Health risk assessment using Monte-Carlo simulations due to arsenic contamination in groundwater in Punjab

Arsenic (As), characterized by its atomic number 33, is a widespread element that exists ubiquitously in the environment, seamlessly integrating into air, water, soil, and rocks (Ali et al. 2019). In developing countries, most of the general population consumes arsenic-contaminated drinking water several times higher than the World Health Organization's (WHO) acceptable limit of 10 μg L⁻¹ (World Health Organization 1996; Hassan et al. 2009). According to numerous reports, over 100 million population globally have been affected by arsenic concentrations of greater than 10 μg L⁻¹ (Rahman et al. 2005; Nriagu et al. 2007). The acceptable limit of arsenic in groundwater is set at 50 μg L⁻¹ in many developing countries (Nickson et al. 2005), and 42 million people are exposed to more than 50 μg L⁻¹ arsenic concentration in potable water. Due to arsenic's high toxicity, it can have adverse health effects even at low exposure levels, including dryness of the throat and mouth, dysphasia, profuse diarrhea, vomiting, melanosis, gastrointestinal distress, immune system suppression, superficial as well as internal cancers, and cardiovascular and neurological effects (Murphy et al. 1981; Wu et al. 1989; Chiou et al. 1995; Mandal et al. 1996; Knobeloch & Zierold 2006). It is a common metalloid prevalent in igneous and sedimentary rocks on the surface of the Earth's crust (Singh et al. 2010). In addition, human activities can discharge high contents of arsenic into the environment (Saha et al. 2017; Senila et al. 2017). There are four oxidation states of arsenic found in the environment: elemental arsenic (0), arsenate (+5), arsenite (+3), and arsenic (−3) (Ammann 2011). Arsenic (+5) exists in four different forms in aqueous solution, depending on the pH: H₃AsO₄, H₂AsO₄⁻, HAsO₄²⁻, and AsO₄³⁻. Arsenic (+3) exists in five different forms in an aqueous solution: H₄AsO₃⁺, H₃AsO₃, H₂AsO₃⁻, HAsO₃⁻, and AsO₃³⁻. The solubility of arsenic salts depends on pH, but elemental arsenic is insoluble in water (Edwards et al. 1998). Inorganic arsenic compounds contain trivalent and pentavalent arsenic; trivalent arsenic is more toxic and is four to ten times more soluble in water than pentavalent arsenic (Tuzen et al. 2009; Brahman et al. 2013). Analysis of the valency and speciation of soluble arsenic can significantly help in improving the preparation of an arsenic removal strategy. The most prevalent species of arsenic is arsenite, whereas arsenate is stable in oxygenated aquatic environments. Arsenic, in its elemental form, is redox-sensitive. Numerous geochemical parameters, including pH levels, reduction–oxidation processes, aquatic chemistry, distribution of other ionic species, and microbial activity, combine to determine its presence, mobility, distribution, and forms. Groundwater is more susceptible to arsenic contamination than surface water because anoxic conditions in subsurface environments increase arsenic's mobility (Shih 2005).

Groundwater in Argentina, Hungary, Mexico, and various regions of the United States is heavily polluted by arsenic (Thakur & Semil 2013). According to several studies (Mandal & Suzuki 2002; Smedley & Kinniburgh 2002; Agusa et al. 2004; Stanger et al. 2005; Sengupta et al. 2006; Yadav et al. 2014), there is a concerning danger of arsenic pollution in Southeast Asian nations, including India, Bangladesh, China, Nepal, and Vietnam. According to Nriagu et al. (2002), thermal springs and mining operations are important sources of arsenic release into groundwater under oxidizing circumstances. The highest concentration of arsenic in Canada has been found to be roughly 100 mg L⁻¹ in groundwater (Nriagu et al. 2002). Australia has underground water significantly contaminated with concentrations of arsenic in groundwater that have been reported as high as 300 mg L⁻¹ (Nriagu et al. 2002). Mining operations, oxidation of minerals containing sulfides, and volcanic eruptions have all been identified as major causes of arsenic release into groundwater (Boyle et al. 1998). In developing nations including India, groundwater contamination with arsenic is one of the most significant and hazardous environmental problems (Environmental Protection Agency 2001; Prakash & Verma 2021). Kumar et al. (2016) and Shaikh et al. (2016) reported 84.28 μg L⁻¹ arsenic concentration in the groundwater of Bihar state. In Jharkhand, Alam et al. (Singh 2004; Alam et al. 2016) reported the arsenic concentration higher than 10 μg L⁻¹. Sharma et al. (2013) reported that arsenic contamination is greater than 10 μg L⁻¹ in groundwater in the eastern part of India. In the Indian state of Punjab, groundwater is an essential source of drinking water and is used in all aspects of human life, including industries, agriculture, and residences (Gupta 2009). About 95% of the people in Punjab get their drinking water from groundwater (Hundal et al. 2009).

The primary source of arsenic in groundwater is the dissolution of minerals from weathered rocks and soils. In 1967, Punjab had just about 50,000 tube wells, but in 2008, this number surpassed one million (Hundal et al. 2009). In the previous three decades, groundwater extraction has increased by 200 times in this state.

In this paper, a comparative study of both approaches for the risk assessment due to contamination in groundwater of Moga, Faridkot, Fazilka, Patiala, Ferozepur, Rupnagar, and Amritsar districts of Punjab is performed. These regions are chosen based on their geographic locations and the increasing rate of cancer-related mortality due to the intake of water of inferior quality. The dependance of health risk parameters on the input variables is determined using the sensitivity analysis. The concentration of arsenic in these districts is reported in the literature (Central Ground Water Board 2013; Kumar et al. 2017; Sidhu et al. 2018; Virk 2019). In these studies, health risk assessment due to arsenic contamination is not carried out.

In the previous studies, the deterministic approach was used for the human health risk assessment due to arsenic contamination in groundwater. This method considers the site-specific data to estimate the risk of non-carcinogenic adverse health effects in adults and children, but it does not address the inherent uncertainty of input parameters. There are various variables in calculations of health risk for which assuming a point distribution might not incorporate the inherent variability in the input parameters used for health risk assessment. In order to avoid the uncertainty associated with the deterministic methodology, Monte-Carlo simulations were carried out for the non-carcinogenic as well as carcinogenic risk assessment in adults and children due to the presence of arsenic in groundwater.

In order to evaluate the arsenic level in groundwater, Mandeep Sindhu et al. collected water samples using hand pumps from several locations in the Moga and Faridkot districts of Punjab (Sidhu et al. 2018). The author collected a total of 40 groundwater samples from each district using a random sampling approach. For a comprehensive coverage of the region, these samples were obtained from several hand pumps. After collection, 1 milliliter (mL) of hydrochloric acid was added to each sample for preservation after measuring the pH of the sample. Each water sample was tested by pouring 100 mL into a glass beaker, and 2 mL of concentrated nitric acid (HNO₃) was added. The combination was brought to a temperature of 90–95 °C in a water bath, covered with a watch glass, and left there until the liquid evaporated to the extent of 20 mL. After the solution cooled, any insoluble minerals or silicates in the sample were removed by filtering it using the Whatman filter paper (grade I). The samples were prepared for analysis after adding reagent water to get the volume back up to 100 mL. After that, the arsenic concentration of the water was determined using the inductive coupled argon plasma–atomic emission spectroscopy (ICAP–AES). To analyze the level of arsenic contamination in groundwater samples from the Fazilka, the digested filtrates were produced using a diethylenetriamine penta acetic acid extraction procedure (Virk 2019). After being produced, the quantity of arsenic in these filtrates was determined by using an inductively coupled plasma–AES (Thermoelectric ICAP-6300 model). Groundwater samples were taken using a random sampling technique in Patiala, Ferozepur, Rupnagar, and Amritsar. To avoid contamination, HNO₃ was used to clean the polyethylene plastic bottles before the sample collection. The bottles were then given a final washing with distilled water prior to the collection of 1,000 mL of flowing groundwater. Three milliliters of pure nitric acid of analytical grade were added to each sample. The arsenic content of the groundwater was measured using a graphite furnace–atomic absorption spectrometer, more precisely the 240 FS AA model from Agilent Technologies, Santa Clara, California, USA. Before analyzing the samples, the spectrometer was carefully calibrated, and a blank run was performed to ensure accuracy in the readings (Central Ground Water Board). The measured concentration of arsenic in groundwater in all districts is above the permissible limit of 0.01 mg/L.

Human health risks are quantified using the United States Environmental Protection Agency's (USEPA) approved health risk model. A quantitative evaluation of the impact of environmental pollution on human health is known as a health risk assessment. In the present study, the non-cancer risk due to arsenic contamination in groundwater is calculated by evaluating the hazard quotient (HQ) considering ingestion and dermal routes.

An HI of more than one indicates that there may be adverse effects on human health, emphasizing the need for careful consideration and possible mitigating actions.

CSF is a heavy metal-dependent factor based on actual studies demonstrating the adverse health effects of specific carcinogenic pollutants. According to the USEPA recommendation, for arsenic, the CSF is 1.5 mg kg⁻¹ day⁻¹ for the ingestion pathway.

The permissible range of carcinogenic risk is between 10⁻⁶ and 10⁻⁴, whereas the maximum acceptable level recommended by the WHO is 10⁻⁴.

A deterministic approach is used when limited data are available or when the system being studied is well understood. In the deterministic method, the health risk is assessed from the known parameters, and it focuses on assessing the impact of single-value risk. This method considers the site-specific data to estimate the risk of carcinogenic and non-carcinogenic elements on human health in adults and children, but there is inherent uncertainty and variability in the parameters used in the calculations. To calculate a single-point estimate of risk, the various parameters used in calculations are listed in Table 2.

Evaluation of potential risks to human health is inherently ambiguous due to individual human characteristics and environmental variables. A more realistic approach (Monte-Carlo simulations) to risk calculation can be achieved by addressing issues of variability and uncertainty. It is used to assess the risks of carcinogenic and non-carcinogenic risks in adults and children due to direct exposure to contaminated water via ingestion and dermal pathways. To accomplish this, Monte-Carlo simulations are used to estimate the non-carcinogenic and carcinogenic risks based on their probability distributions of various parameters. In this method, input parameters are defined as probability distributions resulting in the average daily dose and the HI. The value of each parameter is randomly chosen from the probability distribution function as given in Table 2. To test the reliability of the results, various independent runs were conducted with 10,000 iterations to estimate the non-cancer risk. The results of non-carcinogenic and carcinogenic risks are given in Table 4. The non-carcinogenic risk falls within an acceptable range if their HI value is less than 1.

The analysis of groundwater pollution by arsenic in Punjab's districts shows significant regional variation. Amritsar has the highest arsenic content, whereas Moga has the lowest. All districts except Moga have arsenic levels over the WHO-recommended 0.01 mg/L threshold. A previous study on groundwater contamination due to arsenic exposure in the area affirms these results (Table 3). The deterministic approach is used to calculate the HI for adults and children in the seven districts (Table 4). The findings demonstrate that children are more vulnerable than adults. In Moga, Faridkot, Fazilka, Patiala, Ferozepur, Rupnagar, and Amritsar, the corresponding HI values for adults are 0.41, 1.62, 3.54, 2.01, 2.58, 3.35, and 7.94. The equivalent values for children are much higher: 1.72, 6.83, 14.87, 8.44, 10.85, 14.07, and 33.26. According to the deterministic analysis, children are more at risk for non-carcinogenic diseases due to arsenic exposure; in all districts, including Moga, the HI value is more significant than 1 for children. On the other hand, adults in Moga have the HI values of less than 1, indicating no risk. All other districts, however, surpass the threshold, with Amritsar's adults being at the highest risk. The elevated HI values in children highlight their heightened vulnerability, especially in areas like Fazilka, Ferozepur, and Amritsar, where the HI levels surpass 10, indicating severe health issues such as cardiovascular disease, skin lesions, and cancer.

Long-term exposure to contaminated groundwater raises the risk of acquiring cancer since arsenic is a proven carcinogen. The ELCR was assessed using probabilistic and deterministic approaches in each of the studied districts. According to the deterministic analysis, the cancer risk for adults in the Moga, Faridkot, Fazilka, Patiala, Ferozepur, Rupnagar, and Amritsar are 1.84 × 10⁻⁴, 7.29 × 10⁻⁴, 1.59 × 10⁻³, 9.01 × 10⁻⁴, 1.16 × 10⁻³, 1.50 × 10⁻³, and 3.50 × 10⁻³, respectively. The ELCR for children are significantly higher: 7.74 × 10⁻⁴, 3.06 × 10⁻³, 6.66 × 10⁻³, 3.78 × 10⁻³, 4.80 × 10⁻³, 6.30 × 10⁻³, and 1.40 × 10⁻², in the same order. Particularly in Fazilka, Rupnagar, and Amritsar, the values in several districts above the WHO's threshold limit of tolerable cancer risk (1 × 10⁻⁴). The cancer risk for adults in Moga, Faridkot, Fazilka, Patiala, Ferozepur, Rupnagar, and Amritsar is estimated using the 95th percentile probabilistic approach to be 3.51 × 10⁻⁴, 1.21 × 10⁻³, 2.15 × 10⁻³, 1.24 × 10⁻³, 1.46 × 10⁻³, 3.10 × 10⁻³, and 4.60 × 10⁻³, respectively. 1.58 × 10⁻³, 5.14 × 10⁻³, 9.41 × 10⁻³, 5.30 × 10⁻³, 6.51 × 10⁻³, 1.36 × 10⁻², and 2.01 × 10⁻² are the similar values for children. More than 10 children out of every 1,000 might get cancer from arsenic exposure in Rupnagar and Amritsar, where the cancer risk for children is more than 1.36 × 10⁻² and 2.01 × 10⁻², respectively. These findings highlight the seriousness of the carcinogenic hazards associated with arsenic exposure, especially for children. Long-term exposure to tainted groundwater may raise the incidence of cancer in these areas, which has serious public health ramifications. Focused remediation techniques and public health initiatives are essential to address this issue effectively.

While this study provides critical insights into the health risks posed by arsenic contamination in groundwater, limitations are acknowledged. This study only looks at arsenic, leaving out other potentially dangerous pollutants, including nickel (Ni), lead (Pb), and iron (Fe), which are often co-contaminants in groundwater. A more thorough examination that considers a number of pollutants should be included in future research. To increase the accuracy of risk evaluations, long-term studies with more detailed exposure data should be performed. Furthermore, a more comprehensive knowledge of the health effects of arsenic exposure in Punjab could be achieved by diversifying the research to include other areas and using data from continuous monitoring.

This study highlights the significant health concerns associated with arsenic-contaminated groundwater in Punjab's seven districts, where the majority of the population, especially children, is at higher risk for both non-carcinogenic and carcinogenic diseases. For children in every district, the HI values for non-carcinogenic hazards are more than one, suggesting possible health problems such as neurological damage, cardiovascular illnesses, and developmental delays. Moreover, high ELCR indicates substantial long-term cancer risks, with certain districts such as Amritsar, Rupnagar, and Ferozepur exhibiting the greatest risk of developing cancer as a result of arsenic exposure. Monte-Carlo simulations have revealed the variability in exposure and risk, helping to provide a more nuanced risk assessment that highlights the disproportionate vulnerability of children. These results highlight how urgent arsenic mitigation measures, including better water treatment, public health campaigns, and continuous groundwater quality monitoring, are needed. In order to protect the health of the impacted communities and reduce exposure levels, particularly for pediatric populations, immediate action is essential. The study's findings provide authorities with a solid basis on which to build their priorities for reducing arsenic and enhancing public health in Punjab's impacted areas.

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

The authors declare there is no conflict.