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 and it is a major problem in the world. It is caused by the rising of the water table due to the evaporation of water from the soil. In drylands, the water loss is reduced due to the removal of vegetation and change in land use patterns that allows accumulation of more salts into the soil and groundwater, which have adverse effects on plant growth and lead to low crop yield (Zaman et al. 2018). The other causes that induce water table rise are restricted drainage systems, excessive recharge of groundwater due to heavy rains and floods, and replacement of deep‐rooted perennial plants with shallow‐rooted annual plants.
3.4.3 Tertiary Salinity
Tertiary salinity is also called irrigated salinity. It is characterised by the rise in the local groundwater level due to repetitive irrigation with large quantities of water over many cycles. This process can add some salts to the soil profile, which may also be mobilised into the groundwater. Each successive irrigation or reuse of saline groundwater keeps adding more salts to the groundwater, which progressively becomes more saline, resulting in higher levels of salinity over several cycles (Zaman et al. 2018). It gets even worse when irrigating from poor quality water or saltwater.
3.4.4 Urban Salinity
Urban salinity is a combination of both dryland salinity and irrigation salinity that has the potential to affect valuable assets. It is characterized by the rise in groundwater level that is possibly due to blocking or changing natural drainage patterns due to urban developmental activities like laying roads, constructing buildings and other infrastructures, and leakage of pipes and drains. The most common sources of urban salinity are untreated urban effluents/industrial wastewater, building materials, fertilizers, and chemicals (Brindha and Schneider 2019).
3.5 Effects on Agriculture
3.5.1 Soil Structure
The bivalent cations such as Ca2+ and Mg2+ tend to flocculate, which facilitates penetration of water into roots, whereas the monovalent cations such as Na+ and K+ disperse the clay particles, which reduces the porosity of the soil. Hence the excess amount of Na+ and K+ has a profound impact on the relationship between soil and water, resulting in soil erosion and crop failure (Chibowski 2011).
3.5.2 Oxidative and Alkaline Stress
In general, the osmotic gradient helps in taking water from the soil by roots. Salinity in soil diminishes the osmotic gradient, which reduces the intake of water by roots and hinders cellular activities. This leads to the loss of vacuole fluid and the plant starts to wilt (Litalien and Zeeb 2019). The alkaline soils, usually saline/saline‐sodic with pH above 8, tend to reduce the absorption of nutrients that due to the redox potential of major nutrients (Husson 2013).
3.5.3 Ion Toxicity
Long‐term saline stress in terms of excess Cl− and Na+ ions in soils induces the accumulation of ions into the plant that leads to ion toxicity. A high concentration of Cl− ions in the soils affect the plant; further, it can affect photosynthesis, which leads to leaf burn and necrosis. Whereas the excess amount of Na+ ions in soil reduces the intake of K+ ions, which is highly desirable (White and Broadley 2001; Barhoumie et al. 2007; Machado and Serralheiro 2017). Boron toxicity is a common issue in the soils of the arid and semi‐arid regions. It affects various aspects of the plant growth, such as metabolism alteration, lowering chlorophyll content in leaves, and decreasing root growth (Nable et al. 1990, 1997).
3.5.4 Nutrient Deficiencies
Continuous salt accumulation in soils over a period of time can cause an ionic imbalance in plant cells that inhibits the absorption of core elements like Ca2+, K+, and NO32−. Accumulation of Na+ and Cl− ions in soil induces nutritional deficiencies in plant tissues, which results in Na+ induced Ca2+ and K+, Ca2+ induced Mg2+ and Cl− induced NO32− deficiencies (Grattan and Grieve 1992). Excess boron in soils results in deficiencies of Ca2 + in plants that cause necrosis of the leaf (Abdulnour et al. 2000).
3.6 Effects on Non‐Agricultural Lands and Other Natural Resources
3.6.1 Subsidence of Land
Salinity has a profound effect on land subsidence, especially in clay‐dominated coastal soils. Higher salinity in water reduces the interconnectivity of the pores by converting clay fabrics into parallel alignments that induce the fast dissipation of pore water. Hence, consolidation is more pronounced (Sarah et al. 2018).
3.6.2 Corrosive Risk
In general, the corrosive risk of freshwater is lower than that of saline water. The water containing a high percentage of dissolved oxygen and sodium and other chlorides makes metals like steel and low‐alloy steels more susceptible to metal corrosion. In fact, these are not only the causes but are also affected by the other dependant factors such as pH, temperature, amount of soluble gases, and pollutants (Zakowski et al. 2014).
3.6.3 Deterioration of Water Quality
Salinity is one of the major issues that affect water resources in various forms. It is possibly due to both natural and anthropogenic activities.
