Distribution pattern and ecological risk assessment of heavy metals in Henan section of the Yellow River Open Access

As the second largest river in China, the Yellow River has a length of 5,464 km and a drainage area of 752,443 km² (Han et al. 2020). A total of 15,274 water storage projects have been built in the Yellow River basin, with a total storage capacity of 90.93 billion m³ (Cui et al. 2021). The Yellow River is the most irreplaceable water resource for northwest/north China with over 14 existing dams, playing an important role in daily lives and productive activities for local residents and industries (Su et al. 2018). Especially, it plays an important role in water supply, including drinking, agricultural and industrial water supply, for Henan province (Liu & Xia 2004; Chen et al. 2020; Li et al. 2021). Its length in Henan is 711 km, and the annual water flow through the province is 20 billion m³, which greatly promotes economic development in Henan (Liu et al. 2021a, 2021b).

In recent years, the rapid economic development of Henan has inevitably affected the water environment of the Yellow River basin. The occurrence of various pollutants, such as antibiotics, organophosphorus flame retardants (OPEs) and polycyclic aromatic hydrocarbons (PAHs), has been reported in the river (Su et al. 2017; Yu et al. 2022). Among all the pollutants, HM pollution has always attracted intense public attention (Islam et al. 2018; Zhang et al. 2018; Li et al. 2022; Zhao et al. 2022), since it has the characteristics of non-degradability, high toxicity and bioaccumulation, and poses a great threat to human health through the food chain (Zeng et al. 2020). The adverse effects of HMs in the environment are mainly related to their total concentration and chemical partitioning (Zeng et al. 2020). For example, previous research has revealed that HMs (such as Cu, Cd, Ni, Pb) can produce toxic effects on organisms, including lipid peroxidation and thiol protein consumption, as well as reactions with nuclear proteins and DNA, which cause damage to biological macromolecular substances (Morcillo et al. 2016; Y. Hong et al. 2020; Z. Hong et al. 2020).

Sediment serves as an important pool for HMs in aquatic ecosystems and can reflect the long-term pollution status for pollutants (Zhang et al. 2021). Meanwhile, it turns into the HM source and releases HMs to the water body when the external physicochemical parameters change, causing secondary pollution (Luo et al. 2020; Zhang et al. 2021). Moreover, sediments are capable of reflecting long-term pollution status and are easy to incubate for pollutant analysis (Yang et al. 2017; Zhang et al. 2021). Therefore, the importance of sediment in HM accumulation and migration cannot be neglected in the aquatic environment.

The HMs in sediments can be generally divided into stable and unstable states. The bioavailability, migration ability and potential toxicity are quite different between these two forms (Fang et al. 2019; Sun et al. 2019; Shao et al. 2020; Pan et al. 2022). The HMs in stable states cannot be biologically utilized, so they can be regarded as basically harmless. In contrast, the strong mobility and biological utilization potentiality of HMs in unstable states indicate potential human health risks (Ferrans et al. 2021; Hao et al. 2021). Therefore, HM fractionation analysis is of great significance to assess pollution degree and clarify risk characteristics.

HM speciation in river sediment has been extensively explored. For example, Xia et al. (2018) investigated the distribution pattern and fraction content of HMs in the Wen-Rui Tang River and found that several HM fractions are strongly correlated with industrial land proportion. Meng et al. (2021) investigated HM pollution in the Yongding River and found that Cu and Pb have higher bioavailability compared with other elements. Budianta (2021) studied the HM pollution level of the Tajum River and found higher exchangeable and carbonate fractions for Cu and Pb. Zeng et al. (2020) found that the health risk caused by Cd is generally high, and HM pollution is closely related to anthropogenic activities.

However, the pollution characteristics of these elements in the Henan section of the Yellow River have not been fully understood. In this research, the BCR method was adopted to determine the speciation and distribution of HMs. The I_geo and ecological hazard index method were adopted to reveal the potential ecological risks caused by HMs. Moreover, the pollution sources of HMs were clarified by the principal component analysis (PCA) method. The results provide a reference for preventing and controlling HM pollution in the Yellow River.

The bedrock in the Yellow River basin was formed in the Archean and Proterozoic. The surface outcrops and shallow rock formations that are widely distributed in the Yellow River basin formed from the Precambrian to Quaternary. Moreover, there are numerous metallic minerals in the Yellow River basin (Ma et al. 2020), indicating the association of the accumulation of precious and other metals with the sulfide minerals pyrite, chalcopyrite, sphalerite, galena, and arsenopyrite (Budianta 2021).

Eleven sediment samples were collected to investigate the variation of HMs in the sediment in the Henan section of the Yellow River. Especially, H₈ was set to assess the influence of a tributary to the mainstream of the Yellow River. The sampling was conducted from 28 to 29 June 2021 at the specific locations shown in Figure 1. H₁ means the first sampling point, and so on. H₁₁ is the last point in Henan province. The sediment sampler (KH0201, Xinbao, China) was used to collect the surface sediment at 10 cm. All the samples were immediately sent to the laboratory in an icebox at a temperature of 4 °C. All the samples were freeze-dried by a low-temperature freeze-dryer (Alpha 2-4 LSCbasic, Christ, Germany).

The content of six HMs, specifically zinc (Zn), lead (Pb), cadmium (Cd), nickel (Ni), manganese (Mn) and copper (Cu), was determined. The studied metals were chosen based on: (1) toxic response factor of HMs (Hakanson 1980); (2) previous studies suggesting that Cd, Pb, Cu, Zn, Ni and Mn are possible pollutants in the sediment of the Yellow River mid-reach (Cheng et al. 2015; Yan et al. 2016; Y. Hong et al. 2020; Z. Hong et al. 2020).

Speciation analysis was conducted with the BCR approach (Chen et al. 2022). In brief, exchangeable and carbonatebound HMs (B₁ state, exchangeable/acid-soluble fraction) were extracted with 0.11 mol/L acetic acid solution. Then, iron–manganese-oxide-bound HMs (B₂ state, reducible fraction) were extracted with 0.5 mol/L NH₂OH.HCl solution, while organic and sulfide-bound HMs (B₃ state, oxidizable fraction) were extracted using 8.8 mol/L H₂O₂ and 1 mol/L NH₄Ac. Finally, the residue (B₄ state, residue fraction) was eluted with HNO₃-HCl-HF into the digestion tank and digested with a microwave digester (Tankbasic, Sineo, China). An inductively coupled plasma optical emission spectrometer (ICP-OES, ICAP pro, Thermo Fisher Scientific, USA) was used to determine the HM concentration.

In this research, the stream sediment standard reference sample (GSD-5a) was used as the quality control sample, and the whole blank was implemented to guarantee the quality in pretreatment and morphological determination. A calibration blank (2% HNO₃) was run before the standards, and a multi-element solution was used (iCAP 6000series, Thermo Fisher Scientific, USA). Three duplicated determinations were performed for the HM concentration among all the samples. The sum of all fraction contents was compared with that of the reference material to form the recovery rates, which were 88.2%–120.6% for the six HMs. All the data of the selected HMs is presented in Table S2.

HM risk assessments were conducted by Microsoft Excel 2016. Spearman correlation analysis and principal component analysis (PCA) were conducted using SPSS 25.0.

Table S5 and Figure S1 provide a statistical summary of the HMs (Mn, Zn, Pb, Ni, Cu and Cd) in the Yellow River sediment. As can be seen, the elemental concentrations are 345.76–875.42 mg/kg for Mn (509.04 mg/kg on average, similarly hereinafter in this paragraph), 46.33–79.56 mg/kg for Zn (68.27 mg/kg), 23.94–64.43 mg/kg for Pb (32.80 mg/kg), 20.26–46.72 mg/kg for Ni (28.81 mg/kg), 9.36–40.96 mg/kg for Cu (18.64 mg/kg), and 0.13–0.19 mg/kg for Cd (0.16 mg/kg). Mn is the dominant HM among all selected HMs, of which the content is higher than that of the sum of the other HMs. The result is similar to that of previous research, and the high background value of Mn might be the underlying reason (Yang et al. 2016; Zhu et al. 2017).

Among the six HMs, the variation coefficient of Cu from each sampling point is the highest (48.13%), indicating that the content of Cu varies in a wide range (Table S5). Moreover, the result indicates the content of Cu is unbalanced in terms of its spatial distribution, which may lead to high local content. In contrast, the variation coefficient of Cd is the lowest, indicating a relatively uniform distribution of this element in the Yellow River. The variation coefficient of other HMs ranks from high to low as Pb > Mn > Ni > Zn. The HM contents in the Yellow River are similar to those in the reservoir of Henan province (Li et al. 2019a; Li et al. 2019b). Moreover, the contents are lower than those in the urban lakes of China (Cheng et al. 2020). Except that the concentration of Cd is significantly higher than the background value (1.43 times higher than the background value), the average content of other HMs involved in this research is only slightly higher than the background value of HMs in the soil of Henan province. The above results suggest the modest overall HM pollution of the Yellow River sediment in Henan section.

The highest and lowest total HM content are found at H₈ and H₁₁, respectively. The HMs content before H₈ has no obvious variation pattern in general. However, a rapid increasing in HMs content is found at H₈. In particular, the highest values of Zn, Pb, Ni, Mn, and Cu are found at H₈, and the Cd content of this sampling point is also high (the fourth highest). HMs content in the sampling points after this point decrease gradually and finally decrease to the lowest at H₁₁. The above results indicate that the HM pollution level in H₈ is more serious than that at the other sample points selected in this research. H₈ is the place where the Sishui River flows into the Yellow River, indicating that the inflow of tributaries may be an important reason for HM pollution in the Yellow River.

The speciation analysis results of HMs are shown in Figure 2. The B₁ fraction of HMs accounts for 34.85% of the total amount, which is lower than that of other environmental media. The detection rates of Ni (100%), Mn (100%) and Zn (81.82%) are high, while the detection rates of the B₁ state of Cd (0%) are low. The average value of the B₁ state of Mn is 197.27 mg/kg, accounting for 38.72% of the total amount of Mn, indicating that the B₁ state content of Mn accounts for a high proportion in the sediments of the Yellow River. The B₁ state proportion of the other HMs in the total content ranks from large to small as Ni (2.82%) > Zn (2.01%) > Cu (1.22%) > Pb (0.53%) > Cd (0%), indicating that the proportion of the B₁ HMs state is generally at a low level. The B₁ state is extremely sensitive to changes in external water environment, and can be released into water under weak acid and neutral conditions. Thus, the state has high bioavailability and can have the greatest influence on the environment among the four states (Liu et al. 2018; Tang et al. 2018; Alfaro et al. 2022).

The bioavailability of Mn is the strongest among all HMs in this research (37.79%–73.02%, with the average proportion being 51.75%). Mn is an essential element for plants and can promote their growth at low concentrations. However, high-concentration Mn may have negative effects on plants (Santos et al. 2017). Compared with other environmental media, both the detection rate and content of the B₁ state in the Yellow River basin are low (Bi et al. 2018; Gao et al. 2018). Therefore, the pollution caused by the B₁ state of the Yellow River basin is relatively low.

The B₁, B₂ and B₃ states are collectively referred to as available HMs (Tang et al. 2018; Shi et al. 2022). Available HMs can be utilized indirectly or directly by organisms and are the main sources of HM pollution in sediments (Saleem et al. 2018). The detection rate and average proportion of the B₂ state are 100% and 17.01%, respectively, significantly higher than those of the B₁ and B₃ states. Similar to the B₁ state, the B₂ state HMs can also be directly utilized by organisms and cause harm to organisms (Yu et al. 2021). Therefore, the high detection rate and content of the B₂ state deserve more attention. It is worth noting that the average proportion of the B₂ state of Pb accounted for 35.92% of the total amount of the element, and the potential health risk should receive more attention. Previous studies have shown that Pb can be strongly adsorbed by Fe/Mn, and the adsorption behavior is affected by organic matter and soil viscosity (Soliman et al. 2018). The high B₂ state proportion found in the Yellow River is consistent with research on reservoir and river sediments (Wang et al. 2020), but different from that on marine sediments. The difference in sedimentary physicochemical properties may be an important reason for this difference (Delgado et al. 2011; Pignotti et al. 2018).

The B₄ fraction mainly contains primary and secondary minerals and is hard to be released in solution in the sediments (Lasheen & Ammar 2009). The average proportion of HMs ranges from 48.24% to 89.34%, with an average value of 71.04%, indicating that the HMs of the Yellow River are mainly contained in the B₄ fraction.

There is an obvious difference in HMs speciation distribution in the Yellow River. The B₁ fraction of Zn is the highest at H₉. However, for the other HMs, the value of B₁ fractions is the highest at H₈, which is the junction of the Sishun River and Yellow River. Moreover, the B₄ fractions of two HMs (Pb and Cd) are the lowest at H₈. B₁ fractions of HMs have a close relationship with anthropogenic inputs (Wang et al. 2020), and the speciation results indicate that the branch afflux might be one of the important ways for anthropogenic activities to influence the Yellow River.

The evaluation results of the I_geo and potential ecological risk method are demonstrated in Figure S2 (a) and (b), respectively. Notably, both the methods indicate that the content of Cd is relatively high among all investigated HMs. The result is consistent with previous studies investigating river, lake and coastal areas. This indicates that HM is the main factor causing ecological risk in surface sediments (Xiao et al. 2019; Rao et al. 2021; Arisekar et al. 2022). We further investigated the Cd pollution level in the rivers of China (Figure S3), and found that Cd is the main potential ecological risk contributor. Although the Cd content of the Yellow River meets the value of the Soil Environmental Quality Risk Control Standard for Soil Contamination of Agricultural Land (GB 15618-2018), the value of the Cd is still higher than the background value of Henan province. Moreover, the toxicity of Cd is strong. The above factors lead to the high ecological risk of Cd.

Despite the ecological risk caused by Cd, the total ecological risk index of HMs at each sampling point ranges from 75.76 (H₇) to 120.05 (H₈), with an average value of 92.22, which is far lower than the threshold of medium risk. Moreover, this research compares the value of each HM in the Yellow River with other rivers in China (Figure S3). It is found that the value in the Yellow River is lower than that in the other rivers in most cases. In addition, the HM content of this research is much lower than that of the Inner Mongolia section of the Yellow River, which has rich mineral resources and intensive human activities (Hao et al. 2021), and the geological conditions and human activities might the underlying reason. In conclusion, the results indicate that the potential ecological hazards caused by HMs are at a low level in the Yellow River sediment in the Henan section. Previous research has indicated that anthropogenic activities could have significant influence on HM pollution in the environment (Wong et al. 2017; Qu et al. 2018; Zeng et al. 2020). In recent years, Henan province has gradually strengthened the control of soil pollution sources, regulating the pollution sources of HMs such as the steel, non-ferrous metals and dyeing industries (Ding & Meng 2021). The results of this research indicate that the above policies can benefit the water environment of the Yellow River in Henan province (Ding & Meng 2021).

Spearman correlation was adopted for the potential source identification of HMs in sediment samples in the Yellow River. The HMs have significant positive correlation, which usually comes from similar sources. Table S6 displays the correlation analysis results of the investigated HMs. The strong positive correlations are among Zn, Pb, Ni, Mn and Cu (the minimum correlation coefficient is 0.782, p < 0.01), indicating that these HMs may have similar sources. However, there is no significant correlation between Cd and the other HMs.

PCA was adopted to further explore the pollution sources of HMs (Kumar et al. 2017; Islam 2021). The KMO adequacy is 0.721 and the significance value is 0.000, respectively (Table S7), suggesting the correlation among HMs, which is consistent with the correlation analysis result. Two principal components (PCs) were extracted from the six HMs in the sediments and the two PCs together contributed 96.02% of the total variance. Zn, Pb, Ni, Mn and Cu have high loadings on PC1, explaining 79.19% of the variance. Zn, Pb and Mn in the riverine ecosystem are considered to be from parent material weathering and the pedogenic process, or sorption and desorption behavior (Currell et al. 2011; Díaz et al. 2016). Moreover, the results from section 3.1 of this research suggest that HM pollution is generally light in the Yellow River. Also, the average values of Zn, Pb, Ni, Mn and Cu are close to the background values and lower than those in the research of Feng et al. (2015), in which the sediment samples were collected in 2006 from the Henan section of the Yellow River. The result indicates that the pollution level of HMs has been alleviated in recent years, and the water body in the basin receives less influence from anthropogenic activities. In addition, a recent study reported that sediment properties, such as TOC and TN contents, are low in the channel sediment of the Yellow River (Hou et al. 2021). Especially, the content of TN is at the first level according to investigation and evaluation on sediment pollution of rivers, lakes, and reservoirs in China (Table S8) (Li et al. 2019a; Li et al. 2019b). The water quality indices, including pH, TN and TP, are at the second level according to Environmental Quality Standards for Surface Water (GB 3838-2006) (Song et al. 2020). The above results combined with the HMs content result of this research indicate that the pollution threat of the Yellow River has gradually been alleviated.

This research investigates the pollution levels of Zn, Pb, Cd, Ni, Mn and Cu in the sediment of the Yellow River. The main conclusions of this research are summarized as follows.

• 1.

  The B₁ fraction of Mn and the B₂ fraction of Pb are high, and the potential health risk requires more attention.

• 2.

  The potential ecological risk caused by HMs is low in general, while the risk caused by Cd cannot be neglected.

• 3.

  Most of the HMs mainly come from natural formations.

This work was supported by the National Natural Science Foundation of China (42007209) and the Nanhu Scholars Program for Young Scholars of XYNU.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.