Spatial Distribution and Health Risk Assessment of Nitrate in Drinking Water: A Case Study in the Central Plateau of Iran

ORIGINAL ARTICLE Introduction : This study aimed to determine nitrate levels in water wells supplying drinking water in Taft city, Iran, and assess the associated health risks using the method proposed by the US Environmental Protection Agency. Materials and Methods: In 2021, the average annual nitrate levels were determined in 48 drinking water wells which were located in Zone 39 (Taft city). Health risk assessment and sensitivity analysis were conducted to identify the most influential variables. Results: The mean nitrate content in the water wells under study was 32.88 ± 18 mg/L. Out of the 48 examined water wells, 10 had nitrate levels higher than the standard value (50 mg/L) established by the Iranian Institute of Standardization (Standard No. 1053) and WHO. The calculated Hazard Quotient (HQ) for children and adolescents was greater than 1, while it was less than 1 for adults. Nitrate concentration in drinking water was found to be the most important influencing variable in the calculated HQ for children and adolescents. Conclusion: The results indicated that children and adolescents’ health in the studied area is at risk, and appropriate measures must be implemented to avoid and control the exposure of these vulnerable groups; they can be continuous monitoring of nitrate levels using on-site treatment methods where nitrate concentrations exceed the standard level, and decommissioning wells with high nitrate levels.


Introduction
Nitrate is extensively applied in agriculture as a mineral fertilizer and also as a preservative agent in food products 1,2 .Surface waters usually contain low concentrations of nitrate (0-18 mg/L); however, it varies with the change of seasons.
From the available evidence, it is believed that nitrate concentration in surface and groundwater inceases as a result of surface runoff, especially runoff from farmlands.In aerobic conditions, the concentration of nitrate in groundwater is affected by soil properties.
In many European countries, nitrate levels in water resources have increased over the past few decades and have doubled in some cases in the last 20 years.For example, some rivers in the UK have experienced an average annual increase of 0.7 mg/L of nitrates 3 .In the United States, the levels of nitrate and nitrite in groundwater typically remain below 4-9 mg/L and 0.3 mg/L, respectively 4 .Increased uncontrolled agricultural activities can easily increase nitrate concentrations 5 .For example, a concentration greater than 1500 mg/L of nitrate has been observed in groundwater around farmlands in some areas of India 6 .
Although nitrate carcinogenicity has not been reported in laboratory animals, it has been reported that exposure to high concentrations of nitrite can increase cancerous tumors in animals 7 .The reduction of nitrate to nitrite is the main route of toxicity in humans, followed by the conversion of hemoglobin to methemoglobin which disrupts oxygen transfer, leading to methemoglobinemia after more than 10 percent of the hemoglobin is converted to methemoglobin 8,9 .
As methemoglobinemia progresses, suffocation, and finally, death occur.Normally, the concentration of methemoglobin in the body is less than 2%, and in the children under 3 months old, this concentration is less than 3% 2 .In many situations, such as the influx of agricultural runoff and industrial wastewater, there is a chance of increasing levels of nitrate in groundwater 10,11 .
Using electronic systems and computer software helps to better control water resources and also estimate the health risks of pollutants [12][13][14] .Moreover, estimating the spatial distribution of groundwater quality makes it possible to manage water resources and prevent groundwater pollution 15 .Geographic Information System (GIS), as a new technology 16 , can be applied to analyze and interpret the pollutant's distribution in water resources 17 .
In order to assess the spatial distribution of pollutants, a useful and practical technique of ArcGIS software is inverse distance weighting (IDW) which simulates the concentration of pollutants in other parts of the investigated area with the given information about the distance between the points and the concentration of the pollutants everywhere 18 .Various studies have utilized GIS software to analyze and determine the types of pollutants present in different zones 19,20 .
The first purpose of the present work is to investigate nitrate concentration in groundwater in Taft city located in Yazd province, Iran.Second, the zoning of 48 wells supplying drinking water was done by determining the concentration of nitrate in them, and finally, the noncarcinogenic risks of nitrate exposure were estimated in three population groups such as children, adolescents, and adults.

Materials and methods
The study area A part of groundwater in Taft city located in zone 39 was studied, with a population of about 47,267 people and 48 wells which had 22.22 ± 2.63 average flow; the wells were located at 31˚15'N -31˚50'N and 53˚35'E -54˚05'E.Figure 1 indicates the geographic location of water wells in the city.

Nitrate determination
Nitrate concentration in drinking water was determined by the 4500 -NO3 -method.The samples were collected in polyethylene bottles and stored unacidified for up to 48 h at the temperature below 6 °C without freezing.Analysis of the collected samples was carried out by the Ultraviolet Spectrophotometer technique at the wavelength of 220 nm.

Spatial analysis and risk assessment
ArcGIS 10.1 software developed by ESRI was used to map the drinking waters with heavy metals in a city 21,22 .The interpolation technique or IDW was applied for zoning and preparation of an independent rostrum layer associated with the concentration of the pollutants in various points of the study area 21 .Furthermore, an estimation of the non-carcinogenic risks for nitrate was done using the following equation 23,24 : ,where HQ is the hazard quotient, EDI defines estimated daily intake, and RfD is the oral reference dose.Moreover, the calculation of EDI was performed utilizing the subsequent equation 23 : ,where C is the nitrate concentration, CF represents the conversion factor, IR is the drinking water ingestion rate, EF indicates exposure frequency, ED stands for exposure duration, BW represents body weight, and AT denotes the averaging time.Table 1 shows the necessary parameters for EDI calculation.

Nitrate concentration
Based on the results, the average nitrate concentration in the investigated water wells was 32.88 ± 18 mg/L.Out of the 48 wells examined, 10 wells had a nitrate concentration higher than the standard value (50 mg/L) determined by the Iranian Institute of Standardization (standard 1053).
Figure 2 shows the nitrate concentration in the studied wells and the number of replicates for each concentration.According to the data in Figure 2, the maximum concentration was 100 mg/L, and the most frequent concentration was 30 mg/L.

Spatial distribution analysis
The IDW technique of ArcGIS software was used to map nitrate concentration in the study area.According to the zoning, wells 20, 9, 33, 37, 39,  38, 48, 36, 30, and 21 had the highest concentration of nitrate (above 50 mg/L), respectively, while well 14 had the least nitrate concentration at 4 mg/L.Figure 3 shows the mapping of the nitrate concentration in the 48 studied wells.

Discussion
The present study investigated nitrate levels in 48 regional water supply wells which provide water for a population of 47,260.The results indicated that the concentration of nitrate in 10 of the wells under study was exceeded the standard level (50 mg/L).According to the nitrate concentration zoning, 8 out of of 48 of the studied wells (16.66% of the total investigated wells) in the northern part of the study area contained nitrate levels that exceeded the standard level.Moreover, The clustered pattern of nitrate distribution in the examined area was revealed by Moran's index, showing the point distribution of the contamination source.
An examination of the area, where the wells with high concentrations of nitrate were located, showed that most of the activities in that area was limited to agricultural activities.In the mentioned area, irrigation was based on an ancient method called flood irrigation which produced large volumes of runoff, mainly contaminated with nitrogen fertilizers.The use of nitrogen fertilizers and the infiltration of runoff into groundwater led to contamination of groundwater with nitrate 25, 26 .Fallahzadeh et al. reported that using nitrogen fertilizers in agriculture was the main reason for groundwater contamination with nitrate in areas with agricultural activity 26 .Livestock activities were another cause of groundwater pollution with nitrate 27,28 .Releasing and penetrating domestic and human wastewater was also another source of groundwater pollution with nitrate 29,30 .
Non-cancer risk assessment among the studied groups indicated that the calculated HQ for children and adolescents was above 1, which suggested a high risk for developing the disease through drinking water.The calculated risk for child population was three times higher than the ones calculated for the adolescent population.Various studies have shown that the risk of developing the disease in children is higher than other age groups during exposure to high concentrations of pollutants 23,31 .Factors such as high surface area to volume ratio increase the calculated risk for children 23 .High nitrate concentrations have caused an HQ of above 1 regarding adolescents.
The sensitivity analysis results indicated that nitrate concentration was the most influential variable increasing non-cancer risks, with rates of 54.9% and 56.9% in children and adolescents, respectively.Previous studies also highlighted the high impact of the pollutant concentration variable on increasing health risk when the pollutant concentration is higher than standard 32,33 .Reducing pollutant concentrations could reduce non-carcinogenic risk.Therefore, drainage and prevention of surface water infiltration into groundwater in areas where agricultural activities were the main activity of the region that could significantly reduce the concentration of pollution entering groundwater 34 .

Limitation
The main limitations of this study was lack of nitrite measurement.Moreover, it is recommended to measure other pollutants such as heavy metals in future studies.

Conclusion
The obtained results revealed that 10 water wells had higher nitrate concentrations than the standard limit provided by the Iranian Institute of Standards.Moreover, the obtained HQ in the children and adolescent population was higher than 1.0.The sensitivity analysis also indicated that high concentrations of nitrate in drinking water increase health risks.In conclusion, minimizing pollutant concentrations can remarkably decrease the calculated non-cancer risk.

[Figure 1 :
Figure 1: The geographical location of the study area

Figure 2 :
Figure 2: Histogram plot of the concentration of nitrate in the study area and the number of repetitions

Figure 3 :
Figure 3: Spatial distribution analysis in the studied area

Figure 4 :
Figure 4: Results of Moran index

Table 1 :
Necessary parameters for EDI calculation Therefore, drainage and prevention of surface water infiltration into groundwater in areas with high agricultural activities are suitable ways to reduce nitrate concentration in groundwater.Mahvi AH, Meybodi MN, et al.Application of photo-electro oxidation process for amoxicillin removal from aqueous solution: modeling and toxicity evaluation.Korean J Chem Eng.2019;36(5):713-21.14.Dehghani F, Omidi F, Fallahzadeh RA, et al.Health risk assessment of occupational exposure to heavy metals in a steel casting unit of a steelmaking plant using Monte-Carlo simulation technique.Toxicology and industrial health.2021;37(7):431-40.15.Ehsani H, Javid A, Hasani A, et al., editors.Evaluation of nitrate variation and total dissolved solids trend in drinking water using GIS Hamedan plain ground.10th National Conference on Environmental Health; 2007.16.Alam A, Kumar A, Singh A. A GIS approach for groundwater quality evaluation with entropy method and fluoride exposure with health risk assessment.Environmental Geochemistry and Health.2024;46(2):47.17.Merchant JW.GIS-based groundwater pollution hazard assessment: a critical review of the DRASTIC model.Photogramm Eng Remote Sensing.1994;60:1117.18. Cressie N. Statistics for spatial data.John Wiley & Sons; 2015.19.Gaus I, Kinniburgh D, Talbot J, et al.Geostatistical analysis of arsenic concentration in groundwater in Bangladesh using disjunctive kriging.Environmental geology.2003;44(8): 939-48.20.Jafari M, Mohammad AH, Tahmoures M, et al.Assessment of soil property spatial variation