Presence of fluoride and arsenic in groundwater has affected more than 300 million people all over the world (Bhattacharya et al., 2020; Bibi et al., 2017). Groundwater from deep and shallow tube-wells is mainly used for drinking purposes, agricultural production, and industrial processing (Mishra and Bhatt, 2008). About 110 million people residing in south Asia are majorly affected by fluoride and arsenic through groundwater (Ain et al., 2017; Brammer and Ravenscroft, 2009). The Indo-Gangetic and Brahmaputra flood plain in India and Bangladesh have been identified as a major hotspot of both these contaminants (Bhattacharya et al., 2020; Kumar et al., 2016, 2017).

Fluoride-contamination in groundwater has been reported worldwide (Maity et al., 2021); nevertheless, arid and semi-arid regions are enormously affected (Moghaddam and Fijani, 2008; Raj and Shaji, 2017). Fluoride endemic areas have been identified all across the southern and western parts of India comprising 20, out of 29 states and a substantial population, approximately 401 million (40% of Indian population) live in fluoride endemic 201 districts (Brindha et al., 2011; Chakraborti et al., 2016). Fluoride-contamination in groundwater initiates mainly from weathering of minerals like fluorite, mica, topaz etc. (Yadav et al., 2018). Likewise, in the Bengal basin, silicate minerals are reported to be responsible for fluoride leachability in groundwater (Datta et al., 2000).

Approximately, 70.4 million people from six Indian states in the Ganga-Brahmaputra Plain (GB-Plain) are potentially at stake due to arsenic toxicity through groundwater above 10 ug/L (Chakraborti et al., 2016). The lower parts of the Indo-Gangetic flood plains mainly West Bengal, India and Bangladesh are marked as a hotspot of arsenic-contamination along with several newly identified areas around the countries (Mukherjee et al., 2021; Podgorski et al., 2020). Various theories have been postulated on sources and mobility of arsenic in groundwater from Bengal Delta (Polya and Charlet, 2009). Reductive dissolution of arsenic-rich iron-oxyhydroxide (Acharyya et al., 1999; Bhattacharya et al., 1997; Bundschuh et al., 2021; Liu et al., 2009, 2013; Maity et al., 2011) in presence of dissolved organic carbon (DOC) in the subsurface aquifers leads to release arsenic into groundwater (Harvey et al., 2002; Kar et al., 2011; Van Geen et al., 2003).

In increased pH conditions, arsenate and fluoride both can contaminate groundwater simultaneously through desorption from negatively charged metal oxides (Kim et al., 2012; Maity et al., 2017). The dissolution of Fe (hydr)oxide plays a potential role in the mobilization of arsenic during the recharge of aquifers through rainfall (Chakraborti et al., 2016; Patel et al., 2019); while, mobilization of fluoride occurs due to the dissolution of silicate and calcium minerals or degassing of CO₂ (Bundschuh et al., 2013, 2017; Rao, 2011; Sahu et al., 2018).

The essentiality and toxic effects of fluoride are differentiated via a narrow margin (Kimambo et al., 2019). A lower level of fluoride (<0.5 mg/L) results in dental caries (Ijumulana et al., 2021; Keshavarz et al., 2015); even can cause dental fluorosis, especially in warmer countries due to higher consumption of contaminated water (USEPA, 1985). Though, the presence of fluoride in drinking water in the range 0.6–1.2 mg/L is beneficial for the development of bones and teeth; whereas, it is harmful when crosses 1.5 mg/L (Brindha and Elango, 2011; Mohapatra et al., 2009; USEPA, 2010; WHO, 1985). Higher concentration of fluoride (>1.5 mg/L) causes tooth cavities, dental, skeletal, and crippling fluorosis, deformation of bone structures (Bhattacharya et al., 2020; Iheozor?Ejiofor et al., 2015; Mridha et al., 2021; WHO, 2011). Higher arsenic concentration causes severe health problems including cancer, peripheral vascular and respiratory diseases (Chakraborti et al., 2016; Samal et al., 2013). The prolonged consumption of these two toxicants through drinking water leads to high health risk (Bhattacharya et al., 2020), which further increases through food chain contamination (De et al., 2021; Kazi et al., 2019; Samal et al., 2021). In the lower Gangetic plains of West Bengal, presence of high concentrations of fluoride and arsenic in groundwater and related health hazards have been reported earlier (Chakrabarti and Bhattacharya, 2013; Kar et al., 2010; Samal et al., 2015).

Mandal et al. (1998) reported the groundwater arsenic-contamination and its adverse human health effect from the gram panchayat areas of Sonarpur block, South 24 Parganas district, West Bengal. However, no evidence was available from the Rajpur-Sonarpur Municipality area in Sonarpur block, which consists of two separate towns Rajpur and Sonarpur including 35 wards administered by Kolkata Metropolitan Development Authority (KMDA). Based on the previous investigation by Public Health Engineering Department, Govt. of West Bengal, the groundwater in the region of Sonarpur block was reported as fluoride-contaminated (PHED, 2007). However, the exact location of contamination and its extent were not reported. Hence, the present study has been focused on the spatial and depth-wise distribution, prevalence and health risk of fluoride and arsenic in groundwater from 35 wards of Rajpur-Sonarpur Municipality to determine the exact location of contamination and its extent. Principal component analysis (PCA) has been performed to identify the relationship among the four elements present in groundwater including two major elements like iron and calcium along with arsenic and fluoride. The acute exposure has been evaluated through estimation of the toxic elements in urine. Fluoride and arsenic induced health risk assessment (Targeted Hazard Quotient and Cancer Risk) has been measured using modelled ADD response of these toxicants through drinking water among the three different age groups i.e. children (3–10 years), teenagers (11–19 years) and adults (20–70 years). To the best of our knowledge, this study might be a unique one that provides a clear insight into the distribution and prevalence of fluoride and arsenic in groundwater and associated human health risk from 35 wards of Rajpur-Sonarpur Municipality.