Water quality analysis and health risk assessment of the Lanzhou section of the Yellow River Open Access

In the 21st century, the demand for water resources has increased year by year because of population growth and continuous development of industry (Zhao et al. 2018). According to forecasts, demand for water resources will reach 6.9 × 10¹² m³ by 2030, which is approximately 140% of the current supply (Gilbert 2010). What's worse, with the growing demand, severe water pollution has contributed to the global growth of water shortages (Gilbert 2010).

The safety of drinking water is directly related to people's health. However, with the rapid development of economy and urbanization, human activities (metallurgy, mining, coal combustion, metal smelting and petrochemical plants) have brought many toxic elements into the river water system (Meng et al. 2016). These factors increase toxic substances (heavy metals and toxic organic substances) in the water, endangering human health (Norback et al. 1995; Krishna et al. 2009; Cakmak et al. 2014). To be exact, industrial and personal wastewater is polluted by heavy metals and organic wastes that have been directly discharged into rivers and lakes (Cakmak et al. 2014; Wang et al. 2015; Bamuwamye et al. 2017). Nowadays, almost all rivers in industrial cities that flow through economically developed areas are polluted by heavy metals and organic pollutants to some extent (Islam et al. 2015; Zuo et al. 2016). In addition, 90% of rivers flowing through towns are significantly polluted by ammonia, in particular the Yellow River (Li et al. 2006). Therefore, in recent years, many scholars at home and abroad have been keen to study the relation between drinking water pollution and health risks. That means understanding the content and source of toxic substances in rivers is of great importance for the assessment of risk to human health.

Lanzhou, as an important industrial town in northwest China and the only provincial capital city where the Yellow River flows, covers a population of over 3.6 million (Bai et al. 2016). The city's daily living water source for residents comes from the Yellow River, in which domestic sewage is also discharged. Many industrial enterprises, such as the petrochemical industry, are distributed along the river and use it for water and drainage (Xu et al. 2006; Wang et al. 2021). Therefore, the water quality of the Yellow River is related to the health of thousands of families. In 2013, benzene pollution occurred in Lanzhou City because upstream enterprises polluted the source water, which seriously affected people's lives and productivity.

A health risk assessment is done to evaluate the risk to the health of individuals exposed to the factors by estimating the probability of adverse effects on human health. The main risk factors considered in the health risk assessment process are heavy metals, pesticides and endocrine disruptors. Individuals are exposed to the risks through drinking water and skin contact (Giri and Singh 2014; Moolgavkar et al. 2014). The risk types of harmful metal elements are divided into health risks caused by chemical carcinogenic metals and health risks caused by non-carcinogenic metals. Based on the testing done by the World Health Organization (WHO) and the International Agency for Research on Cancer (IARC), a comprehensive analysis and evaluation of the degree of reliability of the carcinogenicity of chemical substances is done, in which 10 metal elements are studied, measured and divided into chemical carcinogenic metal elements (As, Cd and Cr) and chemical non-carcinogenic metal elements (Al, Cu, Fe, Hg, Mn, Pb and Zn).

At present, the research of water quality evaluation methods mostly focuses on their advantages and disadvantages, improvement and application of single factor evaluation, Nemerow index evaluation and fuzzy comprehensive evaluation (Yan et al. 2013; Liu et al. 2015; Shang et al. 2015; Li et al. 2016). However, studies on the combination of water quality assessment and health risk assessment in the evaluation of the environment quality of the Yellow River are few. Combining the water quality assessment with the health risk assessment could help determine the water environmental quality and the water environmental status, strengthening water risk management and control of the Yellow River. It would also be helpful in planning and implementing corresponding pollutant control strategies. In this paper, the Nemerow index method is combined with health risk assessment. The routine indicators of five key monitoring sections in the Lanzhou section of the Yellow River are sampled and tested in flood, dry and normal water periods respectively, while the water quality and health risk assessment are carried out. The goal was to provide comprehensive reference materials for the water environment quality assessment and scientific basis for water environment risk management and control, water resources protection, guaranteeing human health in the study area.

The region is in the upper reaches of the Yellow River and extends from Xincheng in Xigu County on the western side to Qingcheng in Yuzhong County on the eastern side. The research area has a temperate continental climate with an average annual temperature between 9.1 °C and 10.9 °C. It includes agricultural and chemical industries, among others. The research area is shown in Figure 1.

To evaluate water quality, five representative water samples were collected from the Lanzhou section of the Yellow River between September 2018 and March 2019. The first sample was from Xincheng Bridge (XC, 36°11′2″, 103°28′3″), with the second from Yintan Bridge (YT, 36°4′20″, 103°41′0″), the third from Zhongshan Bridge (ZS, 36°4′37″, 103°48′49″), the fourth from Baolan Bridge (BL, 36°30′25″, 103°56′40″) and the last from Qingcheng Bridge (QC, 36°20′17″, 104°11′10″).

In this study, 54 samples from each of the six sections during different river periods were collected from September 2018 to March 2019. Among them, three were taken from each section during the wet period (September), normal period (December), and dry period (March) respectively. The Water and Wastewater Monitoring and Analysis Method was used, which was issued by the Ministry of Environmental Protection of the People's Republic of China (MEP) in 2002, formerly known as the State Environmental Protection Administration (SEPA). The samples were analyzed through a combination of self-testing and submission for inspection. The water quality pH, water temperature and turbidity index were measured on site. This was done using titration, Nessler's reagent method, TN detection method, TP detection method, nitrate-nitrogen addition measurement method, nitrite detection method and detection of COD_Mn, NH₃-N, TN, TP, NO₃-N and NO₂-N in different water samples. Heavy metals tested by testing companies, including Chromium (Cr), Copper (Cu), Zinc (Zn), Selenium (Se), Arsenic (As), Hydrargyrum (Hg), Cadmium (Cd) and Lead (Pb).

In the formula: C is heavy metal content in water samples (mg/L); IR is daily water consumption for adults, taken at 2.0 L/d; EF is exposure frequency, taken as 365 d/a; ED is exposure duration, and non-carcinogens is for 31.3 years and carcinogens for 73 years; BW is per capita weight, taken as 60 kg; AT is average exposure time: 11,268 days for non-carcinogens and 26,645 days for carcinogens. The R_fD and SF values are derived from the Environmental Protection Agency (EPA) Integrated Risk Information System (IRIS) and related documents (US EPA 1996). The specific parameter values are shown in Tables 3 and 4.

The test results for the conventional indicators of different sampling sections are shown in Table 5. However, cyanide, volatile phenol and anionic surfactant were not detected and are not listed in Table 5. According to the Class III water quality standard limit specified in the ‘Environmental Quality Standard for Surface Water’ (GB 3838-2002, China Environment Protection Department 2002), TN, by the conventional water quality indicators of the five nationally controlled sections, exceeded the standard rate in all of them by 100%. Besides, in measuring the TN concentration of each section, the maximum values reached 2.41, 2.82, 3.26, 2.91 and 3.94 times the standard limit for each. The excess rate of other indicators in the control section was zero, and the limit of the environmental quality standard for surface water was met. A concentration of petroleum pollutants was detected in Baolanqiao and the Qingcheng Bridge National Control Section, but the concentration was lower than the limit specified in the Surface Water Class III Environmental Quality Standard. The coefficients of variation of NH₃-N and TP in the Xincheng Bridge, Yintan Bridge, and Baolan Bridge sections all exceeded 50%, and the NH₃-N coefficient of variation for Zhongshan Bridge exceeded 50%, showing that NH₃-N and TP are present at Xincheng Bridge, Yintan Bridge and Baolan Bridge. The sampling sections at Lanqiao and Zhongshan Bridge all showed a certain degree of change.

The statistics on the mass concentration of metal elements in the surface water of different sections are shown in Table 6. However, metal elements such as chromium, copper, zinc, selenium, arsenic, mercury, cadmium, and lead have not been detected under the ‘Surface Water Environmental Quality Standards’. The data are calculated by adding together the machine detection limit and L. Arsenic was only detected in the Yintan Bridge section. Therefore, in the Lanzhou section of the Yellow River, there is no problem with metal and non-metal elements exceeding the standard. Arsenic was detected in the Xincheng Bridge to Yintan Bridge section, but the concentration was much lower than the standard value. The maximum concentration of arsenic was 0.01 times the detection limit, and the coefficient of variation was 20.33%, indicating that the sampling was relatively stable. This analysis was mainly related to the Xigu District, which is in the upper reaches of the Lanzhou section of the Yellow River. This area is a key national petrochemical industrial base with many industrial enterprises, such as petroleum, chemical, machinery and metallurgy, which may cause an increase in arsenic content in the YT section, but because of water self-purification and sedimentation, arsenic was not detected in other sections.

From the distribution characteristics of the average mass concentration of the conventional surface water quality indicators across different monitoring sections (Figure 2), it can be found that the pH, TN, COD_Mn and BOD₅ contents of each section are the highest, and the pH value of the Lanzhou section of the Yellow River does not change much. Entering the Lanzhou section Xingcheng bridge (XC), the average pH value is 8.22. Outside Lanzhou, it is 8.37, which is stable. The TN concentration increased slightly in the Qingcheng Bridge section. The average TN concentration in the entry section was 2.02 mg/L, while the average TN concentration in the exit section was 2.78 mg/L. Therefore, the TN concentration in the section of the Yellow River flowing through Lanzhou increased by 0.76 mg/L. The average concentration of COD_Mn showed a downward trend along the way, going from 2.06 mg/L at the entry section to 1.37 mg/L at the exit section, which is a decrease of 0.69 mg/L. BOD₅ changed a little, with an average concentration of 2.36 mg/L at the entry section and 2.70 mg/L at the exit section.

Without considering the effect of TN, the results of the Merrow composite index method for different sections of the Lanzhou section of the Yellow River are shown in Table 7. The evaluation result is that the Zhongshan Bridge Section (ZS) has a maximum score of 1.04. According to the classification results, the water quality of each section in the study area is uniform. The water quality is good, and the evaluation results of the entrance and exit sections of the Lanzhou section are consistent, which reflects the conclusion in Figure 2. When considering the impact of TN, the evaluation results of each section are poor, because TN exceeded the standard when entering Lanzhou. The major reason for this may be related to the agricultural non-point source pollution in Linxia City, Gansu Province and Qinghai Province before entering Lanzhou. Additionally, there are many pastoral areas, and pollution is caused by large amounts of nitrogenous substances entering the river after animal dung and plants rot.

In the light of the analysis in Tables 5 and 6, and Figure 2, the amounts of pH, petroleum, BOD₅ and NH₃-N increase slightly after the Yellow River flows through the study area, though all within the surface water Class III limits for bodies of water. Moreover, fluoride content did not change, the concentration of COD_Mn and TP decreased, and the metal and non-metal elements from the detection phase remained unchanged and undetected. The Lanzhou section of the Yellow River includes the Xigu Petrochemical Area (Zhao et al. 2020) as well as four sewage treatment plants and dozens of flood drains. Agricultural non-point source pollution has not had a significant impact on the water quality of the Lanzhou section of the Yellow River (Zhang et al. 2010).

The average personal health risk from drinking water (Table 8) and skin contact (Table 9) was calculated based on the health risk assessment model by using its parameters and metal element mass concentration data. Since most of the metal elements in the study area were below the detection standards, the undetected metal elements were all measured at the detection limit concentration. The calculated average personal health risk is lower than the actual average personal health risk value.

Table 8 shows the average annual carcinogenic risk to individuals caused by metal elements in drinking water from each part of the Lanzhou section of the Yellow River. The carcinogenic risk of metal elements in each section of the study area is at a medium to high level, while the total carcinogenic risk of each section is 10⁻⁷∼10⁻⁵. The highest total carcinogenic risk is in Yintanqiao (YT), while other sections have equally lower carcinogenic risk. The maximum acceptable risk level recommended by the International Commission on Radiation Protection (ICRP) is 5.0 × 10⁻⁵ a⁻¹. The average annual health risk of As and Cd for each section is in the order of magnitude of 10⁻⁶–10⁻⁷, which is far lower than the recommended maximum risk. The carcinogenic risk comes from metallic Cr, and the average annual risk value of Cr in each section is higher than 5.0 × 10⁻⁵ a⁻¹. The mass concentration of As, Cd, and Cr in each section is lower than the detection limit of surface water type III. Even though the value is lower than the machine detection line, it has a higher carcinogenic risk because the carcinogenic risk is not only related to the mass concentration of As, Cd, and Cr, but also to the carcinogenic intensity coefficient, the average amount of drinking water in the human body, the frequency and duration of exposure, per capita weight and the average human life span. Cr is the greatest source of carcinogenic risk, followed by As and then Cd.

Table 9 shows the average annual personal health risk caused by skin contact with metal elements for each part of the Lanzhou section of the Yellow River. The non-carcinogenic risk of each monitoring section ranges from 10⁻¹³ to 10⁻¹⁰ because the metal elements of each monitoring section are below the detection limit. The health risk value is calculated based on the detection line value, so the total risk value of each section is equal, and the order of magnitude is 10⁻¹⁰, which is less than the acceptable health risk value level of 10⁻⁶ a⁻¹. The order of the sources of health risk for each section is Pb > Cu > Zn > Hg > Se > Cd > Cr > As. These eight kinds of non-carcinogenic metal elements have a small average annual personal health risk to humans and will not cause obvious health hazards to people exposed to them.

The risk value to health that is caused by the metal elements in the Lanzhou section of the Yellow River is at a medium to high level. This is mainly due to the metal Cr. Although most of the metal elements in each section are below the machine detection line, it is still calculated in accordance with the surface water category III limits. Cr has been detected in the Lanzhou section of the Yellow River, and its health risk value is higher. In terms of the ways in which people are exposed to metal elements, the health risks caused by drinking water are five orders of magnitude higher than those caused by skin contact, which is similar to the water quality evaluation results of Zhou et al. (2019) and others in the Xiangshui area of Chongzuo City. This indicates that drinking water is the main form of exposure to metal elements. In terms of carcinogenic risk level, the total carcinogenic risk is five orders of magnitude higher than the non-carcinogenic total risk, which indicates that the main source of health risk in the Lanzhou section of the Yellow River is carcinogenic metal elements, led by Cr. The distribution of carcinogenic risk represents the main health risk in the study area. This result is similar to Zhao et al. (2018) assessment of heavy metal health risks during different seasons in the Yellow River. Therefore, Cr should be the focus of risk decision management.

• (1)

  In the Lanzhou section of the Yellow River, many pollutants, with the exception of total nitrogen, have reached the Class III standard of surface water environmental quality. Toxic substances, such as heavy metals, cyanide, petroleum and anionic surfactant are far below the standard limit. When the Yellow River passes through Lanzhou City, there is no obvious increase in the concentration of various pollutants. The Nemerow index method was used to evaluate this, with the maximum value being 1.04, meaning that the water quality was good.

• (2)

  The total health risk caused by metal elements in the Lanzhou section of the Yellow River is higher than the maximum acceptable risk level (5.0 × 10⁻⁵), and the total carcinogenic risk (higher than 5.0 × 10^-5 a⁻¹) is five orders of magnitude higher than the total non-carcinogenic risk (less than 10^–6 a⁻¹). The health risk of carcinogenic elements is ranked from highest to lowest: Cr > As > Cd, while that of non-carcinogenic elements is ranked as: Pb > Cu > Zn > Hg > Se > Cd > Cr > As.

• (3)

  Based on the Nemerow index method and the health risk assessment, the concentration of conventional pollutants in the Yellow River is low. There are no excessive amounts of heavy metal pollutants in the surface water, though the carcinogenic risk of the Cr element is the highest. Considering drinking water safety, Cr should be the key focus of risk decision management, and Cr pollutants should be controlled to protect drinking water.

This work was financially supported by the National Key R&D Program of China(Grant No. 2019YFC0507405) and the Key Research Program of Gansu (Grant No. 20ZD7FA005) and the National Natural Science Foundation of China (52060012) and Lanzhou Jiaotong University Tianyou Innovation Team (TY202005).

Data cannot be made publicly available; readers should contact the corresponding author for details.