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SUN Zhigao, LI Jing, TIAN Liping, CEHN Bingbing, HU Xingyun, 2021. Spatial Variation and Risk Assessment of Arsenic and Heavy Metals in Surface Water and Suspended Particulate Matter in Tail Reaches of the Yellow River, China. Chinese Geographical Science, 31(1): 181−196 doi:  10.1007/s11769-021-1182-z
Citation: SUN Zhigao, LI Jing, TIAN Liping, CEHN Bingbing, HU Xingyun, 2021. Spatial Variation and Risk Assessment of Arsenic and Heavy Metals in Surface Water and Suspended Particulate Matter in Tail Reaches of the Yellow River, China. Chinese Geographical Science, 31(1): 181−196 doi:  10.1007/s11769-021-1182-z

Spatial Variation and Risk Assessment of Arsenic and Heavy Metals in Surface Water and Suspended Particulate Matter in Tail Reaches of the Yellow River, China

doi: 10.1007/s11769-021-1182-z
Funds:  Under the auspices of National Natural Science Foundation of China (No. 41971128, 41371104), the Award Program for Min River Scholar in Fujian Province (No. Min [2015]31)
More Information
  • Corresponding author: SUN Zhigao. E-mail: zhigaosun@163.com; LI Jing. E-mail: lijing_92@126.com
  • Received Date: 2020-02-21
    Available Online: 2021-12-31
  • Publish Date: 2021-01-05
  • To determine the pollution levels and potential toxic risks of arsenic (As) and heavy metals (Cr, Ni, Cu, Zn, Pb and Cd) in water and suspended particulate matter (SPM) in tail reaches (including freshwater reach and low-salinity reach) of the Yellow River as the Flow-Sediment Regulation Project (FSRP) has been carried out for approximately 15 yr, the surface water and SPM were sampled at pre-flood (April) and post-flood seasons (October). Results showed that similar changes of As and metal levels in water and SPM were observed along the tail reaches at pre-flood or post-flood season. Compared to pre-flood season, the levels of As, Cu, Cr and Ni in freshwater reach and the concentrations of Cr and Ni in low-salinity reach rose greatly at post-flood season. The levels of As and metals in SPM of freshwater reach or low-salinity reach at pre-flood season were significantly higher than those at post-flood season (P < 0.01). The pollutions of As and metals in surface water of tail reaches at pre-flood or post-flood season were not serious. The SPM in freshwater reach at pre-flood season were polluted by Cd, As, Cr, Cu and Ni while those in low-salinity reach were polluted by Cd and Cr. The SPM in freshwater reach at post-flood season were polluted by Cd and Pb while those in low-salinity reach were polluted by Cd and Cr. Cd was identified as heavy metal of primary concern at both pre-flood and post-flood seasons. Combined with the existed data reported in present research, this study found that the toxic risk of As and metals in SPM of tail reaches at pre-flood season was higher than that at post-flood season, implying that the implementation of FSRP during flooding season, to a great extent, reduced the toxic risk of these elements. With the long-term implementation of FSRP, the pollution levels of As and metals (particularly for Cd) in SPM of tail reaches might be elevated and the potential toxic risk primarily produced by Cr, Ni and As might be increased if effective measures were not taken in future.
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Spatial Variation and Risk Assessment of Arsenic and Heavy Metals in Surface Water and Suspended Particulate Matter in Tail Reaches of the Yellow River, China

doi: 10.1007/s11769-021-1182-z
Funds:  Under the auspices of National Natural Science Foundation of China (No. 41971128, 41371104), the Award Program for Min River Scholar in Fujian Province (No. Min [2015]31)

Abstract: To determine the pollution levels and potential toxic risks of arsenic (As) and heavy metals (Cr, Ni, Cu, Zn, Pb and Cd) in water and suspended particulate matter (SPM) in tail reaches (including freshwater reach and low-salinity reach) of the Yellow River as the Flow-Sediment Regulation Project (FSRP) has been carried out for approximately 15 yr, the surface water and SPM were sampled at pre-flood (April) and post-flood seasons (October). Results showed that similar changes of As and metal levels in water and SPM were observed along the tail reaches at pre-flood or post-flood season. Compared to pre-flood season, the levels of As, Cu, Cr and Ni in freshwater reach and the concentrations of Cr and Ni in low-salinity reach rose greatly at post-flood season. The levels of As and metals in SPM of freshwater reach or low-salinity reach at pre-flood season were significantly higher than those at post-flood season (P < 0.01). The pollutions of As and metals in surface water of tail reaches at pre-flood or post-flood season were not serious. The SPM in freshwater reach at pre-flood season were polluted by Cd, As, Cr, Cu and Ni while those in low-salinity reach were polluted by Cd and Cr. The SPM in freshwater reach at post-flood season were polluted by Cd and Pb while those in low-salinity reach were polluted by Cd and Cr. Cd was identified as heavy metal of primary concern at both pre-flood and post-flood seasons. Combined with the existed data reported in present research, this study found that the toxic risk of As and metals in SPM of tail reaches at pre-flood season was higher than that at post-flood season, implying that the implementation of FSRP during flooding season, to a great extent, reduced the toxic risk of these elements. With the long-term implementation of FSRP, the pollution levels of As and metals (particularly for Cd) in SPM of tail reaches might be elevated and the potential toxic risk primarily produced by Cr, Ni and As might be increased if effective measures were not taken in future.

SUN Zhigao, LI Jing, TIAN Liping, CEHN Bingbing, HU Xingyun, 2021. Spatial Variation and Risk Assessment of Arsenic and Heavy Metals in Surface Water and Suspended Particulate Matter in Tail Reaches of the Yellow River, China. Chinese Geographical Science, 31(1): 181−196 doi:  10.1007/s11769-021-1182-z
Citation: SUN Zhigao, LI Jing, TIAN Liping, CEHN Bingbing, HU Xingyun, 2021. Spatial Variation and Risk Assessment of Arsenic and Heavy Metals in Surface Water and Suspended Particulate Matter in Tail Reaches of the Yellow River, China. Chinese Geographical Science, 31(1): 181−196 doi:  10.1007/s11769-021-1182-z
    • Heavy metals are critical pollutants in rivers and can cause a variety of environmental problems due to their toxicity, persistence and bio-accumulation. Over the past 30 yr, large amounts of pollutants (e.g., arsenic (As) and heavy metals) were imported into rivers through runoff and land-based point sources (Duarte et al., 2010), which produced great influences on the habitat of flora and fauna in aquatic ecosystem (Xie et al., 2014) and might have significantly toxic effects on aquatic organisms or human through food chains (Guan et al., 2016).

      Heavy metals in rivers are mainly transported by suspended particles (Zhou, 2016), and, during transportation, numerous variations occur due to their dissolution, precipitation and sorption in natural media (Mil-Homens et al., 2006). In tail reaches of river or estuary, the material-energy exchanges between continental river water and marine salt water generally occur and the mixing of freshwater and seawater usually leads to flocculation and accumulation processes of As and metals (Moran et al., 1996). As and metals discharged into rivers are mainly distributed in overlying water, suspended particle and sediment, which are greatly affected by the changes of physico-chemical conditions such as hydrodynamic force, salinity, pH and redox (Calmano et al., 1993; Bai et al., 2012).

      The Yellow River is well known as a sediment-laden river in the world. In recent years, the water quality in some reaches has been severely polluted and the safety of water quality has been seriously threatened by loading excessive pollutants (Zhang et al., 2013). According to the statistical data , the effluent discharge of the Yellow River basin reached 44.01 × 108 t in 2015 and the contributions of city resident living, secondary industry and tertiary industry were 31.50%, 57.70% and 10.08%, respectively (Yellow River Conservancy Commission of the Ministry of Water Resources, 2015). As jointly affected by the input of water and sand from the upper and middle reaches of the Yellow River and the rapid development of industry and agriculture in the Yellow River basin, large amounts of pollutants in the upper and middle reaches are discharged into estuary and Bohai Sea by lower reaches, which may significantly affect the spatial variations and toxic risks of pollutants in lower reaches and estuarine ecosystem (Bai et al., 2015). In the past decade, although considerable efforts have been conducted in investigating the pollutants such as As and metals, mercury, organochlorine pesticides, polychlorinated biphenyls, polybrominated diphenyl ethers and brominated flame retardants in different reaches of the Yellow River (Sun et al., 2005; Xiao et al., 2009; Bai and Bao, 2014; Ma et al., 2015a; Pei et al., 2018; Tian et al., 2018), most studies focus on the distributions of these pollutants in sediments of the upper and middle reaches, while information on pollutants in water and suspended particle in the lower reaches (especially tail reaches) is very limited.

      During 1997–2002, the annual runoff of the Yellow River was below 10.0 billion m3 and the break-off discharge frequently occurred which reached 226 d in 1997 (Cui et al., 2009). To scour the sediment in the Xiaolangdi Reservoir and in the riverbed of lower reaches, the ‘Flow-Sediment Regulation Project’ (FSRP) was implemented by China since 2002 through regulating the water and sand processes of the Yellow River (Bai et al., 2012). The FSRP is generally conducted from June to July at each year, and, till to 2016, approximately 18 times of flow-sediment-regulation have been implemented. Long-term implementation of the FSRP not only greatly changed the hydrodynamic environment of lower reaches, but also significantly influenced the migration and transformation processes of pollutants in water, suspended particle and sediment. Although many studies have been conducted in the lower reaches of the Yellow River to explore the influences of FSRP on sediment delivery and hydrological process (Wei et al., 2016; Wang et al., 2015; 2017) and spatial variation of pollutants (especially heavy metals) in sediment (Wu et al., 2013; Zhang et al., 2015; Tian et al., 2018), information on changes of As and metals in water and suspended particle is poorly documented as the FSRP has been carried out for 15 yr. Even if at the same year, the variations of As and metals in water and suspended particle along the lower reaches might be different between pre-flood and post-flood seasons. However, study on pollution levels and toxic risks of As and metals in water and suspended particle of the lower reaches (including tail reaches) before and after flooding season is still lacking.

      In this study, the surface water and suspended particle in tail reaches of the Yellow River were sampled at pre-flood and post-flood seasons in 2016, and the concentrations of As and metals in water and suspended particle were determined. The primary objectives of this paper were: 1) to determine the spatial variations of As and metals in water and suspended particle in tail reaches before and after flooding season, 2) to identify the key factors influencing the spatial variations of As and metals in water and suspended particle, and 3) to investigate the contamination levels and the potential toxic risks of As and metals in water and suspended particle. Results of this study was favorable for illustrating the effects of FSRP on ecological risks of As and metals in water and suspended particle in tail reaches of the Yellow River.

    • This study was carried out in tail reaches of the Yellow River, which was located in Dongying City, Shandong Province (Fig. 1). The tail reaches generally start from Lijin hydrological station and end in the boundary between river and sea, which approximates 104 km (Wang et al., 2015). In present studies, the reach from Xintan Floating Bridge to the demarcation line of river and sea was usually regarded as low-salinity area, in which the salinity varied from 0.45‰ to 25.00‰ in a seaward direction (Liu et al., 2008; Guo et al., 2015).

      Figure 1.  Study region and sampling stations in tail reaches of the Yellow River

    • Surface water and suspended particulate matter (SPM) samples were collected in 24 stations (denoted by S1, S2, S3, ..., S24; S1−S19 located in freshwater reach; and S20−S24 located in low-salinity reach) along the tail reaches of the Yellow River (Fig. 1) during two sampling campaigns in April 16−18 and October 10–13 in 2016, representing pre-flood and post-flood seasons, respectively. Water samples were collected using pre-cleaned water sampler (polyethylene bucket). At each station, 250 mL surface water samples were filtered in situ through 0.45 μm cellulose-acetate filter (Whatman), and the filtrate water was stored in an isothermal freezer at 4 ℃ after adding 2.5 mL of 65% concentrated HNO3 (HJ 493−2009 Standard of China). The pH and electrical conductivity (EC) of water were determined in situ by portable pH meter (HACH-PHW37-SS, USA) and EC meter (ECTestr11+ Multi Range), respectively. The SPM was intercepted after filtering with a weighed 0.45 μm cellulose-acetate filter paper and then was treated with freeze-drying, grinding and sieving to preserve drily.

    • A portion of 100 mL filtrated water was used to measure the concentrations of As and metals (Cr, Ni, Cu, Zn, Pb and Cd) by ICP-MS (X Series II, Thermo Fisher Company, USA). A 0.040 g homogenized SPM sample (sieved through a 200-mesh nylon sieve) was digested with 1.5 mL HNO3, 0.25 mL HClO4, and 1.5 mL HF at 150℃ for 12 h and the residue was diluted to 10 mL with deionized water. All the pretreated solutions were stored at 4℃ until measurement. The levels of As and metals in all samples were determined by ICP-MS. Quality assurance and quality control were assessed using duplicates, method blanks and standard reference materials (GSBZ 5009−88 and GSS-1) from the National Research Center for Standards in China with each batch of samples (two blank and one standard for each ten samples). The recoveries of samples spiked with standards ranged from 83.2% to 111.2%.

    • The pollution evaluation criterions of As and metals in surface water of S20–S24 stations (at low-salinity reach) adopted the Class I Criteria of Seawater Quality in China (GB 3097−1997) issued by National Standard of the People’s Republic of China (NSPRC) in 1997 (As, 20 μg /L; Cr, 50 μg/L; Ni, 5 μg/L; Cu, 5 μg/L; Zn, 20 μg/L; Pb, 1 μg/L; and Cd, 1μg/L) (State Environmental Protection Administration, 2004), whereas the values in other stations (at freshwater reach) adopted the Class I Criteria of Environmental Quality of Surface Water in China (GB 3838−2002) issued by NSPRC in 2002 (As, 50 μg/L; Cr, 10 μg/L; Ni, 20 μg/L (limit value for drinking water source); Cu, 10 μg/L; Zn, 50 μg/L; Pb, 10 μg/L; and Cd, 1 μg/L) (State Environmental Protection Administration and General Administration of Quality Supervision, Inspection and Quarantine of the People’s Republic of China, 2002). The index of geoacumulation (Igeo) was used to assess the pollution level of As and metals in SPM. The calculation and classification of Igeo referred to Ma et al. (2015b) and Rao et al. (2018).

      The toxic units (TUs) were used to normalize the toxicities of different metals to allow for the comparison of the relative effects, defined as the ratios of the determined levels to PEL (probable effect level) values (Pedersen et al., 1998). The PEL values at S20−S24 stations adopted the guideline values for marine and estuarine ecosystem (MacDonald et al., 1996), whereas the values at other stations adopted the guideline values for freshwater ecosystem (MacDonald et al., 2000). The potential acute toxicity of the As and metals in a sample could be estimated as the sum of the toxic units (ΣTUs).

    • Data analysis was performed using SPSS Version 11.0 Statistical Software Package (SPSS Inc., Chicago, USA). The analysis of variance (ANOVA) tests was used to determine if pH (EC or SPM) in surface water and As and metal levels in water (or SPM) differed significantly between pre-flood and post-flood seasons. Pearson correlation analysis was conducted to determine the relationships between As and metal levels and environmental variables. In all tests, differences were considered significantly only if P < 0.05.

    • The pH in surface water at pre-flood season showed great fluctuations along the tail reaches. Thereinto, the values in low-salinity reach presented a decreasing trend. By comparison, the pH in surface water at post-flood season demonstrated little fluctuations along the tail reaches (Fig. 2a). The EC in surface water at pre-flood or post-flood season generally showed an increasing trend in a seaward direction and the values at pre-flood season were significantly higher than those at post-flood season (P < 0.01) (Fig. 2b). Both the concentrations of SPM in surface water at pre-flood and post-flood seasons fluctuated greatly along the tail reaches. With a few exceptions, the values at pre-flood season were significantly higher than those at post-flood season (P < 0.01) (Fig. 2c).

      Figure 2.  Spatial variations of pH (a), EC (b) and suspended particulate matter (SPM) (c) in surface water in tail-reaches of the Yellow River. Different letters indicate significant differences at the level of P < 0.05

    • Except for Zn, similar variations of As, Cr, Cu, Ni, Cd and Pb levels were observed in surface water of freshwater reach at pre-flood or post-flood season (Fig. 3). The levels of As and metals at pre-flood season increased greatly at the demarcation line of freshwater reach and low-salinity reach (S20), after which the values fluctuated significantly and the maximums of As, Ni, Zn, Cu, Pb and Cr occurred at S22 station (Fig. 3a). At post-flood season, the levels of Zn and Ni in the demarcation line increased significantly while those of As, Pb, Cr and Cu decreased greatly, after which the values of As and six metals showed similar variations (Fig. 3b). The mean levels of As, Cr, Cu, Ni, Pb and Zn in freshwater reach at pre-flood season were lower than those in low-salinity reach, but only the value of Ni at post-flood season was lower than that in low-salinity reach (Appendix Table 1). Compared to pre-flood season, the levels of As, Cu, Cr and Ni in freshwater reach and the concentrations of Cr and Ni in low-salinity reach increased greatly at post-flood season. Except for Cu, fluctuations of As, Cr, Ni, Zn, Pb and Cd levels at pre-flood season were more obvious than those at post-flood season (Fig. 3, Appendix Table 1). Only the levels of Cr, Ni, Zn and Pb between pre-flood and post-flood seasons showed significant differences (P < 0.01).

      Figure 3.  Spatial variations of As and metals in surface water in tail-reaches of the Yellow River at pre-flood (a) and post-flood (b) seasons

    • Similar changes of As and metal concentrations in SPM were observed along the tail reaches at pre-flood or post-flood season (Fig. 4). For the levels of As and metals in SPM along the tail reaches, significant differences were observed between pre-flood and pos-flood seasons (P < 0.01). The maximums and minimums of As and metal levels at pre-flood season generally occurred in S1 and S9 stations (Fig. 4a), while those at post-flood season were observed in S18 and S23 stations, respectively (Fig. 4b). The mean levels of As, Cd, Cu, Ni, Pb and Zn in SPM of freshwater reach at pre-flood season were lower than those of low-salinity reach, but the values of As and six metals at post-flood season were higher than those of low-salinity reach. The concentrations of As and six metals in SPM of freshwater reach or low-salinity reach at pre-flood season were much higher than those at post-flood season (Suppl. Table 2).

      Figure 4.  Spatial variations of As and metals in suspended particulate matter (SPM) in tail-reaches of the Yellow River at pre-flood (a) and post-flood (b) seasons

    • At pre-flood season, the Igeo values (0 < Igeo $ \le$1) for Cd, Pb, Zn, Ni, Cr, Cu and As in SPM of freshwater reach accounted for 5%, 35%. 45%. 50%, 60%, 60% and 65% of the corresponding Igeo values, respectively. Approximately 5% of Igeo values for Cr and 65% of Igeo values for Cd varied from 1 to 2 and about 30% of Igeo values for Cd exceeded 2. For the low-salinity reach, 60% of Igeo values for Pb and 20% of Igeo values for Cr, Ni, Cu, Zn and As varied from 0 to 1. All Igeo values for Cd were larger than 1 and about 60% of Igeovalues exceeded 2 (Fig. 5a). At post-flood season, 5% of Igeo values for Ni and Cu, 10% of Igeo values for Zn and Cd, and 15% of Igeo values for Cr in SPM of freshwater reach varied from 0 to 1, and more than 90% of Igeo values for Cd were larger than 1. For the low-salinity reach, only 20% of Igeo values for Cr varied from 0 to 1, and approximately 80% of Igeo values for Cd exceeded 1 (Fig. 5b).

      Figure 5.  Spatial variations of the geoaccumulation indices (Igeo) for As and metals in suspended particulate matter (SPM) in tail-reaches of the Yellow River at pre-flood (a) and post-flood (b) seasons. Horizontal dotted lines represent the thresholds for unpolluted to moderately polluted (0 < Igeo$\le $ 1), moderately polluted (1 < Igeo $\le $2), and moderately to strongly polluted (2 < Igeo $\le $ 3)

    • At pre-flood season, 50% of stations for ΣTUs in freshwater reach exceed 4 (only S1 station exceeded 6), whereas the values at all stations in low-salinity reach were less than 4 (Fig. 6a). At post-flood season, except for the ΣTUs at S18 station that varied from 4 to 6, the values at other stations in freshwater reach and at all stations in low-salinity reach were less than 4 (Fig. 6b). Over all sampling stations, Cr, Ni and As in SPM of tail reaches at pre-flood and post-flood seasons showed great contributions to ΣTUs (Fig. 6).

      Figure 6.  Spatial variations of the toxic units (TUs) and the sum of TUs (ΣTUs) in suspended particulate matter (SPM) in tail-reaches of the Yellow River at pre-flood (a) and post-flood (b) seasons

    • This study found that the variations of pH in surface water at pre-flood season changed greatly along the tail reaches, while those at post-flood season showed little fluctuations (Fig. 2a). Previous studies have indicated that the variation of pH was mainly dependent on the buffering capacity of water, which was correlated with the balance systems of carbon dioxide (CO2) in water (e.g., gas dissolution and escape, precipitation formation and dissolution, and the reactions of acid and alkali substances) (Dai, 2006; Zhang and Zhang, 2007). At pre-flood season, the great fluctuations of pH in surface water might be related to the weak hydrodynamic condition of tail reaches. As shown in Fig. 7, the runoff and sediment loading of the tail reaches at pre-flood season (April) were very low, implying that the transportation of nutrients and pollutants might be unsmooth, which might cause the water quality in some sampling stations (e.g. S2, S5−S9 and S17) to be deteriorated. The increased nutrients in water might provide abundant material basis for the thriving of hydrophyte (particularly phytoplankton), which induced the CO2 dissolved in water to be greatly consumed (Zhang and Zhang, 2007), resulting in the elevation of pH. The lower pH in S15 and S16 stations at pre-flood season might be related to the longer retention time of organic matter and its sufficient degradation process since the gradient ratios of the two stations declined slightly (Li et al., 1991). Due to the sufficient degradation of organic matter, mass CO2 were generated and dissolved in water (Guo et al., 2015), resulting in the decline of pH. At flood season, large amounts of freshwater and sediment were discharged into estuary (Fig. 7) due to the heavy rainfall in the Yellow River basin and the implementation of FSRP. Simultaneously, the original status of hydrodynamic condition along tail reaches was broken (Sun et al., 2016) and the transportation of nutrients and pollutants was smooth, which might induce the slight fluctuations of pH along the tail reaches at post-flood season (Fig. 2a). Moreover, compared to flood season, the hydrodynamic condition at post-flood season weakened (Fig. 7) and the possibility of sediment re-suspension declined (Tang, 2011), which induced the SPM in surface water along the tail reaches to be decreased greatly (Fig. 2c).

      Figure 7.  Temporal variations of runoff and sediment loading in tail-reaches of the Yellow River in 2016. Data source: Yellow River Conservancy Commission of the Ministry of Water Resources (2017)

      This study also found that the fluctuations of As and most metals in surface water at pre-flood season were more obvious than those at post-flood season (Fig. 3, Appendix Table 1). One possible reason might be related to the difference in hydrodynamic conditions of tail reaches between pre-flood and post-flood seasons. As mentioned above, the runoff and sediment loading of the tail reaches at pre-flood season were very low (Fig. 7) and the transportation of pollutants in some local reaches was unsmooth. Under such hydrological conditions, the fluctuations of pH, EC and SPM along the tail reaches were more evident at pre-flood season compared to post-flood season. Particularly, significant differences in EC and SPM occurred between pre-flood and post-flood seasons (P < 0.01) (Fig. 2). All these might induce the great fluctuations of As and most metals along the tail reaches. Although the runoff and sediment loading of the tail reaches at post-flood season (October) was slightly lower than those at pre-flood season (April) (Fig. 7), the transportation of pollutants in tail reaches might be more smooth after flood season (Liu et al., 2008), which might induce the slight fluctuations of As and most metals along the tail reaches.

      This study indicated that the levels of As, Cr, Cu, Ni, Pb and Zn in freshwater reach at pre-flood season were lower than those in low-salinity reach, but only the value of Ni in freshwater reach at post-flood season was lower than that in low-salinity reach (Appendix Table 1). The probable reason was related to the increasing of salinity (represented by EC) in a seaward direction (Fig. 2b), and this could be partly explained by Pearson correlation analyses which showed that, in freshwater reach, significantly positive correlations were observed between As (Cr or Ni) and EC (P < 0.01 or P < 0.05) at pre-flood season and between Ni and EC at post-flood season (P < 0.01) (Table 1). Previous studies have reported that the salinity in water could promote the mobility of heavy metals through complexation with salt anions and ion exchange between the cations and the metal ions (Du Laing et al., 2008; Li et al., 2011). Thus, compared to low-salinity reach, the quite lower salinity in freshwater reach might induce the lower metal concentrations. It was also reported that, in the process of river water and seawater mixing, negative correlation generally occurred between the dissolved-particulate partitioning coefficient of metals (e.g., Zn) and the salinity, which implied that the partitioning coefficient decreased with increasing salinity and the metals were apt to transfer from particulate to dissolved state (Turner, 1996). In this study, the maximums of As, Ni, Zn, Cu, Pb and Cr occurred in S22 station at pre-flood season and the higher values of Ni and Zn occurred in S20 station at post-flood season (Fig. 3), to a great extent, might be dependent on the above mechanisms. Pearson correlation analyses also indicated that, in surface water of low-salinity reach, significantly positive correlations occurred between Cd and pH at pre-flood season (P < 0.05) (Table 1). At neutral and alkaline conditions, the soluble Cd2+ (CdSO4) were more easily precipitated (Wang et al., 1981; He et al., 2011). In this study, the pH in surface water at pre-flood season generally showed a decreasing trend in low-salinity reach and the values varied from 7.10 to 7.68 (Fig. 2a), which might induce the Cd levels in surface water of low-salinity reach to be lower than those of freshwater reach (Appendix Table 1).

      PeriodsItemsIndicesReachAsCdCrCuNiPbZn
      Pre-floodseason Surface water pH F 0.06 −0.11 0.02 0.04 0.12 −0.07 −0.20
      L 0.37 0.90* 0.27 0.36 0.22 0.76 0.83
      EC F 0.49* 0.37 0.57** 0.13 0.46* 0.43 0.41
      L 0.24 −0.10 0.16 0.17 0.18 −0.18 −0.02
      SPM F 0.25 −0.05 0.19 −0.02 0.12 0.03 −0.06
      L −0.45 −0.55 −0.89 −0.46 −0.53 −0.39 −0.55
      SPM pH F 0.02 0.06 −0.04 0.05 −0.05 −0.08 −0.03
      L −0.01 0.23 −0.08 −0.14 −0.01 −0.13 −0.12
      EC F −0.21 −0.16 −0.02 −0.13 −0.18 −0.25 −0.16
      L −0.15 −0.01 −0.23 0.02 −0.28 −0.36 −0.14
      SPM F −0.13 −0.19 0.35 −0.17 0.12 −0.03 −0.04
      L 0.13 −0.16 0.23 0.06 0.26 0.33 0.15
      Post-flood season Surface water pH F −0.28 −0.37 −0.22 −0.38 0.25 −0.51* −0.08
      L 0.40 0.74 0.17 0.36 0.27 0.77 0.68
      EC F −0.15 0.11 −0.12 −0.23 0.57** −0.21 0.12
      L 0.36 0.11 0.53 0.40 −0.34 −0.05 −0.27
      SPM F 0.35 0.09 0.29 0.38 −0.18 0.34 0.11
      L 0.09 0.34 −0.17 0.04 0.29 0.70 0.42
      SPM pH F −0.15 0.05 0.24 0.18 0.05 0.20 0.21
      L 0.74 0.64 0.59 0.72 0.74 0.67 0.59
      EC F −0.035 −0.18 0.12 0.02 −0.10 −0.10 0.16
      L −0.76 −0.87 −0.97** −0.76 −0.84 −0.84 −0.08
      SPM F −0.31 −0.43 −0.15 −0.45* −0.31 −0.41 −0.30
      L 0.74 0.85 0.87 0.70 0.81 0.70 0.66
      Notes: F, Freshwater reach (n = 20); and L, Low-salinity reach (n = 5). **P < 0.01; and *P < 0.05

      Table 1.  Correlation coefficients between As or metals and pH, EC and suspended particulate matter (SPM) in tail-reaches of the Yellow River

      This study also indicated that the concentrations of As, Cd, Cu, Ni, Pb and Zn in SPM of freshwater reach at pre-flood season were lower than those of low-salinity reach (Appendix Table 2). Previous studies have shown that SPM was the main carrier of metals in water and it played an important role in metal migration and transformation (Moran et al., 1996; Zhou, 2016). The metal levels in SPM were directly affected by its substance composition, concentration and particle-size and indirectly influenced by physico-chemical conditions such as hydrodynamic force, salinity, pH and redox potential (Calmano and Hong, 1993; Bai et al., 2012). Du (2011) found that, at pre-flood season, the contents of sand, coarse silt, medium silt, fine silt and fine silty clay in SPM of Lijin hydrological station were 3.0%, 6.1%, 13.5%, 25.8% and 51.6%, respectively. Zhou (2016) reported that the contents of clay, silt and sand in SPM of the Yellow River estuary (low-salinity reach) at drought season varied from 20.2% to 29.7%, between 55.5% and 72.5%, and from 0.6% to 22.1%, respectively. Obviously, both fine silt and clay were the main composition of SPM in freshwater reach or low-salinity reach at pre-flood season. In low-salinity reach, the low flows of the Yellow River at pre-flood season caused a significant decrease in coarse particles and the increased fine particles promoted the sorption of As and metals. Moreover, as affected by the interaction of hydrodynamic forces between the Yellow River and the Bohai Sea, the increased salinity in a seaward direction (Fig. 2b) might result in the higher concentrations of As and metals in SPM through complexation with salt anions and ion exchange between the cations and the metal ions (Du Laing et al., 2008; Li et al., 2011). This study also implied that the levels of As and six metals in SPM of freshwater reach at post-flood season were higher than those of low-salinity reach (Appendix Table 2 ). Compared to pre-flood season, large volume of runoff discharge (4.07 billion m3, accounting for 40.28% of annul runoff) and higher sediment loading (0.08 × 108 t, accounting for 52.12% of annul loading) at flood season (Fig. 7) produced a significant dilution on the levels of As and metals in SPM of low-salinity reach. The above reason could also be used to explain the higher levels of As and metals in SPM at pre-flood season compared to post-flood season (Appendex Table 2). The lower levels of some metals in SPM of low-salinity reach at post-flood season might also rest with the variations of EC and SPM. Pearson correlation analyses showed that significantly negative correlation occurred between Cr and EC in low-salinity reach (P < 0.01) (Table 1). The higher EC in low-salinity reach could better explain the lower Cr levels in a seaward direction (Fig. 2b, Fig. 4b). Moreover, the lower levels of As and metals in S23 station at post-flood season (Fig. 4b) might be partly illustrated by the lower content of SPM (Fig. 2c) since positive correlations occurred between As and metals and SPM in low-salinity reach (Table 1).

    • The status of As and metal pollution in surface water of freshwater reach and low-salinity reach were assessed by comparing As and metal concentrations with the Environmental Quality of Surface Water in China (GB 3838–2002) (State Environmental Protection Administration and General Administration of Quality Supervision, Inspection and Quarantine of the People’s Republic of China, 2002) and the Seawater Quality in China (GB 3097–1997) (State Environmental Protection Administration, 2004), respectively. At pre-flood season, only the Zn level in S14 station at freshwater reach exceeded the criteria of Class I recommended by GB 3838–2002, while the Cu level in S22 station, the Zn levels in S20–S22 stations and the Pb levels in S20–S22 and S24 stations at low-salinity reach exceeded the criteria of Class I recommended by GB 3097–1997. At post-flood season, the Cr levels in S1, S5, S8–S11, S15 and S18–19 stations, the Cu level in S3 station and the Ni level in S18 station at freshwater reach exceeded the Class I criteria of GB 3838–2002, whereas the Ni levels in S21, S23 and S24 stations and the Pb levels in S21 and S24 stations at low-salinity reach exceeded the Class I criteria of GB 3097–1997. The pollution level of As and metals in SPM of freshwater reach and low-salinity reach were evaluated by comparing As and metal concentrations with the Sediment Quality Guidelines (SQGs) for coastal ecosystem in China (GB 18668−2002) (General Administration of Quality Supervision, Inspection and Quarantine of the People’s Republic of China, 2002), the mean levels of chemical elements in sediments of rivers in China (Yan et al., 1995) and the TEL (Threshold effect level) /PEL SQGs (MacDonald et al., 1996, 2000) (Table 2). According to the background values of As and metals in loess materials of the Yellow River (CNEMC, 1990), the mean levels of As and metals in SPM of freshwater reach and low-salinity reach at pre-flood or post-flood season exceeded their background values. In freshwater reach, only the average level of Pb in SPM at post-flood season was slightly lower than that in sediments of rivers in China, but, in low-salinity reach, only the mean levels of Cd, Cr, Cu and Ni at pre-flood season exceeded the Class I criteria of SQGs for coastal ecosystem in China. According to the TEL/PEL SQGs, 35% stations contained Cr, 30% stations contained Ni, 45% stations contained Cu, 10% stations contained Zn, 50% stations contained As, 20% stations contained Pb and 15% stations contained Cd in SPM of freshwater reach at pre-flood season exceeded the corresponding TEL values and approximately 65%, 14% and 50% of stations contained concentrations exceeded the PEL values for Cr, Ni and As, respectively. For low-salinity reach, 20% stations contained Ni, 80% stations contained Pb and 100% stations contained Cr, Cu and As exceeded the corresponding TEL values and about 80% stations contained levels exceeded the PEL value for Ni. At post-flood season, 85% stations contained Cr and Ni and 100% stations contained As in SPM of freshwater reach exceeded the corresponding TEL values and only 15% stations contained concentrations exceeded the PEL values for Cr and Ni. In low-salinity reach, 85% stations contained Cu and 100% stations contained Cr, Ni and As exceeded the corresponding TEL values. Overall, As and Cr were identified as contaminants of primary concern in freshwater reach and low-salinity reach at pre-flood or post-flood season. Moreover, Cu, Cd and Ni in low-salinity reach at pre-flood season, Ni in freshwater reach and Cu and Ni in low-salinity reach at post-flood season were of primary concerns.

      ItemsAsCdCrCuNiPbZnReferences
      Mean levels of As and metals in SPM of tail reaches of the Yellow River / (mg/kg)
      Freshwater reach Pre-flood season 17.42 ± 3.59 0.49 ± 0.11 108.62 ± 34.13 34.62 ± 8.13 45.26 ± 14.22 29.24 ± 5.95 92.79 ± 22.43 This study
      Post-flood season 12.79 ± 1.39 0.36 ± 0.05 82.21 ± 11.95 23.87 ± 4.01 31.79 ± 4.70 21.79 ± 2.69 74.28 ± 20.79
      Low-salinity reach Pre-flood season 18.57 ± 3.22 0.54 ± 0.10 107.35 ± 19.09 39.20 ± 7.60 49.93 ± 11.53 31.12 ± 5.23 103.72 ± 19.15
      Post-flood season 11.44 ± 2.38 0.30 ± 0.10 79.71 ± 14.87 22.16 ± 7.19 30.66 ± 6.41 21.07 ± 3.86 70.69 ± 20.40
      Background values / (mg/kg)
      Loess materials 10.7 0.095 59 21.1 27.8 21.6 64.5 CNEMC (1990)
      Sediment quality guidelines / (mg/kg)
      SQGs for freshwater ecosystem TEL 5.9 0.596 37.3 35.7 18 35 123 MacDonald et al. (2000)
      PEL 17 3.53 90 197 36 91.3 315
      SQGs for coastal ecosystem TEL 7.24 0.68 52.3 18.7 15.9 30.2 124 MacDonald et al. (1996)
      PEL 41.6 4.21 160 108 42.8 112 271
      SQGs for coastal ecosystem in China Class I 20 0.5 80 35 34a 60 150 General Administration of Quality Supervision, Inspection and Quarantine of the People’s Republic of China (2004)
      Class II 65 1.5 150 100 40a 130 350
      Mean concentrations of As and metals in fluvial deposit of China Means 9.1 0.14 38 21 24 25 68 Yan et al. (1995)
      Notes: a Sediment quality benchmark in Hong Kong SAR (Environmental Protection Department of Hong Kong, 2005); TEL, Threshold effect level, it was applied to present chemical concentrations below which adverse biological effects rarely occur; and PEL, probable effect level, it was used to present chemical concentrations above which adverse biological effects frequently occur

      Table 2.  Comparison for the concentrations of As and metals in suspended particulate matter (SPM) of tail reaches, loess materials and sediment quality guidelines (SQGs)

      Based on the analyses for Igeo values, it was found that the SPM in freshwater reach at pre-flood season were polluted by Cd, As, Cr, Cu and Ni, while those in low-salinity reach were polluted by Cd and Cr. At post-flood season, the SPM in freshwater reach were polluted by Cd and Pb, whereas those in low-salinity reach were polluted by Cd and Cr. Although the conclusion was generally in agreement with the results of pollution assessment, Cu, Ni and As pollution status at low-salinity reach might be underestimated since both the SQGs for coastal ecosystem in China and the TEL/PEL showed that the sampled stations were polluted by these elements. Moreover, Cd pollution was serious in both freshwater reach and low-salinity reach at pre-flood or post-flood season. This study also found that, in freshwater reach, 50% of stations for ΣTUs at pre-flood season and the value at S18 station at post-flood season exceed 4 (Fig. 6), implying that these stations showed high potential toxicity (Pedersen et al., 1998). The ΣTUs of all stations in low-salinity reach at pre-flood or post-flood season were less than 4 (Fig. 6), which showed no potential toxicity (Pedersen et al., 1998). The above analyses further showed that the potential toxic risk of As and metals in SPM of tail reaches at pre-flood season were much higher than that at post-flood season, indicating that the implementation of FSRP during flooding season, to a great extent, reduced the potential toxic risk caused by the combined effects of these elements in SPM. Over all sampling stations, Cr, Ni and As in SPM of tail reaches at pre-flood and post-flood seasons showed great contributions to ΣTUs (Fig. 6). It was worth noting that, along the tail reaches, Cd showed a lower toxicity contribution at pre-flood or post-flood season (Fig. 6) despite its higher pollution level on the basis of Igeo assessment (Fig. 5), indicating that the evaluation of TUs approach might underestimate its toxicity because of the higher PEL value for Cd.

      To investigate the pollution status of As and metals in surface water or SPM in tail reaches as affected by the long-term implementation of FSRP, the results of this paper and present studies were compared (Fig. 8). The existed data set for As and metal levels in surface water indicated that the levels of Cr, Cu and Pb in freshwater reach at pre-flood or post-flood season generally increased during 2009–2016. For the low-salinity reach, the Zn levels at pre-flood season significantly decreased during 2005–2010, after which the values increased slightly. The levels of Cu, Pb and As at pre-flood season decreased significantly during 2005–2009 and then increased greatly before 2010, after which the values decreased greatly again. The levels of Zn, Cu, Cr and Pb at post-flood season significantly increased during 2004–2005, after which the values decreased significantly. The Cd levels at pre-flood and post-flood seasons generally decreased during 2004–2016 (Fig. 8a). Comparison of existed data set for SPM implied that the levels of Cr, As, Pb and Cu in freshwater reach at pre-flood season generally increased during 2009–2016. By contrast, the concentrations of Zn, Ni, Cu and Pb at post-flood season decreased before the implementation of FSRP (2000–2001), after which the values of Ni, As and Zn generally increased. Only the levels of Cu, Pb and Cd at post-flood season during 2009–2016 decreased greatly. For the low-salinity reach, the levels of As and six metals at pre-flood season generally increased during 2010–2016. By comparison, the concentrations of Zn, Pb and Cu at post-flood season increased significantly during 2010–2013, after which the values decreased greatly before 2016.,The levels of Cr at post-flood season increased during 2013–2016, while those of Ni decreased at the same period. Moreover, the levels of As and Cd generally showed an increasing trend during 2010–2016 (Fig. 8b). In summary, the levels of Cr, Cu and Pb in surface water of freshwater reach at pre-flood and post-flood seasons generally showed an increasing trend, while those of As and most metals in low-salinity reach showed a decreasing trend. For the SPM in freshwater reach or low-salinity reach, the levels of As and most metals at pre-flood or post-flood seasons showed an increasing trend in recent years. Although the pollutants imported into the Yellow River estuary decreased greatly in recent years, the loading of As and metals still maintained a high level (Sun et al., 2015; Tian et al., 2018). With the long-term implementation of FSRP in future, the pollution levels of As and metals (particularly for Cd) in SPM in tail reaches might be elevated and the potential toxic risk primarily produced by Cr, Ni and As might be increased if measures were not taken to control the loading of pollutants. In addition, further studies should be strengthened to identify the potential sources of Cr, Ni and As, and this would provide scientific basis for minimizing their toxic risks on aquatic organisms.

      Figure 8.  Existed data set for As and metal levels in surface water (a) or suspended particulate matter (SPM) (b) in tail reaches of the Yellow River reported in this paper and present studies. Data sources: Huang et al. (1992), Qiao et al. (2007), Wu (2007), Liu et al. (2008), Tang et al. (2010), Tang (2011), Zhang et al. (2013) and Zhou (2016). Data in 2016 were provided by this study

    • This paper investigated the pollution levels and potential toxic risks of As and metals in surface water and SPM in tail reaches of the Yellow River. Results have demonstrated that: 1) similar variations of As and metal levels in water and SPM were observed along the tail reaches at pre-flood or post-flood season. The levels of As and metals in SPM of freshwater reach or low-salinity reach at pre-flood season were significantly higher than those at post-flood season (P <0.01). 2) the pollutions of As and metals in surface water of tail reaches at pre-flood or post-flood season were not serious. Cd was identified as heavy metal of primary concern in SPM of tail reaches at both pre-flood and post-flood seasons. And 3) the toxic risk of As and metals in SPM of tail reaches at pre-flood season was higher than that at post-flood season, implying that the implementation of FSRP during flooding season reduced the toxic risk of these elements. This study found that, with the long-term implementation of FSRP in future, the pollution levels of As and metals (particularly for Cd) in SPM of tail reaches might be elevated and the potential toxic risk primarily produced by Cr, Ni and As might be increased.

    • PeriodsElementsFreshwater reachLow-salinity reachTail reaches
      RangeMean ± S.D.CV (%)RangeMean ± S.D.CV (%)RangeMean ± S.D.CV (%)
      Pre-flood seasonAs0.80–2.791.20 ± 0.4639.170.63–3.391.53 ± 1.2078.450.63–3.391.27 ± 0.6651.87
      Cr0.51–2.021.03 ± 0.4139.280.55–10.333.02 ± 4.11136.160.51–10.331.43 ± 1.94136.87
      Ni1.22–3.752.17 ± 0.7032.311.48–13.024.39 ± 4.87110.941.22–13.022.60 ± 2.3289.23
      Cu1.56–8.583.28 ± 1.6048.831.54–10.224.04 ± 3.6891.191.54–10.223.38 ± 2.1262.78
      Zn6.96–62.1822.85 ± 14.5863.8116.85–40.3029.61 ± 11.5038.846.96–62.1823.62 ± 14.0659.53
      Cd0.03–0.930.10 ± 0.20188.730.02–0.120.08 ± 0.0561.740.02–0.930.10 ± 0.18182.11
      Pb0.96–9.132.27 ± 1.7074.990.90–4.462.35 ± 1.4662.270.90–9.132.28 ± 1.6672.69
      Post-flood seasonAs0.71–2.361.63 ± 0.5332.510.75–2.051.43 ± 0.5538.420.71–2.361.62 ± 0.5131.34
      Cr4.15–11.988.66 ± 2.7331.564.15–9.657.69 ± 2.4331.664.15–11.988.64 ± 2.5529.53
      Ni3.19–31.888.15 ± 7.1087.164.18–31.8811.15 ± 11.65104.493.19–31.887.78 ± 6.5283.83
      Cu1.51–19.794.05 ± 3.8595.001.58–4.253.01 ± 1.1337.511.51–19.793.94 ± 3.5389.50
      Zn6.56–23.2113.58 ± 4.8335.587.91–16.0011.46 ± 3.3028.796.56–23.2113.04 ± 4.6535.63
      Cd0.02–0.100.05 ± 0.0235.000.03–0.060.04 ± 0.0125.590.02–0.100.05 ± 0.0233.23
      Pb0.55–2.031.11 ± 0.4035.790.42–1.540.90 ± 0.4146.200.42–2.031.08 ± 0.4037.33
      Note: CV, coefficient of variation; S. D., standard deviation

      Table 1.  Concentrations of As and metals in surface water in tail reaches of the Yellow River / (μg/L)

      PeriodsElementsFreshwater reachLow-salinity reachTail reaches
      RangeMean ± S.D.CV (%)RangeMean ± S.D.CV (%)RangeMean ± S.D.CV (%)
      Pre-flood seasonAs8.02–22.9417.42 ± 3.5920.6113.05–20.8718.57 ± 3.2217.328.02–22.9417.84 ± 3.3518.78
      Cr56.60–216.13108.62 ± 34.1331.4273.67–120.62107.35 ± 19.0917.7856.60–216.13109.81 ± 30.5127.78
      Ni18.59–73.4145.26 ± 14.2231.4129.57–57.2749.93 ± 11.5323.0918.59–73.4146.89 ± 13.1828.12
      Cu12.49–46.0434.62 ± 8.1323.4826.73–46.9839.20 ± 7.6019.3812.49–46.9835.90 ± 7.8821.94
      Zn37.03–142.4192.79 ± 22.4324.1871.10–121.40103.72 ± 19.1518.4737.03–142.4195.97 ± 21.3222.22
      Cd0.17–0.650.49 ± 0.1122.970.39–0.620.54 ± 0.1017.880.17–0.650.50 ± 0.1121.28
      Pb16.27–37.9729.24 ± 5.9520.3421.89–34.2631.12 ± 5.2316.8216.27–37.9729.94 ± 5.5318.47
      Post-flood seasonAs10.51–15.1112.79 ± 1.3910.857.37–13.1311.44 ± 2.3820.777.37–15.1112.56 ± 1.6513.11
      Cr61.18–118.3382.21 ± 11.9514.5354.55–91.0079.71 ± 14.8718.6654.55–118.3381.32 ± 12.1314.92
      Ni23.95–42.0931.79 ± 4.7014.7719.49–35.6230.66 ± 6.4120.9219.49–42.0931.57 ± 4.9515.69
      Cu17.82–35.1323.87 ± 4.0116.809.71–27.4322.16 ± 7.1932.469.71–35.1323.56 ± 4.6719.81
      Zn51.46–142.8974.28 ± 20.7927.9834.65–82.1170.69 ± 20.4028.8534.65–142.8973.27 ± 20.2827.68
      Cd0.25–0.430.36 ± 0.0515.220.13–0.370.30 ± 0.1032.500.13–0.430.35 ± 0.0719.11
      Pb16.97–26.6321.79 ± 2.6912.3314.22–23.3021.07 ± 3.8618.3014.22–26.6321.57 ± 2.8613.24
      Note: CV, coefficient of variation; S. D., standard deviation

      Table 2.  Concentrations of As and metals in suspended particulate matter (SPM) in tail reaches of the Yellow River (mg/kg)

Reference (53)

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