To further explore pollution characteristics, based on the measured concentrations, and to compare with other river basins with similar conditions, this research focused on the relationship between the concentration of absorbed heavy metals and the structure of SPM. The Igeo, RI, and Qm−PEC indices were used to determine the degree of pollution, and the pollution sources were inferred from the CA and PCA. The effects from the river system and external factors to SPM were analyzed, and then the effective limit strategies were provided.
3.1 SPM heavy metal concentrations
The concentrations of heavy metals are shown in Fig. 2, and corresponding background values are shown in Table 2. Concentrations in SPM from Beiyun River were compared with other typical urban rivers. The average Cr concentration from 12 SPM samples was 70.72 mg/kg, which is higher than concentrations from Luan River (107.17 mg/kg) and lower than that of Bortala (51.55 mg/kg) (Wang et al. 2014; Zhang et al. 2016b). The average concentration of Ni was 27.88 mg/kg, lower than that of Huai River (32.79 mg/kg) and Haihe Basin (41.38 mg/kg), and the same as that of Yellow River sediment (29.4 mg/kg) (Kong et al. 2018; Li et al. 2014; Wang et al. 2015b). The average concentration of Cu was 31.55 mg/kg, which is similar to Bortala Basin (30.09 mg/kg), but higher than that of Luan River (14.92 mg/kg) (Wang et al. 2014; Zhang et al. 2016b). The average concentration of Zn was 115.70 mg/kg, which is lower than that of Xiaobai River sediment (0.16 mg/kg), and higher than that of Yellow River (81.8 mg/kg) (Li et al. 2014). The average concentration of As was 27.77 mg/kg, higher than that of Bortala (9.67 mg/kg) and Haihe Basin (7.56 mg/kg) (Kong et al. 2018; Zhang et al. 2016b). The average concentration of Cd was 0.23 mg/kg, lower than that of Xiaobai River (0.37 mg/kg), and similar to that of Haihe Basin (0.2 mg/kg) (Kong et al. 2018; Li et al. 2014). The average Pb concentration was 29.62 mg/kg, slightly higher than that of Luan River (24.13 mg/kg), but much lower than that of Huai River (53.43 mg/kg) (Wang et al. 2015b; Wang et al. 2014). Of the seven tested metals, Cu, Zn, As, Cd, and Pb were 1.44, 1.48, 2.81, 2.51, and 1.38 times greater than background values, respectively. In terms of concentration distribution, the lower reaches had significantly higher concentrations, which increased with the river flow direction. In summary, the concentrations of Ni, Cu, Zn, and Pb in SPM in this study area were all below the average values of rivers and lakes in China mentioned above. However, the concentrations of As and Cd are much higher than concentrations of these metals in rivers and lakes of China. The concentration of Cr is between the average value from China and rivers across the globe (Fu et al. 2014). Compared with previous studies, the concentrations of Cr, Ni, Cd, Zn, and other elements in the sediments of Beiyun River are slightly different from those of SPM, indicating that SPM is the main component of sediments, especially the top sediment layer (Baoqing et al. 2011; Walling 2005; Weixiao et al. 2013). The results of this study are consistent with those of Atkinson et al. (2007) and Bracken (2010).
3.2 Heavy metal concentration and SPM properties
SPM is an essential source of heavy metals in rivers. Cr, Ni, Cu, Zn, Cd, and Pb concentrations in SPM of Beiyun River show an increasing trend, from upstream to downstream, accumulating and migrating downstream, continuously induced by flow (Feng et al. 2017). SPM microstructure changes due to resuspension: more pollutants such as heavy metals and phosphorus, are adsorbed on the surface of SPM and transported downstream, resulting in the accumulation of heavy metals in rivers (Stephens et al. 2001). Heavy metals and SPM can adsorb together as they are affected by van der Waals force. The double-electron-layer structure and large surface area of SPM means that more heavy metals can be adsorbed on the surface (Jin et al. 2018). In addition, the chemical bonds of SPM means they can irreversibly adsorb heavy metals and other pollutants (Chanudet and Filella 2007). Metal oxides in SPM can combine with heavy metals spontaneously, such as Fe and Mn oxides combining with Pb on the inorganic layer, and Mn oxides combining with Zn on the organic layer (Zhang et al. 2019). The shear stress of overlying water means that heavy metals on particles are more stable if they are absorbed by a chemical process, and will exist in the river for a long time.
3.3 Risk assessment
Pollution level, risk level and toxicity are all derived from Table 1.In terms of sample distribution, that sites B1, B3, B4, B7, B12, B13, and B15 have moderate-strong pollution, while B5, B6, B8, B9, and B14 have moderate pollution. In terms of element type, the SPM of Beiyun River is moderately polluted with Ni, moderate-strongly polluted with Cr, Cu, Zn, and Pb, and strongly polluted with As and Cd (Fig. 3b). The comprehensive potential ecological hazard index (RI) and mean potential effective concentration quotient (Qm−PEC) of Beiyun River are shown in Fig. 3c and d. From the perspective of different heavy metal elements, the mean values decreased in the order Cd > As > Cu > Pb > Ni > Cr > Zn. Among these, Cd pollution is the most serious, and belongs to extremely high risk level. The contribution of Cd to the RI of the study area was up to 60.03%, demonstrating that Cd is the most critical ecological risk factor in SPM of Beiyun River. Moreover, agricultural non-point source pollution has a significant impact on the regional pollution, and Cd should be listed in the priority pollutants list in this study area (Ke et al. 2017). Additionally, Cd has been listed as one of the priory limit pollutants by the United States Environmental Protection Agency (US EPA) (Chen et al. 2015a). In terms of spatial distribution, the concentration of Cr and Zn in the Beijing section was higher, which is consistent with the results of Huo (Huo et al. 2011). In the Tianjin section, the RI value of B6 ~ B9 was low, belonging to slight risk level, which indicates that desilting in Tianjin has played a positive role in controlling heavy metals in recent years. B12, B13, and B15 belong to moderate risk levels, because shipping and petrochemical industry near the estuary seriously affects the quality of river SPM. There is a significant difference between the middle and lower reaches, in terms of Cu, Zn, and Cd. The high RI value of the three metals above is due to frequent human activities downstream (Guo et al. 2010). Among all metals, RI is > 600, demonstrating that the whole river is at high risk level, mainly because it is a sewage river. There are 341 sewage outlets along the river, including 42 large-scale outlets, with 209.28 million tons of industrial and residential sewage discharged into the river every year (Zhang et al. 2015). Figure 3d shows that the Qm−PEC of Cr, Ni, and As is > 0.5, indicating the toxicity of SPM. In addition to SPM properties, many factors such as water flow velocity, physical and chemical properties of sediments, salinity, pH, and dissolved oxygen (DO), all affect the release of metals, resulting in differences in heavy metal concentrations in the basin (Atkinson et al. 2007; Huang et al. 2012).
Table 1
The index categories and evaluation criteria of heavy metals
Geoaccumulation index
Igeo
|
Potential ecological risk index
(E、RI)
|
Mean probable effect concentration quotient
(Qm−PEC)
|
Grade
|
Level
|
E
|
RI
|
Risk level
|
Qm−PEC
|
Sediment toxicity
|
≤ 0
|
None
|
≤ 40
|
≤ 150
|
slight
|
<0.5
|
basically non-toxic
|
0–1
|
None–moderate
|
40–80
|
150–300
|
moderate
|
>0.5
|
Toxicity
|
1–2
|
Almost moderate
|
80–160
|
300–600
|
Considerable
|
|
|
2–3
|
Moderate
|
160–320
|
>600
|
High
|
|
|
3–4
|
Moderate–strong
|
>320
|
|
Extremely high
|
|
|
4–5
|
Strong
|
|
|
|
|
|
>5
|
Seriously strong
|
|
|
|
|
|
3.4 Source analysis of SPM heavy metals
SPM adsorbed heavy metals in rivers, and combined with sediment by flocculation and sedimentation. The source and migration of these elements can be inferred from correlation among heavy metals (Wang et al. 2012). The correlation coefficient matrix of Cr, Ni, Cu, Zn, As, Cd, Pb, OM, TC, TN, and TP in SPM was obtained by CA (Fig. 4). PCA showed that chemical properties of SPM samples were significantly correlated with heavy metal concentrations. Significant positive correlations were found between TC, TN, TP, and Cu concentrations. Moreover, OM was significantly positively correlated with Ni, Cu, and Zn, indicating that TC, TN, TP, and OM have common sources, and are affected by chemical parameters combined with heavy metals (Zhang et al. 2017b). Among these seven heavy metals, Zn has extremely strong correlation with Cu and Pb. In addition, some metals have strong correlation with each other, such as Zn and Cd, Cr and Cu, and Cr and Ni. This indicates similar pathways and mechanisms of SPM absorbing and transforming these heavy metals from the environment. Cr and Cu in this area are mainly from the electroplating industry (John et al. 2016). The Beiyun River basin is also a typical intensive agricultural area. Zn and Cd are added to chemical fertilizers, pesticides, and feed additives, then accumulate in farmland, livestock, and poultry manure (like cattle dung), and are finally discharged into the river through sewage (Franco-Uria et al. 2009; Wang et al. 2015a). There was a negative correlation between As and Cu, As and Zn, and Cd and Pb, which suggests that As was different from other heavy metals in enrichment and transformation pathways.
In the PCA, the cumulative variance contribution rate of three principal components was 88.12%, which reflected total pollution. The first principal component (PC1) with high variance was Zn, Cu, Pb, and Ni, which explain 57.77% of the variance. The second principal component (PC2) was As, Cr, and Ni, that explains 21.56% of total variance. For the third principal component (PC3), the special element explained 8.79% of the variance. Cr had a higher load in PC2 and PC3, and the second one is higher than the third one. However, Ni had a higher load in PC1 and PC2, and the second one is higher than the first one, which indicates that Cr and Ni may have different sources and are controlled by two different factors. Combined with the results of background value and PCA, PC1 was strongly correlated with Zn, Cu, Pb, and Ni, which suggested that PC1 was made by anthropogenic activities. Anthropogenic sources are generally considered the main reason for the higher environmental concentrations of heavy metals, compared with background concentrations (Franco-Uria et al. 2009; Zhang et al. 2016a). Most Zn and Ni comes from urban industrial sewage (such as metallurgy and electroplating industry), agricultural drainage, and landfill in the BTH region. Incineration also affects Zn and Ni concentrations (Huo et al. 2011). In Northern China, nitrogen fertilizer, phosphate fertilizer, and coal combustion may release Zn, Cu, and Pb, which are important sources of heavy metal pollution in the Beiyun River Basin (Zhang et al. 2016a). PC2 has strong correlation with As, Cr, and Ni, which can be inferred as the joint action of anthropogenic and natural sources. PC3 has strong correlation with Cr and can be inferred as natural sources, mainly from mineral weathering and atmospheric precipitation (Wang et al. 2015a). CA, Zn, Cu, and Ni have, not only strong correlation, but also positive correlation, and these three elements had a higher load in PC1, with concentrations exceeding the background value, implying that human activities have significant influence.
3.5 SPM impacting factors from the environment
With increasing heavy metal contamination, understanding of aquatic elements cycle needs to be explored so a management system to control pollution can be determined (Ma et al. 2019). Serious pollution is the interaction of multiple factors, and is affected by the interactions among land use, rainfall patterns, soil moisture, and hydrology (Atkinson et al. 2007). It is simultaneously affected by numerous internal factors such as overlying water, sediment, aquatic organisms, and human activities in the river system (Fig. 5). The metabolites of organisms enter aquatic ecosystems, and substances in water are transformed into SPM by adsorption, sedimentation, and flocculation processes. The nutrients in SPM are transformed by organisms to maintain their physiological needs via biological assimilation. Organisms continuously metabolize and excrete metabolites, which then enter into SPM again. After subsiding, SPM enters into sediments, and the sediment suspended in organism after disturbed. This is a reciprocating cycle (Turner and Millward 2002). Pollutants in surface water and sediment affect the concentration of SPM through numerous processes, such as sedimentation, migration, and transformation; thus, soil and heavy metal pollution caused by human activities can accumulate in SPM (Feng et al. 2017). The pollutants in SPM affect environment by different processes like suspension, erosion and deposition (Marttila et al. 2013). Thus, SPM plays a crucial role in the interaction between overlying water and sediment. More importantly, there are intermittent and periodic interplay dynamic turnover processes between SPM and sediment (Ciszewski and Grygar 2016; Glaser et al. 2020; Schwientek et al. 2017). The exchange and transformation of materials in several different media is accompanied by sudden changes in salinity, pH, redox conditions, and dissolved OM concentration, which changes chemical and particulate reactivity (Turner and Millward 2002). Some uncontrollable factors, such as wind-induced resuspension also affect the concentrations and properties of SPM (Shinohara and Isobe 2010). Furthermore, due to the influence of river hydraulic movement, downstream topography, and industrial structure, scour included, the concentrations of heavy metals at the beginning of the river are far lower than in the Bohai Sea estuary. Scour and hydraulic movement lead to sediment deposition, but excessive sediment deposition may cause more serious accumulation of SPM, even heavy metals in downstream areas (Turner et al. 2002). In addition, human intervention along rivers, such as dam construction, soil and water conservation technology, will affect the sediment supply and transport of the river (Chen et al. 2016). There are many gates and dams along Beiyun River, such as Yangwa gate, Yulinzhuang gate, Beiguan gate, etc. A large amount of industrial wastewater and domestic sewage emerged upstream due to the construction of gates and dams. The interception of these gate and dam causes a reduction in the velocity, runoff, and the self-purification capacity of rivers. The increased water pollution further accelerates the accumulation of heavy metals. When the dam is suddenly opened, sewage will discharge at the same time, and the pollutants carried by sewage will induce water pollution events, which is also one of the main factors for heavy metal pollution in the Beiyun River.
Industry, agriculture, and traffic emissions are all significant factors of SPM concentration (Atafar et al. 2010; Harrison et al. 2003; Park and Dam 2010) (Fig. 6). During industrial production, heavy metal concentrations, such as As, Cd, Cu, and Zn increase in the surrounding areas (Park and Dam 2010). Polluted gas is directly released into the air, then forms a dust, combining with other particles (Route ① in Fig. 6). Part of the dust deposits on the road, field, etc., while some enters the river by atmospheric precipitation or rainfall (Route ⑤ in Fig. 6), then participate in the SPM cycle in the fluvial ecosystem. Apart from the industrial waste, polluted sewage from industrial production is discharged directly into the river through the outlets, which aggravates river SPM pollution (Route ② in Fig. 6). In addition, vehicle exhaust emissions is also a fundamental source for the accumulation of SPM. The way of aautomobile exhaust emission (Route ③ in Fig. 6) is similar to that of industrial gas (Route ③ in Fig. 6). High-concentration heavy metals, Cu, Zn, and Pb, are linked to urban transportation (Harrison et al. 2003): traffic-related exhaust emissions account for 34.47% of heavy metals in road dust of Beijing (Men et al. 2018). In agriculture, the use of pesticides and chemical fertilizers increases the accumulation of heavy metals in the soil (Atafar et al. 2010). Irrigation causes pesticide and fertilizer residues to form runoff, which discharges into rivers and aggravates river SPM pollution (Route ④ in Fig. 6). Moreover, flooding alters SPM transport and overbank deposition (Benedetti 2003). Therefore, the air (dust), farmland, and river form a collective cycle, which affects the concentration of SPM. In this study, heavy metal content is high after the Beiyun River confluence with Ziya River. In terms of spatial distribution, the pollution in the downstream area is higher than that of the upper and middle reaches. Gross domestic product (GDP) is a critical indicator to interpret the impact of human production activities on the environment. Specifically, agricultural and industrial activities represent the primary and secondary industries, respectively. The Beiyun River is located in the BTH region, an area in which the economy is rapidly developing and near to the capital, which is highly sensitive to GDP. GDP is a critical indicator to evaluate the impact of human activities on river pollution (Zhang et al. 2017b). The concentration of Cu and Pb are closely related to agricultural and industrial activities. In SPM from Beiyun River, the pollution level of Cu and Pb is 3 < Igeo < 4 and 80 < E < 160, and the potential ecological hazard index takes in strong risk level. Therefore, agricultural and industrial activities have a great impact on heavy metal pollution in Beiyun River.
3.6 Pollution characteristics on SPM and control strategies
Rivers are crucial to agriculture, industry, tourism, transportation, and even flood protection; a balanced river ecology is of great significance to development (Li et al. 2020). Efficient measures must be implemented to manage and monitor pollution from SPM. There are many gates and dams along the Beiyun River, which has a discontinuous flow and a dense population. Furthermore, it contains numerous industrial and domestic sewages from nearby regions. The continuous input of pollution leads to relatively high concentration of heavy metals in rivers, and seriously affects the SPM quality (Huo et al. 2011). Dams accumulate pollutants, while increasing the risk of water and sediment pollution events. For example, heavy metal concentrations from Yulinzhuang sluice and Beiguan sluice to lower point B4 increases significantly. Serious pollution of heavy metals in the lower reaches is extremely high, especially for Zn, As, Cd, and Pb. Of these metals, severe pollution of Cd and Pb is closely related to the production of chemical raw materials in the industrial zone downstream. In addition, Cd and Pb exceeded the standard value in the whole river; therefore, they must be listed in prevention and control objects. The concentration of Cr was between the TEC (43.4 mg/kg) and PEC values (111 mg/kg), thus biological toxicity rarely occurred or even did not occur. Zn is one of the most polluted elements, in the 12 research points, three study points are three times of the background value. Zn pollution is from smelting processing, mechanical manufacturing, galvanizing, organic synthesis, and industrial wastewater discharge. The As concentration exceeded the effective threshold value at some points after leaving the Tianjin urban area, which had adverse effects on benthos by physical digestion in the digestive tract. The significant increase of heavy metal concentration in the estuarine zone may be related to the effects of coastal soil texture and seawater intrusion (Shan et al. 2016).
SPM is a typical heavy metal adsorbent, which makes it an effective way to reduce heavy metal pollution by controlling the concentration (Laurent et al. 2009). In view of the excessive heavy metals in SPM of Beiyun River, the management of external pollution should be increased, and the sources of heavy metal pollution must be reduced. Intercepting installations in all drainage outlets could control the pollution from SPM in this area (Jeong et al. 2020). In the upper reaches, the focus should be on reducing soil erosion by cultivating forests that could increase soil and water conservation, strengthening the protection of existing forest species to reduce soil and water loss and wind sand erosion, and preventing the increase of SPM caused by soil erosion along the Beiyun River (Vercruysse et al. 2017). Soil and water conservation can effectively reduce sediment mixing, and sediment quality can be reversed through forests (Turner and Millward 2002). As for river systems, desilting is a direct method to remove heavy metal pollution (Liu et al. 2016). Machines like silt cleaners and dredgers have been widely used, and desilting is an important method for effectively removing internal pollution, avoiding resuspension, and reducing the threat to ecosystem balance, which can effectively reduce the concentration of heavy metals. In terms of heavy metal limiting, reducing industrial pollution emissions, such as petrochemical and metallurgical industries, in surrounding areas will be an significant strategy.