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).
1.5 Sulphate
Geogenic sources of sulphate are sulphur‐bearing minerals (e.g. gypsum, anhydrite, and pyrite (Grützmacher et al. 2013). Naturally occurring sedimentary rocks containing sulphur are pyrite (FeS2) and gypsum (CaSO4.2H2O) (Berner 1987; Gourcy et al. 2000). Oxidative weathering of pyrite occurring in alluvial sediments is also one of the causes of high SO42− in water (Raju et al. 2011).
Sulphuric acid reacts with calcium carbonate found in weathered zones and produces calcium sulphate, given by the following reaction:
Factors influencing the concentration of sulphate are sulphate‐rich minerals and reducing condition in the aquifer material (Grützmacher et al. 2013)
1.5.1 Health Impact
SO4 concentration above 1000 mg/L can cause purgative effects (WHO 2011).
1.5.2 Remediation
A natural method for removing sulphate is an anaerobic reduction in constructed wetlands. Other technical methods include reverse osmosis, N.F., and ion exchange (Grützmacher et al. 2013).
1.6 Heavy Metals
Heavy metal is a term given to a division of metals and metalloids that are differentiated by high atomic weight and specific density, five times higher to water or greater than 4000 kg/m3 (Hashim et al. 2011; Kura et al. 2018). Heavy metals are toxic even at deficient concentrations and harmful to human health (Madhav et al. 2020). The following heavy metals are addressed as geogenic pollutants in this chapter.
1.6.1 Iron
Iron is the second‐most abundant metallic element in the Earth's outer crust, but its concentration in water is usually low. It occurs in many oxidation states, such as 0, +2, +3, and +6. Many mixed‐valence compounds, like magnetite and prussian blue, have both Fe (II) and Fe (III) centres (Hashim et al. 2011). Igneous rock minerals such as pyroxenes, the amphiboles, biotite, magnetite, and especially the nesosilicate olivine are rich in iron content. In these minerals, iron mostly exists in the ferrous form (Fe3+) but ferric (Fe3+) form also occurs, as in magnetite, Fe3O4. The ferrous polysulfides like pyrite, marcasite, and the less constant species such as mackinawite and greigite might exist in the presence of sulphur and when reducing conditions prevail. Siderite (FeCO3) may form when sulphur is less plentiful, whereas under oxidizing environments, ferric oxides or oxyhydroxides species like haematite, Fe2O3, goethite, FeOOH, or other minerals having these compositions generally occur (CGWB 2014).
In soil, it is commonly present in organic waste and as plant debris. The oxidation intensity and pH firmly control the chemical nature of iron and its solubility in water. The interaction amid oxidized iron minerals and organic matter or dissolution of FeCO3 leads to a higher concentration of iron in groundwater. This type of water is clear when withdrawn, but soon it becomes cloudy and then browns due to precipitation of Fe (OH)3 (Hashim et al. 2011; Achary 2014a). Organic matter removes
