Introduction

Safe water supplies, hygienic sanitation and good water management are necessary for human wellbeing (Dehghani et al. 2017; Khosravi et al. 2018; Qasemi et al. 2018). The access to clean drinking water resource is still a dream for approximately one-sixth of humankind on the earth (Karbassi et al. 2011). Groundwater is common source for drinking for much of the population in many arid and semiarid parts of developing countries; therefore, it is necessary to ensure its high quality at all time, so that the consumer health is not compromised (Dehghani et al. 2013; Fazlzadeh et al. 2017b; Mohammadi et al. 2017). It is an important renewable resource with many intrinsic advantages compared to surface water. The groundwater is used with the least purification and hence there is a significant risk of exposure to contaminants (Gorgij et al. 2017; Shankar and Shanker 2014).

Nitrate is ubiquitous in both surface and groundwater (Alif Adham and Shaharuddin 2014; Gelberg et al. 1999). It represents one of the most serious environmental problems in several countries and also in many parts of Iran due to rapid growing trend of industrialization and urbanization (Asadi et al. 2017; Ehteshami and Biglarijoo 2014; Esmaeili et al. 2014; Nezhad et al. 2017; Rezaverdinejad and Rahimi 2017; Ziarati et al. 2014). Nitrate in water originates from various natural and anthropogenic sources (Pirsaheb et al. 2017). Nitrate exists naturally in soils through microbial conversion of ammonia resulting from organic compounds such as plants, animals, and manure (Migeot et al. 2013; Thomson et al. 2007). Background levels of nitrate in groundwater in natural grasslands of temperate regions are usually lower than 2 mg/L (Aranibar et al. 2011). The extensive use of especially nitrogenous fertilizers in agriculture, untreated wastewater irrigation, discharge of industrial effluents, abandoned landfills, and contaminated lands are the major origins of nitrate contamination in ground and surface waters (Spalding and Exner 1993; Wakida and Lerner 2005). Nitrogenous fertilizers and the accumulation and application of animal manure are the largest correspondent to anthropogenic nitrogen on a global scale and have been exposed as an even more crucial source of drinking water nitrate in many rural areas (Derakhshani et al. 2017; Noroozi et al. 2011). In most of rural areas around the world, especially in developing countries, people keep some farm animals including cows, donkeys and horses, sheep and goats, birds, dogs, etc. for consumption or for commercialization (Wakida and Lerner 2005). Excreta, dung and urine of these animals comprise a possible origin of nitrate which can eventually be moved through the soil and get into the groundwater aquifers by precipitation (Sankararamakrishnan et al. 2008). Once groundwater is contaminated, treatment is difficult and expensive, thus the contamination prevention is the primary strategy used for water quality management (Corniello et al. 2007).

Drinking water is generally a low source of nitrate intake compared to food when the level is small, but it becomes considerable as the level increases to the World Health Organization (WHO) guideline. At levels above 50 mg/L, water is probably the main source of nitrate intake for the community, unless there are other sources of high nitrate intake such as use of some vegetables and fruits irrigated with sewages (Fan 2011). In areas where groundwater is the main source of drinking, domestic, and agricultural water, potentially considerable health risks are associated with the consumption of nitrate contaminated groundwater (Fazlzadeh et al. 2017a). The main health concern from high nitrate concentrations is methemoglobinemia or blue baby syndrome, which can affects infants under 6 months and can eventually result in the death of these infants (Shams et al. 2009; Wakida and Lerner 2005). Methemoglobinemia occurs when nitrite converts the Fe2+ in hemoglobin to the Fe3+ and consequently it cannot bind oxygen (WHO 2011).

Nitrate itself is not harmful, but in human gastrointestinal tract it can be endogenously reduced to toxic nitrite through nitrosation in the stomach with amines and amides to form various types of N-nitroso compounds (NOCs) (Ward 2005). Generally, 40–75% of NOCs are produced in the gastrointestinal tract. NOCs have increased the risk of cancer in a variety of animals in laboratory studies (Espejo-Herrera et al. 2015; Keszei et al. 2012; Zhu et al. 2014). According to previous reports, acute exposure to high levels of nitrate through drinking water can increase the risk of certain types of cancer, such as gastric, esophageal, and stomach cancer (Bao et al. 2017; Bittner 2000). Due to the serious effects of nitrate on human health, the World Health Organization (WHO) has recommended a guideline of 50 and 15 mg/L for adults and infants, respectively, in drinking water (Hashim et al. 2017). The US Environmental Protection Agency (USEPA) has set a maximum contaminant level of 45 mg/L for nitrate in public drinking water supplies (USEPA 2015).

Risk assessment for human health is a kind of evaluation method applied to estimate the degree of the health impacts that may be the result of exposure to non-carcinogenic and carcinogenic contaminants through a variety of exposure routes (Qasemi et al. 2018). Since the health risk of nitrate contamination has not been addressed to date in rural areas of Gonabad and Bajestan, we aimed in this research to investigate nitrate concentrations in groundwater applied for drinking and to evaluate the health risk from consuming the contaminated water in these areas. For health risk assessment of nitrate through the consumption of drinking water, the values of chronic daily intake (CDI) and the hazard quotient (HQ) were determined for the selected rural areas and then the obtained data were compared. The results of this work are expected to indicate specific regions where the health risk exceed the values of standards which would help health professionals, authorities and decision makers to develop more effective health management plans in terms of local and regional groundwater quality protection.

Materials and methods

Description of study area

The field research is focused on 39 rural areas of Gonabad and Bajestan as shown in Fig. 1a–c. Gonabad and Bajestan, located in south of Khorasan Razavi in east of Iran, were selected for the purpose of the present study because of high levels of nitrate reported by the health professionals in groundwater of some of these areas. These two counties have hot summers and cool winters with annual average temperature of 16.4–17.3 °C located in semi-arid regions with low average annual precipitation in range of 149–155 mm. Gonabad and Bajestan have many rural areas of mostly low-income earners with no or limited sewerage facilities. The geology of the studied rural areas varies from location to location but is mainly sedimentary rocks consisting of igneous and metamorphic rocks, metamorphosed to slate, schist, and quartzite and intrusions subvolcanic rocks. The rural areas have many small farms and the farmers actively spread manure and commonly use nitrogenous fertilizers as prerequisites to enhance the agricultural output. Almost all of inhabitants in the areas use unprotected wells for their wastewater disposal which increasing the risk of groundwater contamination with nitrate. In this study, it is assumed that most of the inhabitants in the rural areas derive 100 percent of their drinking water from the selected wells.

Fig. 1
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Map of Gonabad and Bajestan in Iran (a), sampling locations in rural areas of Gonabad (b) and Bajestan (c)

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Groundwater sampling and analysis

Duplicate groundwater samples were collected from each of 39 rural areas during January 2018, with a total of 78 samples being collected. The samples were taken from domestic wells and sampling stations were selected on the basis of lack of appropriate sanitary facilities considering WHO criteria. Groundwater samples were collected in plastic bottles, stored in ice-box, and sent to the chemistry laboratory of Gonabad University of medical sciences for analysis within 24–48 h. Finally, nitrate concentrations of the samples were analyzed using a UNICO-2100 spectrophotometer. The data of nitrate concentration used for the health risk assessments were average of duplicate groundwater samples from each sampling village. As the sampling frequency from each rural area was during one month (January), the observed level variations were negligible.

Health risk assessment

The non-carcinogenic risk assessment was performed using hazard quotient (HQ) by considering the chronic daily intake dose (CDI), calculated for the nitrate based on the results of chemical detections for the groundwater samples in the study areas. The hazard quotient method has been used in many studies for quantitative risk assessment (Edokpolo et al. 2015; Qiming et al. 2012).

For the estimation of chronic daily intake (CDI, mg/kg/day) to an individual contaminant through ingestion, the data measured for nitrate in drinking water were converted to daily intake using the parameters given in Table 1 as well as the following equation (Ghaderpoori et al. 2018; Qasemi et al. 2018):

Table 1 Parameters applied for health exposure assessment in water
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$${\text{CDI}}=\frac{{C \cdot {\text{DI}} \cdot F \cdot {\text{ED}}}}{{{\text{BW}} \cdot {\text{AT}}}}$$

where C is nitrate content in water (mg/L), DI is daily water intake (L/day), F is exposure frequency (days/year), ED is exposure duration (years), BW is body weight (kg), and AT is averaging time for non-carcinogens (days).

The details of selected parameters and their values for determining the chronic daily intake through oral ingestion for adults, children and infants are given in Table 1.

The hazard quotient (HQ) was applied to evaluate nitrate risks using the following formula (Das et al. 2017; Yu et al. 2010):

$${\text{HQ}}=\frac{{{\text{CDI}}}}{{{\text{RfD}}}}$$

where RfD is the reference dosage of nitrate in mg/kg/day.

The reference dose (RfD) in this study was considered 1.6 mg/kg/day for nitrate based on the United States Environmental Protection Agency (USEPA) report (USEPA 2015). Groundwater samples with HQ values above 1 (HQ > 1) pose significant health risk due to nitrate contamination. The higher the value, the greater the probability of harmful non-carcinogenic health impacts (Khan et al. 2016; Ullah et al. 2017).

Results and discussion

As summarized in Table 2, for rural areas of Gonabad, the mean concentration of nitrate in the drinking water in this research was 29.3 mg/L, ranging from 1.8 to 82.2 mg/L. The mean concentration of nitrate in the drinking water of rural areas of Bajestan was 37.95 mg/L ranging from 5.5 to 84.3 mg/L. Totally, the results clearly indicated that nitrate levels in 11 percent of rural areas in Gonabad and 19 percent of rural areas of Bajestan were above 50 mg/L, which was the maximum limit of WHO guideline for nitrate in drinking water. This concluded that the mean nitrate levels in rural areas of Bajestan were higher than that in Gonabad.

Table 2 Comparison of nitrate concentrations found in rural areas of Gonabad and Bajestan
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Figure 2 indicates that sites G2, G3 and G4 have very low nitrate concentrations, and site G15 has very high nitrate concentrations. G15 (Bilond) is a rural area downward of Gonabad city. Its water is supplied from a Qanat system crosses along Gonabad city. Gonabad city has no wastewater collection system, and almost all the residents use absorbing wells for wastewater disposal. The high levels of nitrate in G15 (Bilond) was due to wastewater leakage from these wells. In Bajesten, B2 (Chah Paliz), B13 (Qasemabad) and B18 (Mazar) had high nitrate levels due to the absorbing wells and agricultural activities (Fig. 3).

Fig. 2
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Comparison of mean nitrate concentrations in rural areas of Gonabad with WHO guideline of nitrate in drinking water

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Fig. 3
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Comparison of mean nitrate concentrations in rural areas of Bajestan with WHO guideline of nitrate in drinking water

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Based on risk assessment formula mentioned above, the amounts of CDI calculations are necessary for the estimation of non-carcinogenic health risk assessment. The mean values of CDI for adults, children and infants were 0.79 (0.05–2.34), 2.08 (0.13–6.16) and 2.22 (0.14–6.57) mg/kg/day, respectively, in rural areas of Gonabad. For rural areas of Bajestan, the mean values of CDI for adults, children and infants were 1.03 (0.15–2.40), 2.73 (0.41–6.32) and 2.91 (0.44–6.74) mg/kg/day, respectively. Using the value of CDI and reference dose, hazard quotients (HQs) for nitrate in drinking water were determined. Chronic daily intake (CDI) estimations are given for each individual in Table 3. From the above data, it can be generally stated that the mean values of CDI in rural areas of Bajestan were higher than those of rural areas of Gonabad.

Table 3 Chronic daily intake (CDI, mg/kg/day) estimation
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Figures 4 and 5 illustrate the health risk assessment distribution in rural areas of Gonabad and Bajestan, respectively, for adults, children, and infants. According to the figures, the health risk of infants was higher than those of children and adults.

Fig. 4
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Values of hazard quotient (HQ) for rural areas of Gonabad city for three age groups (adults, children, and infants)

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Fig. 5
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Values of hazard quotient (HQ) for rural areas of Bajestan for three age groups (adults, children, and infants)

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Drinking groundwater showed high levels of nitrate in the study area, with nitrate being detected in all the samples and at excessive concentrations in 34.3% of the samples. The parameters applied in the study were primarily selected from the USEPA guideline for risk assessment; therefore, additional studies are necessary to determine data specific to the study area.

For non-carcinogenic risk assessment, the values of HQ were calculated separately for adults, children and infants. The mean values of HQ for adults, children and infants were 0.49 (0.03–1.46), 1.30 (0.08–3.85) and 1.39 (0.09–4.11), respectively, in rural areas of Gonabad. For rural areas of Bajestan, the mean values of HQ for adults, children and infants were 0.65 (0.09–1.50), 1.70 (0.25–3.95) and 1.82 (0.27–4.21), respectively. For rural areas of Gonabad, 77% of sites showed a HQ above 1 for children and infants (Fig. 4). But only 0.05% of adults had HQ values above 1. For studied groups in Bajestan, 85% of calculated HQ values for infants and children were above 1 (Fig. 5). For adults, 14% of HQs were above 1. Totally, the results indicated that the risk of adverse health effects of nitrate contamination in groundwater in rural areas of Gonabad and Bajestan was high especially for infants and children. Comparison of HQs among the 39 sampling sites showed that the groundwater of rural areas in Bajestan had higher HQs than Gonabad.

Generally, levels of nitrate in surface and groundwater are normally in the range of 0–18 mg/L (Schmoll et al. 2006). However, many rural areas in the current study had much higher levels, highlighting the groundwater contamination in the areas. In the rural areas, high nitrate levels can be a cumulative outcome of the different activities in the areas. In this case, reducing the amounts of nitrogen transferred by effluents, septic tanks and unprotected sewage wells to groundwater, and also equalizing nitrogenous compounds used in farmlands is entirely mandatory. Therefore, in these areas, a stringent sewage disposal policy will be unavoidable. As a rule of thumb, after water contamination, water treatment processes for diminishing nitrate levels cannot be performed in such large areas, and implementation of preventive procedures are the best option. However, the results of the present study clearly indicated that the level of nitrate may pose considerable adverse health effects to the individuals of some rural areas in Gonabad and Bajestan via oral ingestion of water. In a study, groundwater nitrate levels in eight wells in Dehloran, Ilam, Iran, were determined in 2015. The nitrate levels in all of the analyzed samples were lower than 20 mg/L (Almasi et al. 2016). In another study, nitrate levels of groundwater in Lar area, south Iran were studied. The levels of nitrate ranged from 1.47 to 70.66 mg/L. They attributed the high deteted levels of nitrate to widespread use of nitrogenous inorganic fertilizers (Rezaei et al. 2017). In a similar study, the levels of nitrate in Andimeshk and Susa plains in north of Khozestan-Iran were analyzed. The mean nitrate level was 14.2 mg/L which was below the WHO guideline of 50 mg/L (Mahvi et al. 2005).

Moreover, nitrate concentrations were evaluated in 43 active wells of Shiraz, South-central Iran, by statistical models during 2010–2014. The concentrations of nitrate in the samples ranged from 5 to 72 mg/L, and 11% of the samples had nitrate concentrations above the standard level of 45 mg/L as nitrate (Nezhad et al. 2017) regulated by United States Environmental Protection Agency (USEPA). In another work, trends of nitrate occurrence in Zanjan groundwater resources were evaluated during 2006–2010 in 72 wells. The results of this study showed that only one well had nitrate concentration above the standard and other wells were acceptable regarding to nitrate concentrations. The data showed a rising trend in nitrate levels during the study period (Eslami and Ghadimi 2017).

In a study, spatial distribution and the reason behind the increasing nitrate content in central district of Khodabandeh, Iran were evaluated. The results indicated that nitrate contamination in the south and south-west regions was due to geological processes, agricultural activities, and the discharge of human sewage into absorbing wells (Khosravi et al. 2017).

In another study, the concentration of nitrate of drinking water wells in a Tonekabon village, Iran was analyzed. The nitrate concentration ranged from 8 to 33.7 mg/L (Roholamin Kasmaei et al. 2017). In a cross-sectional study, the levels of nitrate in 100 groundwater samples in Talesh in the North of Iran in three consequent months were analyzed. The results revealed that all samples had nitrate contents above 50 mg/L recommended by WHO (Gheshlagh et al. 2013). In another research, the effects of urbanization on groundwater quantity in the Zahedan Aquifer, southeast Iran were evaluated and high levels of nitrate (up to 295 mg/L as nitrate) were observed in groundwater samples (Khazaei et al. 2004). In another study, health-risk related to nitrate in the drinking water in the Sanandaj, Kurdistan County, Iran was assessed for men, women, and children. The level of nitrate in urban and rural drinking water ranged from 0.28 to 27.77 and from 1.28 to 80 mg/L, respectively. The level of nitrate reported in rural samples was more than that of urban samples. The results of this research showed that all 3 studied groups (men, women, and children) were exposed to health risk due to nitrate (hazard quotients above 1) (Rezaei et al. 2018).

As the results of the present study showed, the level of nitrate was higher in some of the studied rural areas which can be attributed to various human activities and natural processes including agricultural activities, cultivation and fertilizers, improper discharge of human sewage into absorbing wells and sewage disposal manners, landfills, farm/feedlot animal wastes, general rural development, degradation of natural vegetation and termites.

Conclusions

Nitrate contamination of drinking water supplies in many parts of Iran is pervasive and very serious. The identification of nitrate-vulnerable regions is the primary step towards the protection of groundwater against contamination. The research compares nitrate contamination of groundwater in rural areas of Gonabad and Bajestan with a special emphasis to determining non-carcinogenic health risk associated to nitrate contamination in these regions for three groups including adults, children and infants. For groundwater in rural areas of Gonabad, the nitrate concentrations ranged from 1.8 to 82.2 mg/L with only two rural areas in excess of the WHO guideline value of 50 mg/L. In rural areas of Bajestan, the nitrate concentrations ranged from 5.5 to 84.3 mg/L and 4 values exceeded the WHO guideline. However, the consumption of high nitrate containing water can appreciably impact the health of individuals. The non-carcinogenic risk assessment in the present work showed that the rural areas of Bajestan had higher CDI and HQ values than Gonabad. The non-carcinogenic risk assessment calculated for the studied groups in Gonabad and Bajestan were as follows: infants > children > adults. This clearly would notify consumers that the groundwater is not safe, especially for infants under 1 years of age. These findings also illustrate that nitrate concentrations in groundwater of the study area should be regularly monitored to ensure it is within the permissible levels. A comprehensive sanitation improvement plan in these areas will have a positive influence on reducing nitrate levels in groundwater resources. The last recommendation is to abandon water consumption of any well that is found to be contaminated with nitrates higher than the WHO guideline of 50 mg/L. Finally, as nitrate is costly to remove from drinking water supplies, control of the anthropogenic activities that contribute nitrate to groundwater including animal operations, crop fertilization, wastewater discharge, absorbing wells septic systems, etc. should be managed appropriately. The results of the present work would allow one to determine more definitively the levels of nitrate contamination in groundwater of rural areas of Gonabad and Bajestan, and thus to propose and select the most effective remediation and prevention alternatives. The results of this study also provide useful information for future work planning and important to assess the potential health consequences from nitrate exposure.