mg/kg in the continental crust, in which 1.5–3.0 mg/kg was in igneous rock followed by 1.7–400 mg/kg in sedimentary (CGWB 2014).
Arsenic contamination is distinguished in important basin river basins of India, the Ganga and Brahmaputra river basins. These two rivers originated from the Himalayas and create the two significant river basins of India. The As contaminants are prevalent in the lowland area rich with either organic or clayey deltaic sediments in the Bengal basin, as well as sites of the entrenched river channels having the similar facies but sometimes in small pockets in the middle Ganga plain (MGP), i.e. Uttar Pradesh, Bihar, Jharkhand, and West Bengal (Chakraborti et al. 2002; Raju et al. 2012; Saha and Sahu 2016; Tirkey et al. 2017; Kumar et al. 2019). Arsenic induced groundwater deterioration in MGP to the lower Ganga Plain including Uttar Pradesh, Bihar, Jharkhand, and West Bengal and consistently created an adverse impact on human health.
Arsenic is more concentrated in the alluvial aquifers, small lenses in the MGP, low‐lying basin of Bengal, and localized contamination through the gneissic aquifers in Chhattisgarh. Geographically, the large area comes under arsenic contamination from the 89 blocks and 57 blocks in West Bengal and Bihar, respectively. Arsenic contamination is also pervasive sequentially in Uttar Pradesh, Jharkhand, Haryana, Punjab, Manipur, and Assam (Aulakh et al. 2009; Saha and Sahu 2016; Lapworth et al. 2017; Tirkey et al. 2017; Kumar et al. 2019). Most of the cases were reported from the younger alluvial aquifer systems (CGWB 2014).
1.2.1 Health Impact
As poses more adverse health impact to human than animals due to different gastrointestinal absorption. The consumption of As‐contaminated water damages human health, including respiratory distress resulting into laryngitis, bronchitis, or rhinitis, cardiovascular effects, and gastrointestinal effects like lips burning, pain while swallowing, abdominal pain, thirst, and nausea. Consumption of the inorganic As increases the risk of lung cancer, including the side effects of it such as headache, lethargy, hallucination, keratosis and hyperpigmentation in skins, seizures, and mental confusion (Mandal et al. 1996; CGWB 2014).
The organic and inorganic arsenic compound has been introduced in the water system through geological and anthropogenic sources. It is available in all geological material in variable concentration. Arsenic mobilization depends on three mechanisms in groundwater, which have been proposed:
1 Oxidation of arsenic‐bearing pyrite minerals.FeAsS +13Fe3+ + 8H2O → 14Fe2+ + SO42‐ + 13H+ + H3AsO4(aq)
2 Dissolution of As‐rich iron oxyhydroxides (FeOOH) under reducing conditions:8FeOOH‐As(s) + CH3COOH + 14H2CO3 → 8Fe2+ + As(d)+ + 16HCO3− + 12H2O
3 Release of As present in the aquifer media exchanged with phosphate (H2PO−) linked with percolation of phosphate ions into the aquifer when excess application is done in farming practices (Acharya et al. 1999; Pokhrel et al. 2009).
The dissolution of FeOOH under reducing conditions reflected the possible reason for the elevated concentration of arsenic in subsurface water (Harvey et al. 2002). Apart from these, arsenic shows a strong affinity for protein; the biological sources also contribute arsenic though the soil and water ecosystem. Arsenic exhibits a strong affinity for proteins, lipids, and other cellular components and as such, accumulates readily in living tissues (Ferguson and Gavis 1972). Besides this, arsenic concentration was observed high in the aquatic organism through the processes of biomagnification.
1.2.2 Remediation
Several conventional methods are available for arsenic removal from the water system, and the most commonly applied technologies include coprecipitation with adsorption onto coagulated flocs and sorptive media, oxidation method, treatment with lime, membrane filter technique, and ion exchange process. In the oxidation method, the dissolved oxygen converts the mobile arsenite into the less mobile arsenate and dissolved ferrous into ferric ion lowered the arsenic concentration in groundwater (Ahmed 2001; Ayoob et al. 2007).
1.3 Fluoride
Fluorine is highly electronegative in nature and cannot stand isolated in the environment due to its high reactivity. It occurs in the form of oxides in the natural system with the oxidation state of −1. It presents in water as fluoride. Fluoride occurrence in groundwater is predominantly geogenic in nature. Fluoride comes in groundwater by the dissolution of F‐bearing rocks, as well as anthropogenic pollution. Fluorite (CaF2), fluore‐apetite (Ca5(PO4)3F and apatite Ca(PO4)3(F/OH/Cl), hornblende Ca2(Mg,Fe,Al)5(Al,Si)8O22(O.H.,F)2, and biotite K(Mg,Fe)3(AlSi3 O10) (F,OH)2 in gneisses are important fluoride‐bearing minerals. Moreover, fluorine may also present as the constituent of clay minerals and through rock–water interaction it liberates as fluoride into the subsurface water (Raju et al. 2009; Banerjee et al. 2011). Moreover, leaching of fluoride depends on the alkalinity of the groundwater (Brindha and Elango 2011). In the alkaline environment, the leaching of fluoride will be higher; the alkaline environment is attributed to the dissolution of silicate minerals and the leaching of organic matters from the soil layer (Hoque et al. 2000). Some amount of fluorides may occur in groundwater because of mineral fluorite (CaF2) dissolution. The reaction is given below (Helgeson 1969):
If the aquifer system has a high mineral content of calcite, it also supports fluoride dissolution from the fluoride‐rich minerals. Therefore F− will release in water if soil and groundwater have lower calcium content (Kundu and Mandal 2009; Brindha and Elango 2011). The factors that influence the fluoride concentration in groundwater include fluoride‐bearing minerals, pH, temperature, anion exchange capacity of aquifer media (O.H.− for F−), residence time, porosity, soil structure, depth, groundwater age, and bicarbonates (Grützmacher et al. 2013). The fluorine concentration varies in different rock types such as the igneous rocks (100–>1000 ppm), sedimentary rocks (100 ppm in limestone to 1000 ppm in shales), and in metamorphic rocks (up to 5000 ppm) (International Groundwater Resources Assessment Centre, Report nr. SP 2004‐2). Apart from this it leaches from the agricultural activities through phosphatic fertilizers and from the effluents of the ceramic industries in which cay has been used, as well as is present in high amounts in the flying ash from the burning of coal. In geological material, the median fluoride concentration present in the sequence of metamorphic rocks ≥ granitoid ≥ complex rock (Manikandan et al. 2014). Besides geogenic sources, phosphatic fertilizer, cow dung, industrial effluents, and other urban waste are responsible for the fluoride in groundwater (CGWB 2014).
1.3.1 Health Impact
Fluoride upholds healthy teeth and bone development in ranges of 0.7–1.2 mg/L. In developed countries, over 50% of the populations fluoridate water up to this range (Alfredo et al. 2014). However, at higher concentration, i.e. above 1.5 mg/L, it can have disadvantageous health effects as it incorporates into budding enamel crystals and substitutes the hydroxyl ions in the apatite structure. Extended consumption of highly fluoride‐contaminated water during the budding phases of life can cause fluorosis problems linked to mottled or brittle teeth, or even more dangerous in the form of extreme skeletal fluorosis linked with porous bone structures. The World Health Organization (WHO) has reported that the consumption of highly fluoridated water (>1.5 mg/L) is a health concern.
1.3.2 Remediation
The generally applied methods for defluoridation are membrane separation, coagulation‐precipitation, adsorption, lime softening, and activated alumina (Grützmacher et al. 2013). Hydrous
