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 bismuth oxides (HBOs) have been examined as a potential adsorbent for the removal of F− from the contaminated water or aqueous solution (Srivastav et al. 2013). Reverse osmosis is comparatively insensitive to pH because it is a membrane separation technique (Arora and Evans 2011). However, it necessitates a cautious assessment of water characteristics and pretreatment to avert fouling. In addition to this, resistance and fouling of membrane increases due to the accumulation of rejected species and particles. In poor and rural areas of developing countries, the membrane is seldom considered a suitable technology because of issues like expense, fouling, operational sophistication, and difficulty of intermittent operation. Adoption of these advanced technologies in rural, resource‐limited areas of the world is not practical. Hence, a technique called the Nalgonda technique, which was first established by the National Environmental Research Institute in India, is applied as a household treatment. This technique is based on the coagulation‐flocculation‐sedimentation process of lime and aluminium sulphate (alum)– for fluoride elimination. Adsorption methods for fluoride removal are primarily based on clay (Mahramanlioglu et al. 2002; Chidambaram et al. 2003), charcoal (Mjengera and Mkongo 2002; Medellin‐Castillo et al. 2007), and aluminium‐based adsorbents (Ghorai and Pant 2004; Sarkar et al. 2007; Alfredo et al. 2014).
1.4 Salinity (Na and Cl)
Commonly salinity problems in groundwater are very prominent in the coastal region, followed by arid and semiarid regions. The coastal areas of Gujarat, Maharashtra, Goa, Kerala, Tamilnadu, Odisha, and West Bengal are facing the problem of saltwater intrusion termed as coastal salinity, and inland salinity problems have been reported in the states of Haryana, Rajasthan, Punjab, and Gujarat, with some limited problems in other states also. Mainly seawater intrusion is responsible for the salinity in groundwater in coastal areas, whereas agricultural wastes, agriculture runoff, heavy uses of fertilizers, and industrial effluents have caused the salinity in arid and semiarid regions. Ion exchange processes, rock–water interaction within subsurface, and the surface water with urban and semi‐urban wastes percolates through the soil and enters into the aquifer system and leads to salinity problems.
The Indian subcontinent has a coastline stretched about 7500 km long. Saltwater can intrude laterally or by coming up from the deeper layer when the groundwater level has dropped below the sea level. Moreover, tides and coastal floods may contribute to salinity in water by infiltration. Seawater intrusion incidents are common and have been observed in several states, including in Tamil Nadu, Pondicherry, and Saurashtra in Gujarat (Mondal et al. 2010; Garduño et al. 2011). There is no uptake of sodium salt by the plants. Only evaporation eliminates the sodium salts from the solution. The most significant source of sodium and chloride in groundwater, particularly in arid and semiarid expanses, is the precipitation of this salt permeating the soil in the shallow water tracts. Na+ and Cl− concentration was reported higher in the coastal zone due to saltwater intrusion. Na+ concentration increases with Cl− concentration, which resulted in an increase in the weathering of halite minerals in the groundwater. Other sources of Cl− includes natural weathering of bedrocks, volcanic activity, natural brines, saline intrusions, and atmospheric deposition along with the geographical locations, i.e. coastal/inland areas (Grützmacher et al. 2013).
1.4.1 Health Impacts
No health‐based guideline value is suggested by WHO (2011), however Cl− concentration exceeding the 250 mg/L may lead to a noticeable taste (WHO 2011). But it has been reported that high salinity in irrigation water can damage the crops, affect the plant growth, reduce the soil fertility, and deteriorate the water quality. High chloride may harm the aquatic life through leaf burn, defoliation in sensitive crops, and disturbing the oxygen distribution in water, as well as increasing the metal concentration in water, although health‐related issues are not critically observed yet. However, high sodium water can pose heart disease and high blood pressure, predominantly in vulnerable entities.
1.4.2 Remediation
Distillation, membrane technology, including reverse osmosis and microfiltration, ion exchange, and treatment using hydrotalcite are generally applied to remediate the high salinity (Grützmacher et al. 2013).