1 and 45-48 cm in C2 indicate a relatively higher energy regime that prevents sedimentation of fine-grained particles. In addition, the sand content shows gradual decrease from C1 to C2, which indicates the variation of flow condition, where finer particles have settled down in the low flow condition of downstream side.
Table 1 Contents of sand and mud in core of C1 and C2
3.2 Sulphur
Down core profiles of total sulphur (S) in C1 and C2 are presented in Fig. 1 and Fig. 2. The enrichment of sulphur at surface layers in C1 and C2 is due to the precipitation of insoluble metal sulphides from the mines and factories, especially for C2. As noted before, Hunan owes the name of “the Country of Nonferrous Metals” and many nonferrous metal mines and industrial areas distribute in Chang-Zhu-Tan region, especially in Zhuzhou City which is the biggest industry city in Hunan Province. Oxidation of Fe-sulphides is responsible for the decrease in S content of C1 (3-45 cm), however, in C2 it is considered to be negligible as compared to the amplitude of the metallic sulfide. Diffusion processes of sulphides alone are unlikely to account for the presence of sulphur below the sub-oxic/anoxic interface, indicating that sufficient oxidants must be present to generate sulphides at some depths and they are the metal oxides, which interact with sulphur species to form sulphides [1]. The complete oxidation of sulphides with Fe3+ has been documented at low pH in nonmarine environment [20]. However, the flux of liable organic matter to the sediments is very low [21], resulting in low values after sulfate reduction.
Fig. 2 Vertical profiles of heavy metals and S of sediment core 1 in Xiangjiang River, Changsha, China: (a) As; (b) Cd; (c) Pb; (d) Zn; (e) Cr; (f) Cu; (g) S
Fig. 3 Vertical profiles of heavy metals and S of sediment core 2 in Xiangjiang River, Zhuzhou, China: (a) As; (b) Cd; (c) Pb; (d) Zn; (e) Cr; (f) Cu; (g) S
3.3 Down core profiles of heavy metals
The heavy metal contents of As, Cd, Pb, Zn, Cr and Cu have a similarity at different depths (Fig. 1 and Fig. 2). The average values from C1 and C2 are As (45.5 and 52.01 μg/g), Cd (13.46 and 11.9 μg/g), Pb (123.6 and 299.1 μg/g), Zn (429 and 138.2 μg/g), Cr (93 and 62 μg/g) and Cu (50.2 and 50.3 μg/g) (Table 1 and 2), respectively. The heavy metal contents are related with factories and mines around the river. For example, the average Cr content of C1 is higher than that of C2 due to Changsha Chromate Plant which is just located at the upper reaches of the C1, while the average Pb content at C2 is significant higher than that of C1 due to the upper reaches of Zhuzhou Smelter and Refinery of Nonferrous Metals which is one of the top enterprises in China in which main productions of lead and zinc were 35×106 and 10×106 t, respectively, accounting for 20 % of total national exports in China in 2007 [10].
Metal peaks along reduced layers are clearly observed by the high values at 36-45 cm in C1, which suggests that the serious heavy metal pollution has happened during that time in Changsha City. However, the metal peaks along reduced layers obviously are observed with the highest values at the surface, and then gradually decrease in C2, which infers that the heavy metal pollution becomes more and more serious in Zhuzhou City in recent years.
The coincident peaks more or less at the same depth are displayed by heavy metals. On one hand, it suggests the contribution of post-depositional effects, such as reduction of sulphides and formation of metalliferous sulphides under anoxic conditions or reprecipitation of metals on Fe/Mn oxides and oxyhydroxides coatings; on the other hand, it suggests an increase in anthropogenic fluxes related to the urban and industrial development of surrounding or upstream areas during that time.
On the whole, the heavy metal contents in the sediment core profiles are changed dramatically, which in the surface or near the surface layers are significantly higher than those of the bottom layers.
3.4 Heavy metal enrichments
Enrichment of trace metals in the present work is calculated by elemental sequence (ES) and anthropogenic factor (AF). In the case of ES, it is calculated with reference to get the average values, whereas in the case of AF, AF reflects the degree of contamination relative to the composition of the respective metal in sediments to a measured background value (Cd) from geologically similar but uncontaminated area. If AF is larger than 1 for a particular metal, it means contamination; otherwise if AF is not larger that 1, there is no metal enrichment of anthropogenic origin. The ES data indicate the dominance of Cd, Zn, As, Pb, Cr and Cu in all the core samples and also emphasize that there are from external sources. The calculated AF values for C1 and C2 are presented in Table 2.
The calculated AF values indicate that all the heavy metals except for Cr in the core samples are enriched, especially for Cd with enriching coefficients of 119.44 and 84.67 in C1 and C2, respectively.
The ES in core samples is placed in the following order:
C1: Cd>Zn>As>Pb>Cu>Cr
C2: Cd>Zn>As>Pb>Cu>Cr
Table 2 Calculated anthropogenic factor (AF) for C1 and C2
The ES and AF values suggest anthropogenic input of industries and sewage load along the rivebank, especially for Cd with significant high AF values from the surface layers to about 50 cm below the ground in both of C1 and C2 (Table 3). It may be the reason for a series of “cadmium” poisoning cases that had happened around the Xiangjiang River in this region. The lower contents of heavy metals such as Cr in C2 are probably attributed to the coarse nature and low retention of heavy metals in sediments of Xiangjiang River.
3.5 Relationship among elements
Results of correlation matrix for each core samples are presented in Table 4. The strong correlation of heavy metals with mud indicates that they are concentrated to the fine-grained particles and are hosted by clay phases. The close relation among contents of Zn, Cd, Pb and Cu in both C1 and C2 (Table 4) infers the source of these metals pollution from nonferrous metal mining and production activities as these elements commonly coexist in base metal ores [22]. The significant relationship of S with heavy metals indicates that these metals are precipitated or exist as metal sulphides and are also responsible for the fixation of metals in core sediments.
Notably, the correlation and geochemical associations of metals reveal a significant source of contamination, reflecting a common origin of industrial activities related to mining, processing and metallurgy of nonferrous ores, especially for the surface layers.
3.6 Factor analysis
The results of PCA for heavy metal concentrations, sulphur, sand and mud in sediment core samples are summarized in Table 5. The two significant statistical factors (F1 and F2) provide a basis for determining the areas influenced by the sedimentological and geochemical variations. F1 accounts for 82.10% and 68.89% of total variance in C1 and C2, respectively. The grouping of heavy metals in F1 with mud in the two core samples indicates that fine particles are responsible for controlling the uptake of elements into the sediments. The presence of sulfur along with the heavy metals in F1 of all the core samples further indicates that they are present as sulphides with fine particles.
Core sediments in F2 account for 8.23% and 15% of total variance in C1 and C2, respectively. F2 has a positive loading of sand and Cd in C2 (Table 5), indicating that the mode of occurrence of Cd is easily adsorbed by the sand in C2 than in C1. It is inferred that the sources of Cd pollution in C2 have some differences form C1, which is due to the difference of industrial structure between Changsha and Zhuzhou.
Table 3 Anthropogenic factors of Cd in C1 and C2
Table 4 Correlations of heavy metal contents as well as sand, mud and sulphur in C1 and C2
Table 5 Factor loadings of principal component analysis (PCA) of heavy metals, sand, mud and sulphur in C1 and C2
4 Conclusions
1) The vertical profile of heavy metals appears to be dependent on a combination of the sources, and the selectivity of adsorbing medium. The above mechanism is well supported by correlation matrix indicating its association with S and mud, and in the factor analysis as a dominant factor in F1.
2) The contents of heavy metals indicate serious pollution in this region and the metal contents in the surface sediments generally increase.
3) The ES and AF values suggest anthropogenic input of industries and sewage load along the riverbank, especially for Cd. It may be the reason for a series of “cadmium” poisoning cases that have happened around the Xiangjiang River in this region.
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(Edited by YANG Bing)
Foundation item: Project(1212010) supported by the China Geological Survey for Ecosystem Geochemistry Assessment in City of Changsha, Zhuzhou and Xiangtan
Received date: 2011-11-01; Accepted date: 2012-03-21
Corresponding author: LONG Yong-zhen, PhD; Tel: +86-731-88877077; E-mail: Jilllongyz@163.com
Abstract: Samples were collected from two core sediments (C1 and C2) of Xiangjiang River, Chang-Zhu-Tan region, Hunan Province, China. The heavy metal contents are relatively higher, especially for the surface or near the surface layers. The calculated anthropogenic factor values indicate that all the heavy metals except for Cr in the core samples are enriched, especially for Cd, with the maximum enriching coefficients of 119.44, and 84.67 in C1 and C2, respectively. The correlation of heavy metals with sulphur indicates that they are precipitated as metal sulphides. Correlation matrix shows significant association between heavy metals and mud. Factor analysis identifies that signified anthropogenic activities affect the region of Xiangjiang River.
