Pollution Level and Ecological Risk Assessment of Heavy Metals in Riverside Sediments of the Grand Canal (Beijing, Tianjin and Hebei section)

By Li Shen, Yan Gan, Chengyu Li, Chao Wang *
November 2019

The authors are Associate Professors at Logistic School, Beijing Wuzi University, Beijing 101149, China   *Corresponding Author

Abstract
Pollution level and ecological risk assessment of heavy metals in riverside sediments of the Grand Canal (Beijing, Tianjin and Heibei sections) were studied. The concentrations of As, Ba, Cd, Co, Cr, Cu, Mn, Ni, Pb, V and Zn in shallow sediments were measured by ICP-AES. It was found that the concentrations of several heavy metals in shallow sediments were higher than the background values of soils in Beijing area. The results showed that the historical heavy metal discharge in the study area still had some adverse effects on river sediments. The results of potential ecological risk assessment indicated that about 34.6% of the sampling sites reached medium ecological risk level. The main hazardous metal was Cd, and thus Cd should be managed in priority.

Keywords: the Grande Canal; shallow sediment; heavy metals; potential ecological risk

1. Introduction

Beijing-Hangzhou Grand Canal is one of the longest ancient canal in the world, which has been used up to now. It starts from Hangzhou in the south and ends at Beijing in the north. It runs through four provinces of Zhejiang, Jiangsu, Shandong and Hebei, and two cities of Tianjin and Beijing, with a total length of about 1797 km. The canal played a great role in the exchange and development of economic and cultural between the north and the south of China. With urbanization, population explosion and rapid development of industry, a large number of domestic and industrial waste water were discharged into the canal, resulting in poor water quality, which seriously hindered the sustainable development of the region and the improvement of people's living standards. In order to improve the ecological environment of the Grand Canal, the government has increased its efforts to improve the environment of the Grand Canal in recent years. However, the water environment management is very difficult because the canal ans many administrative areas, and some pollutants, especially heavy metal elements, have deposition effect. The canal management has a long way to go.

As one of the main environmental pollutants, heavy metals are difficult to degrade, toxic and easy to bio-accumulate, which will have a continuous impact on the water ecosystem [1, 2]. Therefore, the detection of heavy metals in river water and sediment has risen wide spread attention [3-11]. For example, Guo Xiao et al. [8] studied the heav metal pollution in the South Canal and evaluated the ecological risk of heavy metals. The results showed that the potential ecological risk level of heavy metals in the south canal was moderate, and the priority heavy metal pollutants to be controlled in sediments were Cd and As. Zhang Wei et al. [9] discussed the distribution characteristics of heavy metals in sediment of Shahe Reservoir in the upper reach of the North Canal. They found that Cr, Cu, Ni and Zn had a certainty accumulation in the sediment and their contents were over two times higher than the background values of Beijing soil. In the previous reports, there are few studies on the assessment of heavy metal pollution in rivers across administrative regions. And no attempts have been made to assess the metallic pollutions of shallow sediments in Beijing, Tianjin and Hebei section of the Grand Canal.

In this paper, the shallow sediments of the Grand Canal (Tongzhou of Beijing, Xianghe of Hebei, and Wuqing of Tianjin) are sampled and analyzed to fully understand the types and distribution of heavy metal pollutants of the Beijing, Tianjin and Hebei section of the Grand Canal. The potential ecological risks of heavy metals are assessed to evaluate the health of the environment. The work provides a fundamental basis for the river management and ecological environment improvement of Beijing, Tianjin and Hebei section of the Grand Canal.

2. Materials and Methods

2.1 Study Area

Beijing, Tianjin and Hebei section of the Grand Canal refers to the section from Tongzhou, Beijing to Wuqing, Tianjin via Xianghe, Hebei, with a total length of more than 120 kilometers, of which the Tongzhou section of the Grand Canal is about 42 kilometers long, the Xianghe section is about 20.4 kilometers long, and the Wuqing section is about 62.3 kilometers long [12, 13]. Residential areas and agricultural planting areas are mainly near the Canal banks of Tongzhou and Xianghe, and factories are mainly near the banks of Wuqing. According to the relevant data of river water quality issued by the Environmental Protection Bureau, it can be seen that the water function of the Grand Canal (Tongzhou, Xianghe and Wuqing sections) is class V [14].

2.2 Sampling and Analysis

Sampling activities were carried out along the Canal bank in the year 2018. Nine sampling points were selected in Tongzhou section of Beijing, three in Xianghe section of Hebei and fourteen in Wuqing of Tianjin (Fig. 1). About 1 kg of sediment samples were collected at 10-20 cm of the shallow sediments of riparian zone, which were transferred to polyethylene self-sealing bags with a clean plastic shovel. After the impurities such as sand, plant roots and so on were removed, the sediment samples were air dried, ground in the ventilation cabinet, sieved through 100 meshes, labeled and stored in the dryer for testing.

0.1 g shallow sediment sample was digested with 5:2:1 HNO3/HF/HClO4 mixture. The mixed solution was heated at 120 C for 1 h, at 140 C for 1 h, at 160 C for 1 h, and at 180 C for 45 min. Then the solution was evaporated to nearly dry. After cooling to room temperature, the solution was diluted to 25 mL with deionized water. The contents of eleven types of heavy metals (As, Ba, Cd, Co, Cr, Cu, Mn, Ni, Pb, V and Zn) were determined by inductively coupled plasma-mass spectrometry (ICP-MS, PerkinElmer Instruments Co., Ltd NexION300x).

Figure 1: Sketch Map Showing the Study Area and Sampling Sites
Fig 1

2.3 Data Analysis Method

The potential ecological risk index (RI) method proposed by Swedish scholar Hakanson is widely used to quantitatively evaluate the ecological hazards of heavy metals in sediments [15-19]. The method takes into account both concentrations of heavy metals and ecological factors, and the “toxic-response” factor. So the potential ecological risk index can be acquired as follows:

where RI denotes the potential ecological risk factor of multiple metals; denotes the potential ecological risk factor of individual metal i; is the “toxic-response” factor of heavy metal i, as shown in Table 1; denotes the contamination of heavy metal i; is the measured concentration of heavy metal i; denotes the background value of heavy metal i in the uncontaminated soil. The evaluation criteria are shown in Table 2.

Table 1: “Toxic-Response” Factor of Different Heavy Metals [15, 16]
Metal As Ba Cd Co Cr Cu Mn Ni Pb V Zn
10 2 30 5 2 5 1 5 5 2 1

Table 2: Criteria of Heavy Metals Pollution Assessment and Potential Ecological Risk Assessment [16, 19]
RI Ecological risk
40 100 low
40-80 100-200 medium
80-160 200-400 strong
160-320 200-400 very strong
320 400 extremely strong

3. Results and Discussions

3.1 Concentration Distribution of Heavy Metals in Shallow Sediments

The concentration of heavy metals of all samples, as well as associated background values, are displayed in Table 3. The mean concentrations of Ba, Cd, Cr, Cu and Zn of the study regions are about 1.5 times of the background values. The concentration distribution of As, Mn and Pb is relatively complex. Although the mean concentrations of the three metals are lower than the background values, the maximum concentrations of some sampling points exceed the background values. For example, concentrations of As and Pb in the shallow sediment of Xianghe section are lower than the background values, while the maximum concentrations of As and Pb of Tongzhou section and Wuqing section exceed the background values. The concentration of Mn in the shallow sediment of Tongzhou section and Xianghe section is lower than the background value, while the maximum concentration of Mn of Wuqing section exceeds the background value. The results show that, except for Co, Ni and V, the other eight heavy metals in the shallow sediment of the study regions are polluted to varying degrees. The concentration variation coefficients and the dispersions of Ba, Co, Cr, Mn, Ni, Pb and V are small, which indicates that the concentrations of the above heavy metals are relatively stable and the spatial difference is small. The variation coefficients of Cu, Zn in Tongzhou section and As in Xianghe section are more than 0.6, and the variation coefficients of Cd in the three sections exceed 0.4. It indicates that the spatial concentrations of the above heavy metals vary greatly in the relevant regions.

Table 3: Concentrations and Background Values of Heavy Metals in Surface Sediments of the Grand Canal (Beijing, Tianjin and Hebei section) [20]
Metal Reach Range
(mg kg-1)
Median
(mg kg-1)
Mean
(mg kg-1)
Standard
Deviation
(mg kg-1)
Coefficient
of Variation
Background
value
(mg kg-1)
As Tongzhou 2.04–7.18 4.43 4.50 1.53 0.34 7.09
Xianghe 1.89–7.02 3.54 1.89 2.62 0.63
Wuqing 4.32–9.88 7.34 6.81 1.96 0.29
Ba Tongzhou 574–849 688 695 76.9 0.11 425
Xianghe 634–761 730 709 66.1 0.10
Wuqing 566–696 600 611 35.8 0.059
Cd Tongzhou ND–0.419 0.307 0.268 0.117 0.44 0.119

Xianghe 0.084–0.211 0.186 0.160 0.0671 0.42
Wuqing 0.200–0.387 0.200 0.205 0.086 0.42
Co Tongzhou 6.52–9.33 7.46 7.77 0.884 0.11 18.6
Xianghe 7.63–9.95 7.69 8.43 1.32 0.16
Wuqing 7.76–11.4 9.31 9.49 0.96 0.10
Cr Tongzhou 32.0–68.5 48.4 48.6 12.7 0.26 29.8
Xianghe 43.1–50.3 48.7 47.4 3.75 0.079
Wuqing 43.7–69.0 49.9 53.2 7.76 0.15
Cu Tongzhou 13.0–76.3 26.0 36.2 24.5 0.68 18.7
Xianghe 22.3–25.8 23.6 23.9 1.78 0.074
Wuqing 17.2–40.6 29.8 28.6 6.89 0.24
Mn Tongzhou 285–446 365 368 44.8 0.12 585
Xianghe 341–557 378 425 115 0.27
Wuqing 370–640 535 511 89.9 0.18
Ni Tongzhou 13.2–25.3 17.2 18.8 4.23 0.22 26.8
Xianghe 16.2–22.6 16.5 18.4 3.63 0.20
Wuqing 18.6–29.7 22.3 22.9 3.35 0.15
Pb Tongzhou 15.9–30.0 20.8 22.4 4.95 0.22 24.6
Xianghe 18.3–22.6 20.9 20.6 2.14 0.10
Wuqing 18.2–26.4 22.4 22.8 2.80 0.12
V Tongzhou 47.3–66.4 57.0 56.0 5.82 0.10 83.4
Xianghe 52.1–70.2 58.9 60.4 9.15 0.15
Wuqing 54.9–78.8 65.3 65.6 5.98 0.091
Zn Tongzhou 38.7–220.6 110 112 67.8 0.61 57.5
Xianghe 59.9–87.6 61.5 69.7 15.6 0.22
Wuqing 51.1–124 71.5 76.7 21.0 0.2

3.2 Pollution Risk of Heavy Metals in Shallow Sediments

The potential ecological risk factor of individual metal () and the potential ecological risk factor of multiple metals (RI) are shown in Table 4. The values of Cd in 12 sampling points are in the range of 40-80, which belongs to medium potential ecological risks; while values of Cd in 5 sampling points are in the range of 80-160, indicating that Cd has a strong potential ecological risk. Therefore, Cd is the main ecological risk factor of the shallow sediment in the study regions. The potential ecological risk factors of other heavy metals are far less than 40, showing that other heavy metals are at low ecological risk level. The average RI value of heavy metals in the shallow sediment of the whole study section is 89.2, lower than 100, demonstrating that the heavy metals are in a low potential ecological risk. Considering that the RI values of heavy metals in nine sampling points are between 100-200, the potential ecological risks of these nine sampling points are moderate, accounting for 34.6% of the total sampling points. Among the nine sampling points, five are located in Tongzhou section and four in Wuqing section. The heavy mental Cd contributes the most to RI, reaching 58.8% ~ 72.4%, which is directly related to the “toxic-response” factor of Cd element. Although the content of Cd element in the shallow sediment is the lowest, with a mean concentration of only 0.211 mg kg-1, Cd element has the largest contribution rate in the ecological risk calculation due to its maximum “toxic-response” factor. In general, the potential ecological risk factor of individual metal and the potential ecological risk factor of multiple metals indicate that Cd should be managed in priority.

Table 4: The Potential Ecological Risk Factor of Individual Metal and the Potential Ecological Risk Factor of multiple Metals in Surface Sediments
Reach Sampling
point
As Ba Cd Co Cr Cu Mn Ni Pb V Zn RI
Tongzhou 1 6.25 3.24 73.3 2.00 2.50 6.95 0.654 3.21 4.14 1.13 1.90 105
2 5.93 3.37 46.8 2.00 2.15 5.62 0.607 3.01 4.07 1.39 1.03 76.0
3 6.63 3.35 30.8 2.17 3.29 4.69 0.762 3.20 4.23 1.59 0.94 61.6
4 2.88 3.52 25.8 1.75 2.92 3.47 0.592 2.45 3.57 1.39 0.76 49.1
5 6.64 4.00 105 2.01 3.25 11.5 0.590 3.84 4.99 1.16 2.78 145
6 4.59 2.70 1.84 2.53 3.58 0.487 2.76 3.23 1.29 0.67 23.7
7 8.81 3.17 81.8 2.34 4.43 20.4 0.665 4.63 6.10 1.37 3.84 138
8 10.1 3.07 93.4 2.51 4.60 19.5 0.694 4.71 6.07 1.43 3.32 149
9 5.22 3.03 80.3 2.17 3.67 11.3 0.623 3.78 4.49 1.32 2.22 118
Mean 6.34 3.27 67.1 2.09 3.26 9.68 0.63 3.51 4.55 1.34 1.94 96.2
Xianghe 10 9.90 2.99 52.7 2.67 3.37 6.31 0.952 4.22 4.60 1.68 1.07 90.4
11 4.99 3.58 21.1 2.07 2.89 5.96 0.584 3.02 3.73 1.25 1.04 50.2
12 2.66 3.44 46.6 2.05 3.27 6.90 0.646 3.08 4.24 1.41 1.52 75.8
Mean 5.85 3.33 40.1 2.26 3.18 6.39 0.73 3.44 4.19 1.45 1.21 72.1
Wuqing 13 7.00 3.27 39.7 2.55 4.25 10.8 0.771 4.35 5.18 1.59 1.85 81.4
14 6.65 3.15 73.4 2.50 3.96 10.8 0.792 4.17 5.04 1.54 2.15 114
15 6.09 2.96 34.9 2.30 2.98 6.15 0.676 3.47 3.73 1.47 1.06 65.7
16 6.50 3.00 27.7 2.09 3.22 8.01 0.633 3.47 4.50 1.32 1.38 61.8
17 11.1 2.82 52.6 3.05 3.99 6.99 1.10 5.10 4.54 1.89 1.14 94.3
18 9.31 2.88 36.2 2.47 3.34 6.46 0.661 3.96 4.27 1.37 0.89 72.1
19 7.05 2.82 50.1 2.45 3.35 8.55 0.926 4.28 4.49 1.57 1.23 86.8
20 6.86 2.95 16.3 2.28 2.99 4.60 0.712 3.53 3.69 1.37 0.89 46.2
21 13.6 2.66 67.6 2.88 4.63 9.15 0.985 5.55 4.91 1.72 1.41 115
22 10.9 2.70 68.8 2.80 3.93 8.13 0.995 4.57 5.24 1.61 1.44 111
23 10.4 2.79 62.2 2.50 2.93 5.79 0.885 4.07 4.00 1.58 0.96 98.0
24 10.5 2.78 44.4 2.51 3.26 7.13 0.987 4.07 4.94 1.56 1.24 83.4
25 10.9 2.80 30.4 2.35 3.17 5.37 0.915 3.75 4.15 1.50 0.98 66.3
26 13.3 2.71 96.8 2.69 3.58 7.97 1.03 4.73 5.36 1.63 1.56 141
Mean 9.30 2.88 50.1 2.53 3.54 7.57 0.86 4.22 4.57 1.56 1.31 88.4
Population mean 7.88 3.07 54.3 2.35 3.40 8.16 0.77 3.88 4.52 1.47 1.52 89.2

4. Conclusions

In this paper, ICP-MS has been used to determine the contents of 11 kinds of heavy metals in riverside sediments of the Grand Canal (Tongzhou of Beijing, Xianghe of Hebei, and Wuqing of Tianjin). The potential ecological risk factor of individual metal and the potential ecological risk factor of multiple metals are used to evaluate the ecological risk of heavy metals. The conclusions can be drawn as follows: 1) The contents of many heavy metals in the shallow sediment are higher than the background value of the soil, indicating that the historical discharge of heavy metals in the study regions still has some adverse effects on river sediments. 2) 34.6% of the sampling points shows medium potential ecological risk of heavy metals, and the main risk source is Cd, which should be the priority control heavy metal element; 3) According to the concentration distribution of heavy metals of 26 sampling points, it can be seen that the content of heavy metals in the shallow sediment of the starting point and the ending point of the reach in each administrative region is relatively high. Thus, we should focus on the intersection of different administrative areas in the future work of dredging the river.

Acknowledgments

This work was financially supported by Beijing Natural Science Foundation (2182027) and High Level Scientific Research Project Cultivation Fund of Beijing Wuzi University (053100917).

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