Health risk assessment of nitrate pollution of drinking groundwater in rural areas of Suihua, China

Groundwater is one of the precious natural resources, and the amount of groundwater resources accounts for one-third of the total water resources. Groundwater is closely related to people's production and life (Alireza et al. 2020; Tian et al. 2020a). Groundwater is particularly important in areas with insufficient surface water resources (Zakhem & Hafez 2015; Tian et al. 2021). However, as the drinking water source of 70% of China's population, the groundwater has been polluted (Sun et al. 2020a, 2020b), and the shallow groundwater and deep groundwater polluted by nitrides, phosphates and heavy metal ions have reached 80 and 30%, respectively (Sun et al. 2020a, 2020b). Because groundwater pollution is highly hidden and difficult to detect, it is difficult to control the polluted groundwater (Li et al. 2016).

Nitrate (NO₃⁻) pollution in groundwater is a worldwide environmental problem. Nitrate is easy to dissolve and has strong fluidity (Gu et al. 2013; Zhai et al. 2015; Tian et al. 2020b). It can enter underground through leaching and surface runoff (Li et al. 2015). It is a chemical pollutant that widely exists in groundwater (Hand et al. 2013; Sun et al. 2019). Drinking groundwater polluted by nitrate for a long time will lead to digestive system cancer, methemoglobinemia, blue baby syndrome and other diseases, posing a serious threat to human health (Han 2014; Sun et al. 2017; Chen et al. 2020). Nitrate-contaminated groundwater will cause harm to the human body when it enters the human body; therefore, it is necessary to evaluate the harm and degree of nitrate-contaminated groundwater. HHRA is a qualitative and quantitative assessment method for the hazard degree of polluted groundwater to the human body (Ni et al. 2010; Sun et al. 2022a, 2022b).

In 1983, the National Academy of Sciences (NAS) proposed for the first time a ‘four-step method’ based on hazard identification, dose-effect assessment, exposure assessment and risk characterization to assess the risks of environmental pollutants to human health. Then, the United States Environmental Protection Agency (USEPA) made a more detailed description of health risk assessment. At present, the ‘four-step’ health risk assessment model is applied to the health risk assessment of various pollutants in various environmental media. Saravanan et al. (2022) and Zhang et al. (2021) used models to evaluate the impact of high nitrate in groundwater on human health in different regions.

Suihua region is the main food production base in China. In order to increase agricultural production, farmers use nitrate fertilizers more frequently. In rural areas of this region, groundwater is the main source of drinking water. In order to scientifically prevent and control the health risks caused by excessive use of NO₃⁻ fertilizers in this region, the distribution pattern and pollution status of NO₃⁻ in this region are investigated, and the health risks of NO₃⁻ in drinking groundwater are evaluated. It is essential for the health of the rural population in the main agricultural production areas.

The research area is located in the middle of Heilongjiang Province, China, with the geographical coordinates of 124°13′–128°30′E and 45°3′–48°02′N. The terrain inclines from northeast to southwest, and the overall landform is composed of four parts: low mountains and hills, Piedmont platform, low plain and flood plain. It is a continental climate with four distinct seasons. The average annual temperature is about 2.9 °C, the average annual sunshine time is 2,395 h, the average annual precipitation is 552.5 mm, the average annual evaporation is 1,337 mm and the average frost-free period is 140 days. The river network density in the study area is relatively large, belonging to the Songhua River system, and most of its areas are located in the Hulan River basin, the primary tributary of the Songhua River. The average annual total water resources of the city is 44.77 × 10⁸ m³. The stratum in the area mainly exposes Paleozoic carbonate, clastic rock and metamorphic volcanic rock, Mesozoic volcanic rock series, sedimentary rock series and Quaternary Holocene alluvial-proluvial layer. The main fault in this area is the Hulan River fault, which extends in the northeast direction. According to the occurrence conditions and hydraulic characteristics of groundwater, the groundwater in Suihua can be divided into three categories: Quaternary loose rock pore water, Cretaceous loose rock pore fissure water and bedrock fissure water. Infiltration recharge of atmospheric precipitation is one of the recharge sources of groundwater. The runoff and discharge of groundwater are mainly affected by the characteristics of aquifers, and then by topography, geomorphology, geological structure, characteristics of aeration zone and other factors.

No. 1 plastic bottle, volume 2.5 L, analysis types: TDS, CO₂, CaCO₃, H₂SiO₃, Li⁺, Na⁺, NH₄⁺, F⁻, Cl⁻, NO₃⁻, SO₄²⁻, stored at room temperature; No. 2 plastic bottle, with a volume of 500 ml. Analysis types: Cr, Fe, Li, Mn, Ni, Sr. Add 2–3 ml HNO₃ (1:1) protective agent and store at room temperature; No. 3 plastic bottle, volume 250 ml, analysis type: Se, As, with 1 ml HCl (1:1) protective agent added, stored at room temperature; No. 4 brown glass bottle, volume 250 ml, analysis type: Hg, with 8–10 ml of protective agent (5% nitric acid as a solvent, potassium dichromate concentration is 0.5 g/L) added, and stored at 4 °C at constant temperature, and sent to the laboratory within 24 h. The water sample testing was completed by the Harbin Natural Resources Comprehensive Survey Center Laboratory of the China Geological Survey.

The measurement method for TDS is the gravimetric method, which uses instruments and equipment such as a balance, oven and evaporating dish. CO₂ and CaCO₃ are measured using the titration method using instruments and equipment such as burets and triangular flasks. The determination method of H₂SiO₃ is silicon molybdenum yellow spectrophotometry, using instruments and equipment such as a spectrophotometer and a colorimetric cup. The determination method for F⁻, Cl⁻, NO₃⁻, SO₄²⁻, Na⁺, Li and NH₄⁺ is ion chromatography (IC), using instruments and equipment such as IC, cation protection and separation columns, and cation inhibition columns. The determination method of Cr is the catalytic polarographic method, using instruments and equipment such as a polarograph and an exchange column. The method for measuring Hg is cold atomic absorption spectrophotometry, which uses instruments and equipment such as an atomic absorption spectrophotometer and its mercury measuring accessories or instruments, a mercury hollow cathode lamp and a mercury absorption cell. The determination method for Fe, Mn and Sr is by inductively coupled plasma atomic emission spectroscopy, using an instrument and equipment called an inductively coupled plasma atomic emission spectrometer. The determination method of As is hydride generation atomic fluorescence spectroscopy, using instruments and equipment such as an atomic fluorescence spectrometer, an arsenic special hollow cathode lamp, and a steel cylinder argon (purity ≥ 99.99%). The method for determining Se content is the catalytic polarographic method, which uses instruments and equipment such as a polarograph and a three electrode system (drop mercury, silver or calomel electrode and platinum wire).

The water sample test shall be completed by a testing unit with national testing qualification. Na⁺, K⁺, Ca²⁺ and Mg²⁺ were determined by ICP-OES, Cl^– and SO₄^2– were determined by IC, H₂SiO₃ was determined by ultraviolet spectrophotometry (UV), total dissolved solids (TDS) was determined by gravimetric method (GR) and HCO₃⁻ was determined by volumetric method (VOL).

The reliability test of the water sample data is carried out by using the anion and cation balance test methods, and the absolute value of the relative error E of the anion and cation balance is less than 5% as the reliable data. After the test, all data are reliable data.

The groundwater quality health risk assessment is an effective method to quantitatively evaluate the hazard degree of hazardous substances in groundwater to the human body. Generally, the assessment starts with hazard identification and then carries out a dose-effect analysis to evaluate exposure and risk characterization (Zhou et al. 2016; Tian et al. 2020c). On the basis of field investigation, sampling and indoor analysis (hazard determination), combined with the information on the harmful effects of chemicals on human health provided by the International Center for Research on Cancer (IARC), determine the health effects of nitrate on humans (dose-effect relationship) (Chen 2010; Chen et al. 2016).

At present, there are many risk models for evaluating the hazards of groundwater pollutants to human beings, among which the HHRA model recommended by the USEPA (Chen et al. 2017; Zhai et al. 2017; Javaid et al. 2019) is the most widely used in the groundwater pollutant risk assessment (Su et al. 2013; Zhang et al. 2017; Mahmoud et al. 2018). In this paper, the health risk caused by nitrate pollution in groundwater in the Suihua area is scientifically evaluated by using the evaluation model.

The first step in HHRA is to identify hazards, that is, to determine the potential adverse effects of human intake of hazards, the possibility of such adverse effects and the certainty and uncertainty of such adverse effects. The objective is to evaluate the evidence weight of adverse health effects according to the evaluation results of all existing toxicity and action mode data.

In order to achieve this goal, more detailed site-related data and historical information are needed; Concentration data of pollutants in site groundwater and other samples; Analysis data of physical and chemical properties of the site; Climate, hydrological and geological characteristics information and data of the site (location); Relevant information such as land use mode, sensitive people and buildings of the site and surrounding plots.

The acceptable standard value for nitrate concentration is 10 mg/L (USEPA 2001; Yang et al. 2012).

Exposure assessment is a process of measuring, estimating or predicting the intensity, time and frequency of people's exposure to pollutants in the medium. The exposure assessment is the quantitative basis for risk assessment, which is mainly the assessment of exposure environment, environmental concentration, receptor exposure route, environmental medium and exposure.

The meanings of other parameters are shown in Table 1.

In the above three formulas, each parameter represents the following meanings: is the non-carcinogenic oral hazard coefficient (dimensionless); is the non-carcinogenic skin hazard coefficient (dimensionless); is the reference dose of drinking water (mg/kg/d), taken as 1.6; is the reference dose absorbed by skin (mg/kg/d), taken as 1.0; is the intake dose of drinking water (mg/kg/d); is the skin absorbed dose (mg/kg/d) and is the total risk coefficient (dimensionless).

For the groundwater risk control value based on the non-carcinogenic effect, the non-carcinogenic risk threshold recommended by USEPA is 1 (Huang et al. 2017; Wu et al. 2018; Javed et al. 2019).

In order to provide targeted and effective protection according to different groups of people, this paper carries out NO₃⁻ risk assessment for infants (1 year old and below), children (2–17 years old), adult males and adult females, respectively (see Table 1 for specific parameters).

The distribution of groundwater chemical concentration and the correlation of its components were analyzed by SPSS 19. This method reveals the correlation between different chemical components of groundwater.

The quality standard for groundwater of the People's Republic of China is adopted for evaluation (GB/T14848-2017) (QSGC).

Then, according to the F-value, the groundwater quality is classified according to the method specified in the standard (see Table 3).

The minimum, maximum, average, standard deviation and coefficient of variation of substances in groundwater were statistically analyzed using SPSS software. The size of the coefficient of variation showed the uniformity of substance distribution. The smaller the coefficient of variation, the more uniform the substance distribution was. The larger the coefficient of variation, the more uneven the substance distribution was. The distribution characteristics of substances in the study area are shown in Table 4.

It can be seen from Figure 4 that the single index evaluation of groundwater quality in the study area is generally good, most of the indicators meet the Class I water standard, the vast majority of indicators are in Classes I–III, and only six indicators meet the Class V water quality standard, namely: Fe, Mn, NO₃⁻, CaCO₃, F⁻, pH, the order of exceeding the standard and the size of exceeding the standard are Fe > Mn > NO₃⁻ > CaCO₃ > F⁻ > pH, 21.95, 17.07, 17.06, 7.32, 4.88 and 2.44%. There are four indicators that meet the Class IV water quality standard, namely TDS, SO₄²⁻, NH₄⁺ and Ni. The order and ratio of exceeding the standard are: TDS > SO₄²⁻ > NH₄⁺ > Ni, 12.20, 9.76, 7.32 and 2.44%.

According to the calculation results, the quality of groundwater in the area can be divided into four levels, of which Class III water does not. In general, the quality of groundwater is worrying. Class I water accounts for 2%, Class II water accounts for 5%, Class IV water and Class V water account for 93%, of which Class IV water accounts for 71% and Class V water accounts for 22%. Classes IV and V water are greatly affected by human activities, especially the use of NO₃ fertilizers in agricultural production.

The results show that the nitrate pollution degree of groundwater in this area is relatively high, with zoning. In general, the over-standard rate of NO₃⁻ is 24.67%, and the high content of NO₃⁻ cannot be ignored, and a health risk assessment is required.

The adult female hazard zone (HI > 1) roughly overlaps with the male hazard zone. This finding is consistent with the findings in the Weining Plain in northwest China. There are differences in the health risks of NO₃⁻ pollution to different sex groups, and it emphasizes the importance of gender in the HHRA process.

The health risk area of children is larger than that of adults. About 37.5% of the study area belongs to the child health risk area, and the infant health risk area is the largest. About 45% of the area belongs to the infant health risk area, which is distributed in Wangkui County, Beilin District, Qing'an County and the northeast of Suiling County.

It can be seen from Figures 1, 5 and 6 that in the plain area where the pore water of loose rock is distributed, agricultural production activities are relatively developed and the amount of fertilizer used in farmland is large (Amir et al. 2021) After farmland irrigation or rainfall, most of the water, except for partial evaporation and absorption by plants, seeps into the aeration zone or groundwater. The nitrogen components contained in fertilizer will gradually enter the groundwater environment with the infiltration of groundwater, resulting in nitrate pollution in groundwater (Anju & Kavita 2022). Therefore, compared with the mountainous areas with higher altitude and larger topographic gradient, the groundwater in the alluvial-proluvial plain areas with lower altitude and smaller topographic gradient is more vulnerable to agricultural non-point source pollution (Nitika et al. 2021).

Agricultural non-point source pollution leads to the continuous accumulation of nitrate in groundwater, and high nitrate concentration will cause serious harm to human health. Hypermethemoglobinemia is a health problem caused by high nitrate in drinking water. When the nitrate content is high in the body of newborns, cyanosis will also occur, which will lead to mucous membrane blueness and digestive and respiratory diseases in infants (Marghade et al. 2021). The intake of large amounts of nitrate has a complex relationship with cancer, adverse reproductive outcomes and other health problems, which requires further research in the future. In this paper, the health risk assessment of adult men, adult women, children and infants is carried out, respectively, and the corresponding conclusions are drawn, which is conducive to improving people's awareness of the protection of infants and children against nitrate risk.

Du et al. (2022) identified and analyzed the nitrogen source of groundwater in the Beilin area of Suihua Basin based on isotope methods, and concluded that the high concentration areas of groundwater NO₃⁻ are mainly affected by agricultural activities and the infiltration of sewage discharge from upstream urban areas. Tian et al. (2020c) conducted a study on the health risks of groundwater chemistry and nitrate intake in Helen City, and found that human activities are the cause of the formation of chemical composition in shallow groundwater in Helen. The degree of harm caused by NO₃⁻ pollution is in the order of infants > children > adult females > adult males. The above two conclusions are consistent with this study, and the research results can provide technical reference and theoretical support for the evaluation of groundwater nitrate under the same type of hydrogeological conditions.

By analyzing the chemical characteristics, material distribution characteristics, nitrate pollution characteristics and quality characteristics of groundwater, an HHRA was constructed to evaluate the health risks caused by nitrate in groundwater. Scientific conclusions were drawn.

1. The groundwater in the research area is severely polluted by nitrate, which is mainly concentrated in the main human activity areas, especially in the main agricultural production areas.

2. The groundwater quality in the research area is relatively poor, partly due to the excessive use of nitrate-containing fertilizers and organic fertilizers in agricultural production.

3. NO₃⁻ pollution in groundwater poses a health risk to humans. There are significant differences in the health hazards of groundwater NO₃⁻ pollution in different genders and age groups, with the order of risk being infants > children > adult women > adult men.

4. It is recommended to scientifically use nitrate fertilizers in agricultural production to reduce nitrate pollution. The research results provide a basis for preventing human health risks.

The authors thank the Northeast Geological Science and Technology Innovation Center Project of the China Geological Survey (QCJJ2022-43), the Natural Resources Comprehensive Survey Project (DD20211589) and the Hydrogeological Survey Project (DD20230470 and DD20230508). The authors also thank editors and anonymous experts for their support.

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

The authors declare there is no conflict.