Hazard-mapping and health risk analysis of iron and arsenic contamination in the groundwater of Sylhet district

Only 0.1% of the total water on our planet is pure drinking water (De Filippis et al. 2020). Most developing countries rely on groundwater as their principal supply of potable water, as well as water for agriculture and industry (Kadam et al. 2022). Despite meeting more than half of the world's drinking water requirement, rising population, growing urbanization, and mass industrialization have put a strain on groundwater resources (Amiri et al. 2021; Shukla & Saxena 2021). Additionally, depletion and deterioration of the available surface water supplies are also other contributing factors to increasing pressure on the groundwater (Singh et al. 2019). The chemical components of groundwater, which are primarily influenced by the underlying geological structures and activities caused by humans such as urbanization, wastewater discharges, and mining operations in the surrounding areas, determine its quality in most cases. Over the last few decades, growing anthropogenic interference has disrupted the chemical balance and resulted in a depletion of both the quality and quantity of groundwater (Devic et al. 2014; Selvakumar et al. 2017; Wang et al. 2020). In addition, the weathering and erosion of rocks, industrial discharges, agricultural practices, seepage of contaminated water, and the use of geothermal waters all contribute to the contamination of groundwater (Bodrud-Doza et al. 2020). When heavy metals are added to polluted groundwater, the contamination levels rise to even higher levels (Alsubih et al. 2021).

Heavy metals refer to both essential and non-essential trace metals, each of which can pose a risk to living things in varying degrees based on their properties, physical appearance, and concentration levels. These metals have a significant combined effect on the global water supplies (Marcovecchio et al. 2007). Because of their growing abundance in groundwater and risk to human health, arsenic and iron are among the most thoroughly investigated heavy metals by numerous experts worldwide, i.e., India (Ravindra & Mor 2019; Alsubih et al. 2021; Khan & Rai 2022; Sharma et al. 2022), China (Wu et al. 2009; Li et al. 2015; Liu & Ma 2020; Jiang et al. 2022), Thailand (Wongsasuluk et al. 2014), Italy (Sappa et al. 2014), Ghana (Asare-Donkor et al. 2016), and South Africa (Masok et al. 2017).

The presence of these metals in groundwater in colloidal, particle, or diluted phase forms, as well as their deposition in plants and animals, classifies them as components of the human food chain (Wcisło et al. 2002; Wongsasuluk et al. 2014). As far as groundwater contamination is concerned, Bangladesh has been facing a threat to arsenic and iron contamination. The Department of Public Health Engineering (DPHE) and the British Geological Survey (BGS) undertook a nationwide hydro-chemical survey and discovered that numerous drinking wells in Bangladesh exceeded arsenic and iron limits (Tonmoy et al. 2009).

The allowable concentration of arsenic for drinking water indicated by World Health Organization (WHO) is 0.01mg/L, while 0.05 mg/L is the standard limit in Bangladesh. Research has shown that 8.4% of tube wells in the country contain more than 0.3 mg/L arsenic (Smith et al. 2000; Hasan Shahriar & Jim 2019). According to the Bangladesh's Department of Environment (DoE), the acceptable limit for iron is 0.3–1 mg/L (Tonmoy et al. 2009; Hasan et al. 2019). Multiple researchers assessed the elevated arsenic and iron concentrations in Gopalgoanj (Rahman et al. 2018), Rangpur (Towfiqul Islam et al. 2017), Chapai-Nawabganj (Islam et al. 2017a), Patuakhali (Biswas et al. 2014) district and Central West part (Bodrud-Doza et al. 2019) of Bangladesh. These two elements are also found high in Sylhet district where groundwater is used for both drinking and residential purposes (Chowdhury et al. 2017; Ahmed et al. 2019a, 2019b; Begum et al. 2019). In 2020, Bodrud-Doza et al. identified a higher Fe concentration in Dhaka and demonstrated that the quality of subsurface water is influenced by anthropogenic activities, rock–water interaction, and ion exchange (Bodrud-Doza et al. 2020). Surma river basin in the Sylhet region witnesses a significant concentration of iron during the monsoon (Alam et al. 2007). A hydrogeochemical analysis done by Ahmed et al. revealed that silicate weathering, defined by an active cation exchange mechanism, and carbonate weathering increase element concentrations in the groundwater of Sylhet (Ahmed et al. 2019a, 2019b).

In addition to other factors, human activities have the greatest impact on the groundwater quality in this region (Islam et al. 2017a, 2017b; Ahmed et al. 2019a, 2019b). When this groundwater is used domestically, it can endanger people by exposing them to oral and dermal exposure pathways (Sappa et al. 2014; Khan & Rai 2022; Sharma et al. 2022). While many research studies have been done on the water quality of the drinking water along with its impurities and their effect on human consumptions, no thorough study of an overall hazard map of these metals and their risk for both oral and dermal ingestion has been carried out.

This research aims to find the arsenic and iron-contaminated aquifers in the Sylhet district through the development of a hazard map using geographic information system (GIS) and the IDW interpolation method. GIS serves as a powerful tool for managing and interpreting geographical information about water resources, offering efficient means to analyze pollution patterns and relationships (Selvam Manimaran & Sivasubramanian 2013). The resultant hazard map is critical for assessing groundwater contamination, which is vital for safe drinking water and agricultural use, as well as for mitigating serious environmental health issues. In addition, this study conducts a thorough risk evaluation of human health among the residents of Sylhet, including both adults and children. This assessment focuses on the noncarcinogenic hazard and cancer risk (CR) associated with the presence of arsenic and iron in groundwater. By doing so, it aims to provide a comprehensive understanding of the potential health impacts of these heavy metals in the region. This approach combines spatial analysis with health risk assessment, intending to offer a holistic view of the environmental challenges faced by the community in Sylhet, thereby guiding effective management and remediation strategies.

Groundwater samples were collected from 12 upazilas in Sylhet from November 2018 to November 2019, in winter and summer to avoid monsoon dilution. Samples were randomly chosen across different administrative units, with primary data from five samples in each upazila and secondary data from DPHE. The sampling sites were geographically pinpointed using GPS, and the water was collected from deep tube wells, ranging in depth from 150 to 240 m. Samples of 500 mL were taken in pre-washed high-density polyethylene bottles and cleaned with HCl and NaOH. To ensure purity, the samples were filtered through the Whatman 42 filter paper. Acidification to pH <2 using 65% HNO₃ was done to prevent precipitation and adsorption and were stored below 4 °C.

As and Fe levels were measured using a UV-spectrophotometer (DR 6000). For arsenic, a 35 mL sample was treated with hydrochloric acid, potassium iodide, and stannous chloride, reducing arsenic to its trivalent state. Lead acetate-treated glass wool was used in the scrubber, and the sample was treated with silver diethyldithiocarbamate (SDDC) solution and Zn dust. The resultant red complex indicated AsH₃ presence, measured at 520/535 nm. For iron, 100 mL of the sample was mixed with HCl and potassium thiocyanide, and the absorbance was measured to determine concentration.

Hazard maps were created using IDW interpolation. The Universal Transverse Mercator (UTM) projection system within zone 46 N-Datum Geodetic System (WGS) 1984 was used for spatial distribution maps, categorized into five classes (Mosaferi et al. 2014). A combined hazard map was created using an additive weight approach, assigning weights of 0.7 and 0.3 to iron and arsenic, respectively, based on Analytic Hierarchy Process (AHP) analysis considering their frequency, concentration, and impact (Do 2013).

The health risk associated with such elevated levels of iron is a consequence of natural and human-induced factors, with ferric oxides and hydroxides being the primary natural contributors, while human impact stems from urban wastewater discharge and runoff from agricultural lands. It is suggested that geochemical interactions, particularly silicate and carbonate weathering through ion exchange, play a significant role in the escalation of iron content in the groundwater. The evaluation of health risks for each heavy metal is typically divided into carcinogenic and noncarcinogenic. There are two key toxicity variables to take into consideration when determining risk: cancer slope factor (CSF) for characterizing CR, and reference dose (RfD) for non-carcinogen risk (Wongsasuluk et al. 2014). For measuring the CR from carcinogenic pollutants, the carcinogenic approach is solely used, whereas for the noncarcinogenic risk effects, the hazard quotient (HQ) is used (Masok et al. 2017).

The CDI, HQ, and hazard index (HI) are calculated in the current study using USEPA standards to measure the ingestion and dermal rate of pollutants. Tables 1 and 2 refer to the standard values and references used for the calculation of the metrics for human health.

Arsenic concentrations in the study area are categorized into five classes according to safety thresholds, as detailed in Table 3. The ‘Excellent’ category aligns with the WHO's recommended limit of 0.01 mg/L, while the ‘No risk’ category corresponds to the Bangladesh National Standard (<0.05 mg/L) (WHO 2008; Department of Environment 2023). The two categories encompass a combined area of 1,762 km², or 51% of the study region. The remaining areas are classified into medium, high, or extremely high-risk zones, which constitute 29, 17, and 3% of the study area, respectively.

Groundwater iron concentrations have been classified into five risk categories according to established safety thresholds, as shown in Table 4. Approximately 53% of the area falls within the safe zone, with iron concentrations ranging from 0.2 to 1.0 mg/L, considered excellent to no risk. However, the remaining 47% are categorized from medium to high risk.

The very high-risk zone, which includes Gowainghat Upazila, the southeastern part of Companiganj, and the eastern part of Zakiganj, occupies an area of approximately 140 km², accounting for about 4% of the total district. On the other hand, the groundwater in Sylhet Sadar and Fenchuganj, which covers an area of 831 km², is deemed perfectly safe for consumption. The threshold used to define the categories in the composite map has been shown in Table 5.

The estimated HQ values for ingestion of As and Fe polluted drinking water for oral and cutaneous absorption pathways of exposure are shown in Table 6. The noncarcinogenic risk assessment reveals that arsenic poses a greater risk than iron through oral consumption, particularly for children. The HQ values for arsenic are notably higher than those for iron, showing a more significant health concern. This disparity is further emphasized by the finding that certain regions show extremely high HQ values for arsenic, due to increased groundwater contamination from urbanization and agricultural activities.

Iron is crucial for public health, serving vital roles in oxygen transport, DNA synthesis, and electron transport, yet its bioavailability is often limited due to its tendency to form insoluble oxides (Abbaspour et al. 2014). This poses a significant nutritional challenge, particularly affecting children, adolescents, and women of reproductive age, leading to potential impacts on cognitive development, immune function, and pregnancy outcomes (Abbaspour et al. 2014). The fraction of iron absorbed from the amount ingested is typically low but may range from 5 to 35% depending on circumstances and the type of iron (McDowell 1992). Iron absorption occurs by the enterocytes by divalent metal transporter 1, a member of the solute carrier group of membrane transport proteins which takes place predominantly in the duodenum and upper jejunum (Muir & Hopfer 1985). Generally, drinking water is not considered a significant source of iron compared to other dietary sources as the permissible iron content in water according to the WHO guidelines is very low (<0.3 mg/L). Also, the water with more than the permissible amount of iron can cause distaste, staining, and other esthetic problems. Considering that an adult might drink about 2 L of water per day, the contribution of iron from water would typically be much less than 1 mg/day, making it a small fraction of the overall daily requirement. The dietary needs for iron vary among individuals, especially between adults and children, and between males and females. The average adult stores about 1–3 g of iron in his or her body (Abbaspour et al. 2014). About 1 mg of iron is lost each day through the sloughing of cells from skin and mucosal surfaces, including the lining of the gastrointestinal tract (Abbaspour et al. 2014). Menstruation increases the average daily iron loss to about 2 mg/day in premenopausal female adults (Abbaspour et al. 2014).

The comprehensive study conducted in the Sylhet district of Bangladesh provides an insightful analysis of groundwater contamination by arsenic and iron, highlighting critical areas of concern and their associated health risks, especially among adults and children. By constructing hazard maps for arsenic and iron contamination, the research identifies the most polluted regions – Gowainghat, Companyganj, Jaintiapur, the eastern part of Zakiganj, and the southern part of Balaganj – as exceeding both national and WHO guidelines for safe drinking water. Conversely, areas such as Golabganj, Dakkin Surma, and Fenchuganj are recommended for their low risk of arsenic contamination, with certain locations like Sylhet Sadar and Kanaighat Upazilla also being iron-free. The detailed risk analysis reveals the highest hazards in Balaganj and Bishwanath, based on HI_oral values for adults and children, whereas the lowest risks are observed in Dakkin Surma and Fenchuganj, with dermal hazard indices suggesting minimal risk to residents.

Crucially, the study underscores the arsenic noncarcinogenic effects of both iron and arsenic as well as the carcinogenic potential of arsenic across the areas, emphasizing heightened vulnerability of the children. This alarming revelation demands urgent attention toward mitigating these risks through effective groundwater management and the development of public health policies that ensure the provision of safe drinking water, thereby aligning with the sustainable development goal (SDG) 6 aimed at securing clean water and sanitation for all. Furthermore, the spatial analysis not only delineates the zones of significant contamination but also serves as a vital tool for legislative authorities and policymakers in strategizing tube well placements and enacting legislation to combat groundwater pollution. The acknowledgement of arsenic and iron as prevalent contaminants sets a precedent for future research to explore other heavy metals present in the groundwater of Sylhet, aiming to devise comprehensive mitigation strategies against the adverse health effects posed by these pollutants.

In conclusion, this study effectively maps out the hazardous landscapes of arsenic and iron contamination within the Sylhet district, offering a foundational understanding of the health risks involved and proposing a pathway for enhancing water quality management and public health initiatives. By doing so, it not only contributes to the immediate need for safe drinking water but also emboldens the broader objective of sustaining environmental health and well-being, encapsulating a vital step forward in the global endeavor to mitigate water-related diseases and ensure environmental sustainability.

The authors thank the Department of Civil and Environmental Engineering at Shahjalal University of Science and Technology, Sylhet, DPHE and Sylhet City Corporation for their resources and support in data gathering and insights into groundwater issues. Special appreciation is extended to the participants and local communities in the Sylhet district for their cooperation during sample collection.

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

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

The authors declare there is no conflict.