Impact of water-sediment regulation on the transport of heavy metals from the Yellow River to the sea in 2015

Science of the Total Environment 658 (2019) 268–279

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Impact of water-sediment regulation on the transport of heavy metals from the Yellow River to the sea in 2015 Liu Ming a,b,⁎, Fan Dejiang a,b, Bi Naishuang a,b, Sun Xueshi a, Tian Yuan a a b

Key Laboratory of Submarine Geosciences and Technology, MOE, Ocean University of China, 238 Songling Road, Qingdao 266100, PR China Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266061, PR China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Report the variations in heavy metals concentrations and fluxes during WaterSediment Regulation Scheme (WSRS) in 2015 • Metals flux during the WSRS accounted for 42%–54% of the year and the metals transport was dominated by the particulate form. • Sources of particulate and dissolved metals during WSRS were explored. • Variations of metals concentrations and fluxes show a good respond to the WSRS process.

a r t i c l e

i n f o

Article history: Received 12 August 2018 Received in revised form 11 December 2018 Accepted 11 December 2018 Available online 14 December 2018 Editor: Filip M.G. Tack Keywords: Yellow River Heavy metal transport Water-sediment regulation scheme

a b s t r a c t Variations in particulate and dissolved heavy metal (Cu, Co, Ni, Zn, Pb, Cr, Cd and As) concentrations and fluxes as well as their response to the Water-Sediment Regulation Scheme (WSRS) process in 2015 were studied based on a daily water and sediment survey at Lijin gauge. The results showed that the water and sediment flux increased rapidly in the first stage of the WSRS, which was characterized by high water discharge and suspended sediment concentration (SSC). The suspended sediment was coarser than the natural state, while the particulate and dissolved heavy metal contents increased. In the second stage, the SSC decreased rapidly followed by a gradual reduction in water discharge, the suspended sediment became even coarser and both the particulate and dissolved metal contents showed a decreasing trend. The heavy metal flux during the WSRS period accounted for 42%–54% of the total year, and the transport form was dominated by the particulate form. Dissolved metal contents were affected by the release of heavy metals derived from the channel in the lower reaches, while particulate heavy metals mainly came from erosion of the riverbed and their contents were much lower than Xiaolangdi reservoir sediment. Heavy metal transportation was influenced significantly by the WSRS process. Changes in sediment flux resulted in significant differences in the flux of heavy metals and the distribution of metals in different transport forms between the first and second stage of WSRS. © 2018 Published by Elsevier B.V.

⁎ Corresponding author at: Ocean University of China, Songling Road 238#, Qingdao, Shandong 266100, PR China. E-mail address: [email protected] (M. Liu).

https://doi.org/10.1016/j.scitotenv.2018.12.170 0048-9697/© 2018 Published by Elsevier B.V.

M. Liu et al. / Science of the Total Environment 658 (2019) 268–279

1. Introduction More than 90% of the total terrestrial material transported to the sea are discharged by rivers every year (Milliman and Meade, 1983; Meybeck and Vörösmarty, 2005; Yang et al., 2001), which includes many anthropogenic pollutants such as heavy metals (Audry et al., 2004; Radakovitch et al., 2008; Viers et al., 2009). This may cause a significant risk to the aquatic ecosystem in estuarine and coastal areas as the sink of trace elements, due to the environmental persistence, biogeochemical recycling and toxicity of metals (Liu et al., 2016). However, due to human activities, the natural processes of water discharge and sediment loading by rivers have been significantly disturbed, as well as the biogeochemical cycle of many elements, particularly trace elements (Syvitski et al., 2005; Thevenot et al., 2007; Viers et al., 2009; Biemans et al., 2011; Bi et al., 2014). These changes will have a significant influence on the flux of pollutants transported by rivers to the sea and thus have a profound impact on the fragile ecological environment in estuarine and coastal areas. The Yellow River is the second largest river in the world in terms of its sediment load over the last several thousand years and alone represents 6% of the estimated global river sediment flux to the ocean (Milliman and Meade, 1983; Liu et al., 2012). Currently, the Yellow River plays a critical role in the economic development of China, supporting a population of 107 million, irrigating 15% of the agricultural land, and contributing 9% to China's gross domestic product (Miao et al., 2010). The development of industry and agriculture has led to large amounts of pollutants discharged into the Yellow River. Meanwhile, the combination of climate change and extensive human activities has caused a dramatic decrease in the sediment load and water discharge of Yellow River over the past half century (Wang et al., 2006, 2007). By 2001, more than 3147 reservoirs had been built in the Yellow River basin, with a total storage capacity of 57.4 × 109 m3. Large dams and reservoirs regulated and shifted the seasonal distribution patterns of water discharge and sediments loading in the Yellow River (Wang et al., 2007). The Yellow River Conservancy Committee began to use large reservoirs in the middle reaches to implement a Water-Sediment Regulation Scheme (WSRS) since 2002. Using a man-made flood to scour the river channels in the lower reaches and to discharge the sediments trapped in the Xiaolangdi reservoir. Generally, the WSRS Scheme occurs in two stages: first, floodwater is discharged from the Xiaolangdi Reservoir to flush the downstream river channel assisted by artificial disturbances, and second, joint scheduling of the main reservoirs (Xiaolangdi, Sanmenxia, Wanjiazhai and others) occurs to release sediments trapped in the reservoirs and make an artificial gravity flow to achieve sediment discharge. The WSRS greatly changed the flux and seasonal variation in the water and sediment discharge of the Yellow River to the sea. During a short period that lasts for 15–20 days, more than one-third of the total annual water and sediments are transported to the sea, including large amounts of nutrients, organic carbon and dissolved uranium (Wang et al., 2010; Liu et al., 2012; Zhang et al., 2013; Sui et al., 2014). The rapid transport of materials by the large river to the sea, caused by this unique man-made flood, will have a profound impact on the depositional and ecological environment of the Yellow River estuary and the Bohai Sea. Preliminary studies have shown that flood peaks have changed the transport pattern of heavy metals in suspended particulate matter (SPM) from the Yellow River to the sea. Bi et al., 2014 and Hu et al., 2015 showed that fluxes of particulate heavy metals (PHMs) during the 2009 and 2013 WSRS period accounted for approximately 57% and 30% of the annual total flux. Sediment scoured from the lower reaches riverbed and discharged from the reservoirs in the middle reaches were major sources of pollution. Many studies have reported the content and transport characteristics of trace metals in SPM of the Yellow River under natural conditions (Zhang et al., 1988; Huang et al., 1992; Zhang et al., 1994; Liu et al., 2003; Wang et al., 2003; Qiao et al., 2007). However, the transport

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patterns and fluxes of heavy metals from the Yellow River to the sea during the WSRS and its phase changes are poorly understood. In this study, the objectives were: (1) to examine temporal variations in particulate and dissolved trace metal concentrations and their fluxes from Yellow River to the sea during the WSRS; (2) to discuss the sources and stage change of metals during the WSRS, revealing the significant impact of the WSRS on heavy metal transport of Yellow River. This study is valuable for the understanding of materials exchange between the great river and the coastal zone, as well as the “source-sink” process of heavy metals under the influence of human activities. It also provides scientific information for the optimization of the WSRS and the environmental protection of lower reach of Yellow River, the estuary area and the Bohai Sea. 2. Materials and methods 2.1. Sampling Field observations and water sample collection was conducted at the Lijin Hydrological Station during the WSRS period from June 29 to July 20, 2015. The Lijin Hydrological Station is the last hydrological station and accounts for more than 99% of the total drainage area of the Yellow River, which is located approximately 110 km upstream from the Yellow River estuary. There is no tributary input into the Yellow River after Lijin Station and the station is not affected by tides or seawater, thus it represents the Yellow River material flux to the sea. Therefore, Lijin Hydrology Station was selected as a favorable research station for measuring particulate and dissolved trace metal contents (Fig. 1). All water samples were collected each day using acid-washed polyethylene containers in the middle of the river channel transect. Samples were collected at three different water depths, at 0.2 H (H: total water depth), 0.6 H and 0.8 H of the water column, respectively. The daily and monthly water discharge records at the Lijin Hydrographic Station were provided by the Yellow River Conservancy Commission. For comparative study, a total of 7 surface samples in the Xiaolangdi Reservoir were collected during the period from November to December 2017 by the grab bucket. Xiaolangdi Reservoir has a controlled drainage area of 694,200 km2 accounting for 92.3% of the Yellow River Basin, which located in Luoyang City, Henan Province. Approximately 84% of the sediment from the middle and upper reaches has been deposited in the Xiaolangdi Reservoir and it is the most important sediment source during WSRS (Wang et al., 2010; Hu et al., 2015). 2.2. Geochemical analysis methods The water samples were filtered through pre-weighed paired cellulose acetate micropore filters (Whatman, 0.45 μm pore diameter and 47 mm diameter). A few drops of nitric acid were added to the remaining water after filtration and stored in acid-washed high-density polyethylene bottles in a refrigerator at 4 °C. The suspended sediment grain size was measured by a Mastersizer 2000 laser particle size analyzer (Malvern Instruments Ltd., UK). Prior to particle size analysis, the samples were pretreated with 10% H2O2 and 1 mol/dm3 HCl for 24 h to remove organic matter and biogenic carbonate, and then ultrasonic-dispersed with 0.5 M sodium hexametaphosphate [(NaPO3)6] to ensure complete disaggregation. Repeat test relative errors were less than 2%. The grain size parameters were calculated using Folk and Ward's (1957) formula. Both the suspended sediments and reservoir sediments were ovendried and then ground before element concentration analysis. The concentrations of major elements (Al2O3 etc.) were measured using a desktop X-ray fluorescence system (XRF, Spectro, Germany). The analysis and quality control methods used followed that of Liu et al. (2009). An inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7500) was applied to the analysis of heavy metals (Cu, Co, Ni, Zn, Pb, Cr, Cd and As) and trace element Sc. The analysis and methods used

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The Yellow River Basin

Sampling Station

Fig. 1. Map of the Yellow River drainage basin showing the locations of major gauging stations and reservoirs in the main stream. (Modified from Wang et al., 2010 and Bi et al., 2014).

followed that of Liu et al. (2016). National standard sediments (GBW07309, GBW07311 and GBW07314) were used as sediment reference material. The relative standard deviations of the major elements and the heavy metals for the multiple measurements were less than 5%. The trace metal (Cu, Ni, Zn, Pb, Cr and As) concentrations in the water samples were also measured by the inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7500). 2.3. Estimation of the trace metals flux Both the daily water discharge and sediment load data of Yellow River during the WSRS and the monthly Lijin Hydrological Station data from 2015 were collected by the Huanghe (Yellow River) Conservancy Commission. Heavy metal fluxes during the WSRS in 2015 were calculated following Bi et al. (2014), which based on the heavy metal concentrations and the sediment/water fluxes as follows: FM ¼ F s  CM

ð1Þ

where FM is the heavy metal flux, Fs is the sediment/water flux and CM is the heavy metal concentration. Here, the daily trace metal concentration is the average concentration of the three layers.

2.4. Statistical methods In order to decipher the interrelationship among heavy metals and other parameters to identify the possible sources, Pearson correlation matrix (CM) and R-mode varimax factor analysis were performed with a statistical software SPSS 16.0 for Windows (SPSS München, Germany). Pearson correlation analysis was used to determine the correlation among heavy metals and other physical or chemical parameters to confirm the results of multivariate analysis. Factor analysis is a widely used method in environmental studies, whereby a complex data set is simplified by creating several new variables or factors, each representing a cluster of interrelated variables within the dataset. It can incorporate the data standardized from all the heavy metals in all the sites into one unified comparison, and use the optimized minimum factors to explain the original data sets. Then through defining factors obtained with a R-mode varimax rotation, the geochemical characteristics of heavy metals was determined.

3. Results 3.1. Hydrological features and suspended sediment grain size during the WSRS The WSRS in 2015 lasted 15 days from July 2 to July 16. The peak flood from upstream arrived at the Lijin gauge on July 2 with an increase in the daily water and sediment discharge. As shown in Fig. 2a, the water and sediment discharge were much higher during the WSRS period in July than the normal period. As shown in Fig. 2b, June 29 to July 1 is the early stage before the artificial flood arrival and was characterized by low water discharge and SSC. From July 2 to 16, the water and sediment fluxes exhibited a twostage variation. The first stage from July 2 to 9 was characterized by a higher SSC and lower water discharge, with the water and sediment fluxes increase rapidly in the initial 3 days. The water discharge at Lijin gauge increased sharply from 771 m3/s on July 2 to 2210 m3/s on July 5, then gradually increased to a maximum value of 2700 m3/s on July 9. The SSC increased dramatically from 3.4 kg/m3 to a maximum value of 9.1 kg/m3 on July 4 and then remained above 8.6 kg/m3 to July 7, which followed by a rapid decrease to 6.6 kg/m3 on July 9. The second stage from July 10 to 16 was characterized by high water discharge and lower SSC. The water discharged remained above 2000 m3/s before July 13 and then decreased to 930 m3/s on July 16. The SSC continued to decrease rapidly to 4.32 kg/m3 on July 15. From July 17 to 20 was the final stage of WSRS with the water and sediment flux gradually returned to normal status. The SSC decreased slowly from 2.6 kg/m3 on July 17 to 1.3 kg/m3 on July 20 and the water discharge showed a continuous decrease from 747 m3/s on July 17 to 586 m3/s on July 20. The grain size of suspended sediments during the WSRS in 2015 was shown in Fig. 3. Before WSRS, the suspended sediment was fine with a median grain size of approximately 17–20 μm. In the first stage from July 2 to 9, the grain size of suspended sediment began to increase significantly when the artificial flood reached the Lijin gauge and the amount of sand component increased. The median grain size of the surface, middle and bottom layers were 20–24 μm, 22–32 μm and 24–43 μm, respectively. From July 10 to16, the grain size of suspended sediment in the middle and bottom layers increased rapidly while the amount of sand in the sediment increased significantly. The median grain size of suspended sediment in the surface, middle and bottom layers were 18–26 μm, 25–48 μm, 35–54 μm, respectively. After the WSRS (from July 17 to 20), the suspended sediment particle size began to decrease and gradually returned to the same or lower values

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Fig. 2. Variations in the monthly water and sediment discharge in 2015 (a) and daily discharge during the WSRS period (b).

as those before the flood. The median grain size of suspended sediment in the surface, middle and bottom layers were 10–18 μm, 13–21 μm, 16–26 μm, respectively. 3.2. Heavy metal concentration and flux estimation during the WSRS 3.2.1. Variations in dissolved and particulate heavy metal concentrations The concentration of the dissolved heavy metals Cu, Ni, Pb, Zn, Cr and As in the water during the WSRS were shown in Table 1 and their temporal variations in the surface, middle and bottom layers were shown in Fig. 4. Before the WSRS, the dissolved heavy metal contents in the water were relatively lower. With the arrival of the flood, the concentrations of Cu, Cr, Pb, As and Zn increased rapidly in the initial 2 days and remained high level in the first stage, which reached maximum values on July 5–6. The content of dissolved heavy metals in the second stage

of the WSRS decreased gradually with obvious fluctuations of the metals Pb, Zn and Ni. After the WSRS, the concentrations of most metals continued to decrease and returned to the levels before WSRS. The variations in Ni content was not obvious during the whole WSRS process. It was relatively stable at early of WSRS and began to increase from the later of the first stage, which also showed a gradually decreasing trend in the second stage. The concentrations of metals Pb and Zn showed obvious fluctuations during the period of WSRS. The variations of particulate metal contents in different layers were consistent basically with the WSRS process (Fig. 5). The contents of most metals showed higher content before and after the WSRS. Cu, Co, Zn and As in the suspended sediment were gradually decreased during the period of WSRS, especially in the second stages, while the contents of Ni and Cr did not show an obvious change throughout the process of WSRS. However, the content of the element Pb was slightly smaller in the initial day of the first stage, which then gradually

Fig. 3. Variations in the grain size of suspended sediment during the WSRS period in 2015.

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Table 1 Statistics of dissolved and particulate heavy metals concentration during the WSRS. Concentration Dissolved metals (μg/kg)

Surface layer

Min Max Mean Min Max Mean Min Max Mean Min Max Mean Min Max Mean Min Max Mean

Middle layer

Bottom layer Particulate metals (mg/kg)

Surface layer

Middle layer

Bottom layer

Cu

Ni

Pb

Zn

Cr

As

Co

Cd

2.02 3.52 2.63 1.49 2.95 2.51 1.87 3.47 2.58 19.49 28.31 23.34 14.20 26.90 20.60 13.48 24.71 18.48

0.24 1.50 0.77 0.18 1.34 0.77 0.38 1.74 0.73 25.10 38.93 30.04 18.97 38.12 27.52 19.77 39.63 26.46

0.24 0.78 0.47 0.16 0.87 0.47 0.19 0.77 0.45 20.58 31.13 24.66 16.64 30.72 22.95 18.09 28.35 21.64

0.22 4.09 1.31 0.14 4.10 1.82 0.19 4.98 1.54 64.57 110.80 80.42 48.23 105.20 70.45 40.57 84.16 62.18

0.60 2.55 1.40 0.48 2.58 1.57 0.67 2.55 1.48 52.42 66.90 57.38 46.97 79.22 56.08 40.10 69.21 51.69

0.29 1.27 0.82 0.35 1.70 0.91 0.38 1.39 0.88 6.87 13.78 11.35 4.82 12.60 9.86 8.33 13.22 10.25

– – – – – – – – – 10.48 14.20 12.15 8.04 14.14 11.14 8.02 13.15 10.42

– – – – – – – – – 0.09 0.23 0.15 0.09 0.23 0.15 0.09 0.28 0.15

Note: “–” means that the data was not available.

increased in the first and second stage. The element Cd showed a trend of increasing gradually during the entire WSRS period, especially increased significantly after WSRS. 3.2.2. The estimation of heavy metal fluxes Based on daily water discharge measurements and the corresponding SSC, heavy metals fluxes were established according to Eq. (1). The fluxes of heavy metals during 2015 and the WSRS period were shown in Table 2. The heavy metal contents from June 29 to July 1 could represent the content of heavy metals under natural conditions

of the Yellow River, which was roughly equivalent to that of natural state in the 2012–2013 studied by Hu et al. (Hu et al., 2015). Obviously, the heavy metals transported from the Yellow River to the sea were mainly in the particulate form and the dissolved heavy metals only account for 0.5–4.6% of the total annal fluxes. The transportation of metals was still dominated by the particulate form. However, the proportion of heavy metals in the dissolved form in this period was lower than the natural state. Only the elements Cu and As in dissolved form accounted for approximately 2.2% and 1.5% of the total flux, the others accounted for less than 1%.

Fig. 4. Variations in the dissolved heavy metal concentration during the WSRS period in 2015.

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Fig. 5. Variations in the particle heavy metal concentration during the WSRS period in 2015.

heavy metals transported to the sea accounted for 42–54% of the year (Table 2). That is, in a short period of 15 days, the heavy metals transported to the sea accounted for about half of the whole year by the Yellow River. Approximately 56–67% of the metals flux of the

A large amount of water and sediments rapidly discharged to the sea during the WSRS period, which had an important impact on the fluxes of heavy metals. In 2015, the water and sediment flux during the WSRS accounted for 22% and 55% of the annual flux (Fig. 2a), but the Table 2 The estimated annual and WSRS heavy metals fluxes from the Yellow River to the sea in 2015. Metals Annual fluxes

WSRS period

The first stage of WSRS

The second stage of WSRS

Dissolved form (t) Particulate form (t) Annual fluxes (t) Dissolved/annual (%) Dissolved form (t) Particulate form (t) Total (t) Dissolved/total (%) WSRS/annual (%) Dissolved form (t) Particulate form (t) Ratio of dissolved form (%) 1st stage/WSRS (%) Dissolved form (t) Particulate form (t) Ratio of dissolved form (%) 2nd stage/WSRS (%)

Note: “–” means that the data was not available.

Cu

Co

Ni

Cr

Pb

Zn

As

Cd

35 729 764 4.6 7.9 351 359 2.2 47 3.8 232 1.6 66 3.2 119 2.6 34

– 402 402 – – 188 188 – 47 – 123 – 65 – 65 – 35

13 998 1011 1.3 2.3 469 471 0.5 47 1.1 309 0.4 66 0.8 159 0.5 34

16 1716 1732 0.9 4.8 934 939 0.5 54 2.6 598 0.4 64 1.8 336 0.5 36

6 716 722 1.2 1.6 383 385 0.4 53 0.8 241 0.3 63 0.6 142 0.4 37

15 2779 2794 0.5 5.2 1179 1184 0.4 42 2.5 789 0.3 67 2.2 390 0.6 33

11 377 388 2.8 2.7 176 179 1.5 46 1.4 115 1.2 65 1.0 61 1.6 35

– 4.8 4.8 – – 2.5 2.5 – 52 – 1.4 – 56 – 1.1 – 44

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WSRS period occurred in the first stage and the proportion of dissolved metals was approximately 0.3–1.6% of total flux. However, the flux of heavy metals in the second stage accounted for 33–44% of the WSRS, the particulate form was still the dominant means of transportation, but the proportion in dissolved form increased, accounting for 0.4–2.6% of total flux. Such a huge amount of heavy metals discharged into the sea over a short period of time will have a significant impact on the ecological environment of the Yellow River estuary and the Bohai Sea. 3.3. Correlation and factor analysis Heavy metals in sediments are mainly from natural or released by human activities, etc., and their contents are affected by sediment particle sizes and mineral composition (Mecray and Buchholtz Ten Brink, 2000; Zhou et al., 2007). To explore the geochemical behaviors of heavy metals in suspended sediment during the 2015 WSRS, Pearson correlations were determined. Except for the element Cd, the contents of metals showed a significant positive correlation to the element Al the suspended sediment and negative correlation to median grain size (Table 3). Obviously, the composition of the suspended sediment had an important influence on the content of heavy metals. To eliminate the influence of particle size and mineral composition on the content of heavy metals in the suspended sediment, we normalized the contents of heavy metals by Al. Al is a structural constituent of aluminosilicate minerals and an important component of fine-grained sediments such as clay minerals, which have a refractory concentration, a negligible anthropogenic contamination and are also not subject to environmental influences (Clark et al., 2000). Based on the linear relationship between the metals and Al, we normalized metals concentration to the background value of the element Al (the soil background in the Chinese Loess Plateau from Tian et al., 1991) for each sample (Sun et al., 2018). As shown in Fig. 6, the fluctuation of normalized contents of most metals reduced and their variation during the WSRS period also changed. The normalized content of Cu, Co, Ni, Cr, Pb, Zn and As did not decrease but increased slightly in the first stage of WSRS after the normalization, but most of them also showed a decreasing trend in the second stage except for Pb and Cr slightly increased or maintained the higher value. The content of the element Cd was not affected by the suspended sediment composition or grain size, which showed no change after the normalization. After eliminating the influence of sediment composition, R-mode varimax factor analysis was performed on the particulate metal concentrations during the WSRS to identify the combination of elements and attempt to further clarify the sources of metals in the suspended sediment. As shown in Table 4, two factors explained 76.25% of the

cumulative variance, which indicated different sources for the heavy metals in the suspended sediment. F1 accounted for 48.56% of the total variance with high loadings of Co, Ni, Cu, Zn, As, and the Cr. These metals contents were similar to the background value of Chinese loess soil, which indicates that these metals were largely derived from naturally weathered Malan Loess. F2 represented 28.62% of the total variance and was characterized by loadings of Pb, Cd and Cr. The contents of Pb and Cd were higher than the background value of Malan loess, while Cr was also slightly higher than the loess. Therefore, F2 is referred to as the effect of human pollution. The heavy metals discharged by human activities were mostly associated with secondary geochemical forms in the sediments, and were prone to migrate or transform (Sanjay et al., 2011; Yao et al., 2010). With erosion caused by the flood during the WSRS, the metals in the riverbed sediments were continuously released, causing a continuous increase in the contents of Cr, Pb and Cd throughout the process. However, the element of Cd was different from the other metals, as its content gradually increased during the process of the WSRS and was not affected by the sediment composition. The chemical activity of Cd is active. It mainly enters the epigenetic environment with soluble compounds in the weathering process and is easily adsorbed by clay minerals or colloids. The content of Cd is also affected by human activities including emissions of anthropogenic pollutants. The main forms of Cd bound to the sediments of the Yellow River are exchangeable and carbonate, the amount and release capacity of Cd2+ in Yellow River sediments is much larger than other heavy metals (He et al., 2003; Gao et al., 2014; Liu et al., 2016). Therefore, the flooding caused a large amount of Cd deposited in the riverbed to be released again, and then to combine with the suspended sediment, which led to the continuous increase of Cd in the WSRS process. 3.4. Grain size and heavy metal concentrations of the Xiaolangdi Reservoir sediments As shown in Table 5, the sediment in Xiaolangdi Reservoir was fine with the average median grain size of 6.63 μm, which was much finer than that of suspended sediment during the WSRS. The contents of heavy metals in reservoir sediments were significantly higher than in the suspended sediment during the WSRS in 2015 whether normalized by Al or not. There are many tributaries in the middle and upper reaches cover more than 90% of the Yellow River basin, which have been subjected to increased discharge of industrial waste and sewage. A large quantity of pollutants with high heavy metals concentrations was delivered into the mainstream of the Yellow River by the highly polluted tributaries (Wang et al., 2003; Bi et al., 2014; Huanghe Water Resources Conservation Commission, 2016). At the interception of the reservoirs

Table 3 Pearson correlation matrix of the trace elements, Al and MD. Cr Co Ni Cu Zn As Pb Cd Al MD

.576⁎⁎ .706⁎⁎ .673⁎⁎ .529⁎⁎ .255⁎ .715⁎⁎ .384⁎⁎ .464⁎⁎ −.568⁎⁎

Co

Ni

Cu

.772⁎⁎ .933⁎⁎ .877⁎⁎ .597⁎⁎ .681⁎⁎

.737⁎⁎ .674⁎⁎ .409⁎⁎ .523⁎⁎

.896⁎⁎ .556⁎⁎ .761⁎⁎

−0.152 .750⁎⁎ −.827⁎⁎

0.052 .512⁎⁎ −.671⁎⁎

−0.077 .682⁎⁎ −.815⁎⁎

Notes: MD indicates the median grain size (μm). ⁎⁎ Correlation is significant at the .01 level (two-tailed). ⁎ Correlation is significant at the .05 level (two-tailed).

Zn

As

Pb

Cd

Al

.478⁎⁎ .607⁎⁎ −0.205 .698⁎⁎ −.730⁎⁎

.382⁎⁎ −0.161 .298⁎ −.468⁎⁎

.369⁎⁎ .670⁎⁎ −.642⁎⁎

−0.023 0.028

−.696⁎⁎

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Fig. 6. Comparison of particle heavy metals concentrations before and after normalization.

in the middle and upper reaches, most of the sediments with high heavy metals concentration were deposited, caused the high concentration of metals in the sediments of Xiaolangdi Reservoir.

Table 4 Matrix of factor loadings of the heavy metals after normalization. Metals

Cr Co Ni Cu Zn As Pb Cd % of total variance % of cumulative variance

Component matrix F1

F2

0.421 0.935 0.827 0.888 0.864 0.635 0.264 −0.376 48.56 48.56

0.785 0.118 0.098 0.36 0.109 −0.055 0.859 0.832 27.69 76.25

4. Discussion 4.1. The sources of heavy metals The WSRS used an “artificial flood” to flush the lower reach river channel of the Yellow River and transport the sediment to the sea, by the joint dispatch of the reservoirs in the middle and upper reaches. Therefore, the dissolved heavy metals content influenced by the source area of the middle and upper reaches. However, as shown in Fig. 4, the content of dissolved heavy metals increased rapidly during the initial 1–2 days of the WSRS, then maintained higher values in the first stage of WSRS. The various of the metal contents should be due to the release of heavy metal in the riverbed sediments caused by the flooding and disturbance during the WSRS, which caused the dissolved heavy metal content increases rapidly. With the continuously washing, the finegrained material with a relatively high heavy metal content was taken away, coarse-grained “clean” materials were leaving in the riverbed. So, the release of heavy metals in the downstream riverbed sediments was weakened, causing the concentration of dissolved metals in the

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Table 5 Summery of the median grain size (MD), element Al and metals concentration in the sediments of Xiaolangdi Reservior. Sampling site XLD 1# XLD5# XLD8# XLD10# XLD13# XLD15# XLD20# Mean

MD (μm)

Al (%)

Cu (mg/kg)

Co (mg/kg)

Ni (mg/kg)

Cr (mg/kg)

Pb (mg/kg)

Zn (mg/kg)

As (mg/kg)

Cd (mg/kg)

7.72 6.91 8.63 7.08 5.16 7.01 5.10 6.63

6.66 6.71 6.76 6.61 6.60 6.44 6.62 6.80

37.32 38.01 33.81 34.81 34.51 31.53 34.42 34.92

18.51 17.69 16.27 15.92 18.18 17.12 18.45 17.45

43.68 41.30 40.78 40.87 44.33 41.81 44.79 42.51

77.18 76.85 76.98 76.99 85.12 74.50 79.38 78.14

43.20 45.75 46.64 42.36 37.54 35.90 40.31 41.67

97.86 92.22 93.93 93.83 98.93 94.01 97.95 95.53

22.49 19.00 19.23 18.94 22.43 19.86 22.22 20.60

0.15 0.18 0.19 0.15 0.12 0.13 0.12 0.15

Al normalization XLD 1# XLD5# XLD8# XLD10# XLD13# XLD15# XLD20# Mean

Cu

Co

Ni

Cr

Pb

Zn

As

Cd

39.56 39.87 35.36 37.35 37.15 35.21 36.89 37.34

19.53 18.54 16.98 17.09 19.39 18.80 19.58 18.56

45.83 43.08 42.27 43.31 46.87 45.34 47.16 44.84

80.17 79.33 79.06 80.39 88.65 79.41 82.68 81.38

44.99 47.24 47.89 44.40 39.66 38.85 42.29 43.61

107.76 100.44 100.81 105.08 110.62 110.28 108.88 106.27

22.93 19.36 19.53 19.44 22.95 20.58 22.70 21.07

0.15 0.18 0.19 0.15 0.12 0.13 0.12 0.15

water decreased during the second stage and gradually returned to levels of the before by the end of WSRS. The source of particulate heavy metals was related to the source of sediments. However, the suspended sediment of the Yellow River during the WSRS mainly came from the erosion of sediments in downstream rivers by floods or sediments in reservoirs such as the Xiaolangdi in the middle and upper reaches. The two stages in the traditional WSRS always led to two SSC peaks which were monitored by the Lijin gauge: the first high SSC was caused by the scouring of downstream riverbeds by a flood discharging from the reservoirs in the first stage, the second one was caused by sand discharging from the Xiaolangdi Reservoir in the second stage (Hu et al., 2015). However, in the second stage of the 2015 WSRS, although the runoff remained high level for 4–5 days, the SSC at Lijin gauge continued to decrease and the second peak was not monitored (Fig. 2b). The water and sediment fluxes monitored at Lijin gauge during the 2015 WSRS were significantly lower than in previous years, especially in the second stage compared with a typical year, such as in 2013 (Table 5) (Bi et al., 2014; Hu et al., 2015). As shown in Table 5, the concentration of metals was much higher and the grain size was much smaller in the Xiaolangdi Reservoir than the suspended sediment in Lijin gauge during WSRS. Meanwhile, the suspended sediment particle size in 2015 WSRS was also significantly higher than that in 2009 and 2013 (Table 6). It can be inferred that suspended sediment during the WSRS in 2015 was not from the Xiaolangdi Reservoir, but mainly from the erosion of the channel in the lower reaches (Wang et al., 2010). The Yellow River basin had less precipitation and experienced a rare drought from 2014 to 2015. Compared to the average values of previous years, the water and sediment fluxes monitored at the main hydrological gauges were lower by 14–54% and 69–98% in 2015, respectively.

Table 6 Comparison of hydrological characteristics during WSRS between 2015, 2009 and 2013.

2009-1 2009-2 2013-1 2013-2 2015-1 2015-2

Average grain size of the suspended sediment (μm)

Runoff (×109 m3)

Flux of sediment (×106 t)

22.4 (MD) 8.3 (MD) 15.9 (Mz) 7.7 (Mz) 25.9 (MD) 33.9 (MD)

40.99 3.95 30.4 22.9 1.55 1.39

37.2 0.62 16.9 20.9 11.04 0.63

Reference

Bi et al., 2014 Hu et al., 2015 This study

Note: 20**-1(2) represents the first (second) stage of WSRS in 20**, MD indicates the median grain size and Mz indicates the mean grain size.

When compared to the average values over the past 10 years, they were lower by 12–27% and 46–82%, respectively. In 2015, there was no sediment transported come out from the Xiaolangdi Reservoir and nosediments transported to the lower reaches (Huanghe Water Resources Bulletin, 2016). Due to less water stored in the reservoirs of the middle and upper reaches, the amount of water and sediment discharged during the 2015 WSRS was relatively small and the sediment deposited in the Xiaolangdi Reservoir was not discharged during the second stage of WSRS. Therefore, the sediment recorded at the Lijin gauge during the whole WSRS mainly came from the erosion of the channel in the lower reaches. Previous studies have shown that, large number of dikes and levees have been built in the lower reach of Yellow River for the purpose of flood prevention and irrigation, which narrowed the river channel and greatly reduced the flow downstream of the floodplain and the input of heavy metals along the coast of river (Lair et al., 2009). Moreover, the lower reaches of the Yellow River are characterized by a unique elevated riverbed above the horizon of the surrounding relief that prevents the input of pollutants from water and sediment into the main stream through tributaries or sewage channels (Bi et al., 2014). Most of the anthropogenic heavy metals in the Yellow River suspended sediment are derived from upper and middle reaches, which trapped mostly in the reservoirs. Therefore, the coarser and relatively “clean” sediments mainly came from the lower reaches during the 2015 WSRS period led to much lower contents of particulate heavy metals in suspended sediment than the sediments in Xiaolangdi Reservoir. 4.2. Stage characteristics of heavy metal transportation and its response to the WSRS process During the 2015 WSRS, the variations of dissolved heavy metals concentrations influenced by the release of heavy metals in the riverbed sediments of the lower reach caused by the flushing. The dissolved metals contents were relatively low before WSRS, but the erosion and disturbance of the riverbed sediments after the WSRS caused the release of metals from the sediments to the water, the content of dissolved heavy metals increased rapidly during the initial 1–2 days of the first stage and maintained relatively high values. In the second stage, as the riverbed sediments were continuously washed away, fine sediments with relatively high metals were taken away, the release of metals was weakened. As a result, the amount of dissolved heavy metals in the second stage gradually decreased and gradually returned to normal status in the end period (Fig. 4). The particulate heavy metals in the Yellow River during the 2015 WSRS mainly came from erosion of the downstream channel. In the

M. Liu et al. / Science of the Total Environment 658 (2019) 268–279

normal state before the WSRS, the runoff was small and the sediment was relatively fine, the content of heavy metals was relatively high. In the first stage, the erosion of lower reaches riverbed brought a large amount of coarse and “clean” sediments (Fig. 3), resulting in a gradual decrease in the content of heavy metals (Fig. 5). In the second stage, the continued high runoff and scouring resulted in a further increase in the particle size of suspended sediment (Fig. 3) and a sharp decrease in the SSC (Fig. 2b), while the heavy metal content continued to decrease. After the WSRS, as the fluxes of water and sediment decreased, the suspended sediment particle size decreased and the heavy metal content increased significantly (Fig. 5). After normalizing by the element Al to eliminate the influence of sediment composition, the contents of the metals Co, Ni, Cu, Zn and As, which mainly come from natural weathering, increased in the first stage but reduced in the second stage. With the implementation of the WSRS, the scouring by the artificial flood in lower reaches caused heavy metals in the sediments to be released into the water and then combine with particulate matter, thus leading to the increase of particulates and dissolved metals in the first stage. In the second stage, due to the erosion by the flood, the release of metals in the sediment gradually decreased, which resulted in the decreasing of particulate and dissolved metals content. However, the contents of metals such as Cr, Pb and Cd, which were affected by human activities, increased continuously during the WSRS process (Fig. 7). The particle form was the dominant transport form of metals during the WSRS and the ratio of metals transported by the dissolved form was also lower during the WSRS than in the natural state. During the WSRS in 2015, more than 97% of the metals were transported in particle form (Table 2). In addition, the water and sediment fluxes in the first stage accounted for approximately 53% and 65% of the entire WSRS period, the higher sediment transport flux in the first stage caused to a significant larger flux of heavy metal, which was nearly double of the second stage. With the implementation of the WSRS, the distribution of heavy metals in the different transport forms also changed. The proportion of the dissolved form in the total flux was slightly higher in the second stage than the first stage. In the first stage, the SSC and the heavy metal content were higher, led to a higher proportion of heavy metals in the particulate transport form. The water flux in second stage was roughly equivalent to the first stage, but the sediment transport flux was significantly smaller. Although the contents of most heavy metals were reduced in both the particle and dissolved forms, the reduce of the

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particulate metals flux caused the increase of the dissolved metals proportion in the second stage. 4.3. Comparison of heavy metal transport during WSRS in 2015 to 2009 and 2013 The water and sediment fluxes during the WSRS in 2015 were significantly lower and the suspended sediment particle size was significantly higher than in 2009 and 2013 (Table 6). Due to the low water and sediment fluxes in the upper reaches of the Yellow River from 2014 to 2015, continuous scouring of the lower reach channel caused coarser suspended sediment of the WSRS in 2015, especially in the second stage (Table 6). As shown in Table 7, The content of each heavy metal element has a significant difference in the first stage between 2009, 2013 and 2015, but in the second stage, the contents of all heavy metals were significantly lower in 2015 than 2009 and 2013, which was maybe significantly affected by the grain size of suspended sediment. Hu et al. (2015) used the method of the ratio of heavy metals to Sc to eliminate the influence of particle size when studying heavy metals in the 2013 WSRS (Hu et al., 2015). For comparison, we calculated the ratio of heavy metals to Sc in 2015 using the same method (Table 7). Most metals contents were similar in the first stage of WSRS in 2015 and 2013, while Cu, Zn, Pb and Cd were significantly poorer in the second stage of 2015. The suspended sediment in the first stage of the WSRS mostly came from sediments resuspended in the downstream riverbed, which results in a consistent heavy metal content in different years. In the second stage, the sources of suspended sediment were significantly different in 2013 and 2015, which caused the difference in heavy metal contents. At the same time, the decrease in sediment transport in the middle and upper reaches of the Yellow River in 2015 led to a smaller flux of heavy metals to the sea throughout the year. Although the heavy metal flux during the 2013 WSRS was much higher than in 2015, the heavy metal flux during the WSRS accounted for about one-third of the annual flux in 2013, while it reached about half the yearly flux in 2015 (Table 7). 5. Conclusion Variations in water and sediment discharges and heavy metal concentrations in the Yellow River measured at the Lijin gauge during the

Fig. 7. Distribution of heavy metals in particle and dissolved forms in two stages of WSRS in 2015. (Red in the figure represents the first stage and blue represents the second stage). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Table 7 Comparison of metals transport characteristics during WSRS between 2015, 2009 and 2013. Metals concentration (μg/kg)

Contents of heavy metals in two stages of WSRS

Flux of particulate metals (t)

Proportion of particulate metals flux during SWRS in the year (%)

2009-1 2009-2 2013-1 2013-2 2015-1 2015-2 2013-1 2013-2 2015-1 2015-2 2009年 2013年 2015年 2013年 2015年

Reference

Cu

Co

Ni

Cr

Pb

Zn

As

Cd

– – 33.4 45.9 21.1 18.2 Cu/Sc 3.1 3.3 2.3 2.1 – 1538 351 32.8 48.1

– – 12.2 16.4 11.2 10.1 Co/Sc 1.1 1.2 1.2 1.2 – 508 188 29.3 46.8

25.88 46.59 30.9 41.5 28.1 24.5 Ni/Sc 2.9 2.9 3.0 2.8 1601 1281 469 29.1 47.0

44.65 78.23 65.9 82.8 54.1 51.9 Cr/Sc 6.1 5.9 5.8 6.0 2761 2614 934 29.9 54.4

24.03 56.47 26.1 45.1 21.9 22.1 Pb/Sc 2.4 3.2 2.4 2.5 1491 1228 383 30.3 53.5

63.12 114.5 77.4 122.2 71.8 59.3 Zn/Sc 7.2 8.7 7.8 6.8 3904 4058 1179 33.8 42.4

– – 10.4 15.0 10.6 9.6 As/Sc 1.0 1.1 1.1 1.1 – 446 176 27.6 46.7

– – 0.3 0.5 0.13 0.16 Cd/Sc 0.03 0.04 0.01 0.02 – 14 2.5 26.9 52.1

Bi et al., 2014 Hu et al., 2015 This study

Hu et al., 2015 This study Bi et al., 2014 Hu et al., 2015 本研究 Hu et al., 2015 This study

Note: 20**-1(2) represents the first (second) stage of WSRS in 20**.

2015 WSRS exhibited two stages. The first stage was characterized by high water fluxes and high SSC, the suspended sediment were thicker than that in the normal period, while the particulate heavy metal contents were lower and the dissolved heavy metal contents were higher than the metals contents in the normal period. During the second stage, SSC decreased rapidly and then the runoff gradually decreased, the sediments were further thickened, the particulate and dissolved heavy metal contents all showed a decreasing trend. The heavy metal flux during the WSRS period accounted for 42–54% of the annual flux, while the metal fluxes in first stage accounted for 56–67% of the WSRS. The transportation of metals was dominated by the particulate form. Variations of dissolved metal content were influenced by the release of heavy metals in the riverbed sediments of the lower reach during the WSRS. The particulate metals were mainly derived from the erosion of the downstream river channel. Heavy metal transportation was influenced significantly by the WSRS process. In the first stage, the flux of water and sediments from the Yellow River to the sea accounted for 53% and 65% of the WSRS, while the content of particulate and dissolved heavy metals increased. In the second stage, with scouring by the flood, the release of heavy metals from the sediments gradually decreased, the content of particulate and dissolved heavy metals all decreased, and the fluxes of heavy metals were smaller than the first stage. Significant differences in the fluxes of particulate metals caused by the changes of sediment flux, resulted in more heavy metals being transported in the dissolved form during the second stage. Compared to previous years, the water and sediment fluxes during the 2015 WSRS were smaller and the suspended sediment was much thicker. Different sediments sources in the second stage caused the contents of metals Cu, Zn, Pb and Cd were lower in 2015.

Acknowledgements The authors thank the helpful comments and suggestions provided by the anonymous reviewers are sincerely appreciated. We are very grateful to Professor Yao Peng from Ocean University of China for providing sediment samples of the Xiaolangdi Reservoir. This study was supported by the National Natural Science Foundation of China (grant numbers 41606054, 41676036) and key study project of the Chinese Ministry of Science and Technology (grant number 2016YFA0600904). References Audry, S., Schafer, J., Blanc, G., Jouanneau, J.M., 2004. Fifty-year sedimentary record of heavy metal pollution (Cd, Zn, Cu, Pb) in the Lot River reservoirs (France). Environ. Pollut. 132 (3), 413–426.

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