Assessment of heavy metal pollution in surface sediments of the Chishui River Basin, China

Accumulated heavy metals in surface sediments are released into the aquatic environment, causing secondary contamination of the hydrosphere, and increasing the risks to human health. To evaluate the pollution characteristics of heavy metals in the sediments of the Chishui River Basin, in the present study, the concentrations of five heavy metals in surface sediments of the Chishui River Basin in China were investigated using the geo-accumulation index, pollution load index, and potential ecological risk indexes. These indexes evaluated the degree of contamination and the influence of human activities on heavy metal levels in the basin. Cu, Zn, Cd, Hg, and As were found at concentrations of 5.12–120.40, 36.01–219.31, 0.03–1.28, 0.01–1.18, and 1.56–11.59 mg kg–1, respectively, with mean values of 37.43, 91.92, 0.25, 0.07, and 5.16 mg kg–1, respectively, in the order Zn > Cu > As > Cd > Hg. The contamination indices revealed Hg as the principal pollutant based on the spatial distribution, while Pearson’s correlation coefficients suggested that Cu, Zn, and As originated from a similar source. Hg had a different source from the other metals, whereas Cd originated from a different source compared with that of Zn, As, and Hg. This paper showed a Hg and Cd contamination in the Chishui River Basin.


Unfunded studies
Enter: The author(s) received no specific funding for this work.

Introduction
Contaminants containing high concentrations of heavy metals continue to be However, heavy metals in sediments can be released and discharged into aquatic systems via changes in water conditions, such as the hydrodynamics, temperature, and pH, causing secondary pollution (Zoumis et al., 2001). Therefore, the principal objective of this study was to determine the concentrations of five heavy metals, i.e., Cu, Zn, Cd, Hg, and As, in the surface sediments in the Chishui River, China. The extent of pollution due to these metals was characterized using the geo-accumulation, pollution load (PLI), and potential ecological risk (RI) indexes. The findings of this study may be useful for future investigations on heavy metals in river ecosystems, heavy metal pollution management, and policy formulation.

Sediment sample collection
Samples were collected during a dry period in December 2012. At each site, sediment layers from the top 0-10 cm were collected from various points, which were mixed to produce a composite sample. Sediment samples were collected from 32 stations ( Fig. 1) throughout the Chishui River Basin. The samples were sealed in plastic bags, stored at 4 ℃, and transported to the laboratory for heavy metal analyses. In the laboratory, the sediments were spread on plastic films; stones, branches, and other plant materials were removed. The samples were then stored under dry conditions at room temperature. After gently rolling using a wooden stick according to the four-diagonal method (Luo et al., 2014), samples smaller than 200-mesh were collected and stored in polyethylene bags for testing.

Heavy metal analyses
All analyses were performed at the Key Laboratory of Karst Environment and Geohazard Prevention, Ministry of Education, Guizhou University. An HNO3-HF mixture was added to approximately 0.05 g of sediment in a Teflon vessel, and the mixture was subjected to digestion at 140 °C on a hot plate. The sample was then removed from the acid mixture after it appeared white or light-colored. Cu, Zn, and Cd concentrations in the sediments were determined using flame atomic absorption spectrometry (AAS; Contr AA 700, Germany) while Hg and As concentrations were measured via cold vapor AAS using the digested sample (0.3 g; GB/T 22105.1-2008).
To ensure accuracy and precision of the measurements, stream sediment (GBW07309) and soil (GBW07401) standards were used for quality control. Samples were analyzed in triplicate, and the relative standard deviations were < 5 %. Ultrapure water was used for sample preparation for all tests, and all reagents were of guaranteed quality.
The SPSS Statistics software (version 25.0, IBM) was used for analyzing the correlation matrix of the heavy metals present in the surface sediments. Microsoft Excel (version 2019) was used for statistical analysis of the test data; a p-value of < 0.05 was taken as significant.

Geo-accumulation index
Igeo is an effective parameter for assessing heavy metal pollution levels in sediments.
It can be obtained using the following equation: where Ci represents the concentration of the heavy metal n and Bi represents the background level (mg kg -1 ). A factor of 1.5 was used for lithological variations in the background value, based on previously reported values for shales (Muller, 1969). The Igeo classes established by Muller (1969) for heavy metal pollution are presented in Table 1. It reflects the changing trends in heavy metal pollution in time and space. The PLI can be calculated using the followed equations: PLI site = √CF 1 × CF 2 ×······× CF n n , and (3) where CF i represents a pollution coefficient, C i represents the measured concentration of a metal in the sediments, B i represents the background value of a heavy metal, n represents the number of heavy metals investigated, and m represents the number of sampling sites. Table 2 lists the classes of the PLI and their corresponding contamination levels. The potential ecological risk index (RI) is used to assess the level of heavy metal pollution in sedimentary environments (Hakanson, 1980). This is a widely utilized advanced index, which investigates the heavy metal content, the ecological effect of heavy metals, environmental benefits, and toxicology. The potential ecological risk factor, , can be calculated as follows: where represents the toxic-response factor of a given substance, represents the concentration of metal i in the sediments, and denotes the background value of metal i. According to the findings of Hakanson (1980), the toxic response factors for Zn, Cu, As, Cd, and Hg, are 1, 5, 10, 30, and 40, respectively.
The RI is calculated by summing the values, as follows: Background values (BV) for Hg, Cd, As, Cu, and Zn are 0.034, 0.15,7.6, 21.5, and 73.6 mg kg -1 , respectively (Alan, 1993; Wedepohl and Merian, 1991). Table 3 lists the derived five categories of and four classes of RI. Table 3 Indexes of the potential ecological risk and grades.

Descriptive statistics for heavy metals
Concentration levels in sediments showed a variation with the distance from the start of the river (Fig.2). The heavy metal concentration level of upstream is higher than downstream, it indicated that there was no large-scale heavy metal pollution in downstream. Further, a point source pollution may appealed in tributaries as Tongzi River (TZ-1) was affected by As Pollution and Gulin River (GL-1) was polluted by Cd.  Table 5 varied from 43 % for As to 280.37 % for Hg, producing the following sequence: Hg > Cd > Cu > Zn > As. These findings highlight the higher spatial variations for Hg, Cd, and Cu relative to Zn and As.
In this study, the Cu concentration in 65.63 % of the samples exceeded the BV.
Overall, samples obtained from the upstream region showed higher Cu concentrations than those obtained from the middle and lower reaches of the Chishui River. The maximum Cu concentration (sample 7) was 5.6-fold higher than the BV while the minimum concentration (sample 19) was 24 % of the BV. This is because site 7 is in Weixin County, where industrial effluents and other pollutants from human activities are common. Similarly, the maximum Zn concentration measured in sample 12 was approximately three-fold higher than the BV while the minimum value obtained from XS-1 was 49 % of the BV. The concentrations of As in all samples were lower than those of Cu and Zn, but were higher than those of Cd and Hg. The highest As concentration, obtained from TZ-1, was almost 1.5-fold higher than the BV while Cd associated with GL-1 was 8.53-fold greater than the BV. The high Cd levels in the GL-1 samples indicate severe Cd contamination in the Gulin River: the mining industry in this area discharges industrial wastewater into this river, and soils in the area are likely contaminated with Cd. The highest Hg concentration was found in sample 24 collected from Hejiang County, which is located at a site where the Chishui River flows into the Yangtze River; its value was 39.33-fold greater than that of the BV. We note that Hg   Additionally, sediments from the Chishui River exhibited lower Cd and higher As concentrations compared to those from the Thamirabarani River (India) (Ulaganathan et al., 2020). Furthermore, the Cu and Zn concentrations of the sediments from the Chishui River were 4.6-and 3.4-fold higher than those of sediments from the Hunza River, respectively (Pakistan) (Kashif et al., 2020). These differences in the concentrations of heavy metals between the sediments in this study and those from global rivers can be attributed to the sampling sites, levels of contamination, regional characteristics, and anthropogenic activities (Fabio et al., 2021).  Table 6 presents the results of the Igeo assessment, which highlights the extent of 5 pollution associated with various metals. The Igeo values for the elements ranged from 6 -2.66 to 1.90 for Cu (mean = -0.23), -1.61 to 0.99 for Zn (mean = -0.43), -3.11 to 2.51 7 for Cd (mean = -0.28), -3.24 to 4.54 for Hg (mean = -0.5), and -2.86 to 0.02 for As 8 (mean = -1.28). The negative mean Igeo values for all elements indicate unpolluted areas. 9 The Igeo value for Hg from sampling site 24 is an outlier, whereas 18.75 and 9 % of the 10 sampling sites showed Cu and Cd accumulation, respectively. Moreover, upstream of 11 the Chishui River, all heavy metals, except for As, were characterized by accumulation. 12 The Igeo values reported in the present study for Cu, Zn, and Cd were higher than those 13 reported by recent studies on similar sediments in other areas of China (e.g., Ren et al., 14 2021). The accumulation of the examined elements had the following order: Cu > Cd > 15 Zn > Hg > As. 16 The PLI values of the 32 samples from the Chishui River Basin are shown in Fig.3. 19 The values ranged from 0.31-2.47, with a median value of 1.09. According to these 20 values, 9.37 % of the samples were considerably contaminated, 43.75 % were 21 moderately contaminated, and 46.88 % were uncontaminated. The considerably 22 contaminated samples were concentrated upstream of the Chishui River (samples 1, 3, 23 and 7); among these, sample 3 showed the maximum PLI (Fig. 3) The Er(Ecological Risk) and RI values for Cu, Zn, Cd, Hg, and As in the sediments 34 from the Chishui River Basin are presented in Table 7 and Fig. 4 Fig. 4). The RI value for the 14 sites in the GL-1 section was > 300, which 52 represents a considerable ecological risk; six of the 32 RI values were 150 ≤ RI ≤ 300, 53 denoting moderate ecological risk; and 24 of the 32 RI values were < 150, implying a 54 low ecological risk. Hg contributes significantly to the RI values because of its high 55 toxicity and point source pollution, such as at site 24. 56

Heavy metal source apportionment 66
The Pearson correlation matrix is useful for determining the sources and pathways 67 of contaminants in river surface sediments (Bartoli et al., 2012). A correlation matrix 68 for the elements studied is presented in Table 8. The confidence levels between Cu, Zn, 69 and As were up to 99 %, which suggests similar pollution sources for these heavy 70 metals. The weak positive correlations between Cd and the other heavy metals (except 71 Cu) indicates that Cd is likely associated with contaminant sources different from those 72 of Hg, Zn, and As. Moreover, the weak negative correlations between Hg and the other 73 metals (excluding As) imply no relationships among these metals (Song et al., 2011). 74 75

82
The analysis of heavy metals (As, Cd, Cu, Hg, and Zn) in 32 sediment samples from 83 the Chishui River Basin in the present study produced mean concentrations with the 84 following order: Zn > Cu > As > Cd > Hg. The mean concentrations of Cu, Zn, Cd, and 85 Hg exceeded the corresponding BVs, whereas the concentration of As was lower than 86 the BV. According to the Igeo values, 18.75 and 9 % of the sampled sites displayed Cu 87 and Cd accumulation, respectively. The values for the five heavy metals followed 88 the order Hg > Cd > Cu > As > Zn, with 24 samples considered low Er, six being 89 moderate Er, one being considerable Er, and one being very high Er. Hg contributed 90 significantly to the RI values because of its higher toxicity. Overall, the Chishui River 91 Basin is characterized by moderate contamination. According to a Pearson correlation 92 matrix, sediment Cu, Zn, and As likely originate from similar contaminant sources. This 93 study provides a reference for the formulation of policies in Guizhou. As the water 94 source for Guizhou's wine industry, Chishui River is slightly polluted. In addition, Hg 95 and Cd pollution in the Chishui River should be considered a serious problem. 96