India
2School of Environmental Science and Engineering, Chang'an University, Xi'an, China
3School of Environmental Sciences, Jawaharlal Nehru University, New Delhi, India
4Department of Geology, Yogi Vemana University, Kadapa, Andhra Pradesh, India
3.1 Introduction
Salinity is a major social, economic, and environmental menace in climates with low rainfall and high evapotranspiration (Jabbar and Chen 2008; Abuelgasim and Ammad 2018). In general, the surface water supply in arid and semi‐arid climates is scarce, which shifts the attention of policymakers to groundwater. Groundwater in these regions is very limited and considered a major resource for sustaining terrestrial ecosystems (Balaji et al. 2019a, b; Huang et al. 2019). In addition, intense urbanisation, demand for freshwater due to an increase in population, and poor management strategies have generated additional stress to this limited resource which leads to lowering groundwater levels (Cosgrove and Loucks 2015; Nagaraju et al. 2016). Therefore, people dig bore wells to great depths for groundwater that are basically rich in soluble salts (Miglietta et al. 2017; Akinlalu and Afolabi 2018). Among complex environmental issues such as droughts, heavy blowing winds, heatwaves, and floods, salinity is also a major issue. It turns soils and irrigated land more saline, which impairs crop growth and leads to low production and land degradation (Shrivastava and Kumar 2015). Moreover, salinity is a serious public health concern and its consequences are seen mostly in coastal drylands. Consuming a higher amount of salt increases blood pressure, which increases the risk of cardiovascular disease that induces heart stroke and attack. Nowadays, it accounts for a large number of deaths worldwide. Among various direct and indirect sources, salinity is one of many serious issues that affect the hydrological cycle in terms of water quality deterioration (Pulido‐Bosch et al. 2018). In urban landscapes (non‐agricultural lands) it affects the structures by subsidence, corrosion, and water quality deterioration. All these practices result in the loss of arable lands that affects terrestrial habitats, particularly in drylands. It is estimated that the annual loss of arable land due to salinity is 20–50% (Pitman and Läuchli 2002). It is projected that about 50% of the world's arable land that is to be lost by 2050 will be attributed to the salinity problem (FAO 2009; Hussain et al. 2020).
Therefore, some knowledge on salinity‐related health issues, management strategies, and reclamation techniques are needed to tackle this exacerbated situation. Hence, the sources, implications, and reclamation strategies are discussed in this chapter.
3.2 Problem of Salinity
Salinity refers to the amount of soluble salts in water and soils, mostly of ions such as Na+, Cl−, SO42−, HCO3−, K+, NO32−, and F− (Imadi et al. 2016; Artiola et al. 2019). It is likely to be one of the major issues that affect the world economy in the near future, especially in drylands. Figure 3.1 represents the world map showing countries with salinity issues. Salinity could be either natural or human‐induced. Weathering of minerals, sea breeze (mainly in coastal areas), and capillary rise of saline water from lands of low water tables are the natural factors. This can be further accentuated by irrigated agriculture, intense fertilization, and seawater intrusion due to critical groundwater overdrafts, which are human induced. It is a serious issue affecting crop production worldwide. Abrol et al. (1988) reported that more than 932.2 million hectares of the world's fertile land are at risk due to salinization which makes the arable lands unusable for farming. According to UNEP (1992) (cf. FAO‐ITPS‐GSP 2015; Shahid et al. 2018) 1030.1 million hectares of the world's arable land were affected by salinity. About 20–50% of the world's arable land is salt‐affected and degraded (Pitman and Läuchli 2002; Glick et al. 2007). The current scenario of land degradation could be much higher than previously thought.
Figure 3.1 World map showing countries with salinity issues.
3.3 Sources of Aquifer Salinity
Salinity in aquifers can be either inland or coastal induced. The most common sources of salinity in aquifers is given in the following sections.
3.3.1 Inland Aquifer Salinity
Inland salinity is one of the main concerns linked to groundwater pollution and other associated issues worldwide (Greene et al. 2016). It is a daunting challenge for policymakers to handle water resources effectively. Inland salinity may result from a number of mechanisms like weathering of native rocks, erratic water supply due to low rainfall or high evaporation, leaching of poor quality irrigation water and untreated sewage effluents, recharge water quality, the inflow of paleo‐saline water from adjacent formations due to critical pumping of groundwater, rise in water level due to removal of vegetation and poor drainage system, and use of excess fertilizers (Figure 3.2).
3.3.2 Coastal Aquifer Salinity
Saline water intrusion is a global threat to coastal aquifers that leads freshwater ecosystems to be contaminated due to excessive groundwater pumping (Badaruddin et al. 2015). This excess pumping of groundwater near coastal aquifers reduces the hydraulic head of inland groundwater that allows the saltwater to enter into inland aquifers, which leads to aquifer salinization. In fact, it is not only the cause, but it is also affected by global warming, which increases the opportunity for the intrusion of saltwater into coastal aquifers (Figure 3.3).
3.4 Types of Salinity
Based on the causes, salinity has been divided into four types. They are given in the following sections.
Figure 3.2 Salinization of inland aquifers.
Source: Brindha and Schneider (2019), Elsevier.
Figure 3.3 Salinization of groundwater in coastal areas.
3.4.1 Primary Salinity
Primary salinity is also called natural salinity. The most common sources of primary salinity are the rainfall, the characteristics of the parent rock, and seawater intrusion (Podmore 2009). In general, the rain leaves a certain amount of salt in the soils through evaporation. Over many cycles, these salts in the soil reach elevated levels. Rocks such as granites, rhyolites, and marine sediments left by the retreating of seas can contain high salts, which may release into the soil and mobilise into groundwater through weathering. Moreover, salts may be brought into the lands by strong winds and some salts may enter into the coastal aquifer by seawater intrusion.
3.4.2 Secondary Salinity
Secondary salinity is also referred to as dryland salinity