Seawater intrusion in the coastal aquifers is a major concern in India. What are the causes of seawater intrusion and the remedial measures to combat this hazard?

India has a long coastline of 7,516 km, supporting nearly 30% of its population. The coastal aquifers, which act as a critical source of freshwater, are increasingly facing Seawater Intrusion (SWI)—the landward movement of seawater into freshwater aquifers. High-vulnerability zones currently include the Saurashtra coast (Gujarat), Minjur (Tamil Nadu), and the coastal belts of Kerala and Andhra Pradesh.

Causes of Seawater Intrusion

The movement of the freshwater-saltwater interface inland is driven by a combination of anthropogenic and natural factors, disrupting the hydrodynamic balance.

1. Anthropogenic Causes (Primary Drivers):

• Over-extraction of Groundwater: Coastal agriculture and rapid urbanization lead to heavy pumping. According to the Ghyben-Herzberg principle, a 1-meter drop in the freshwater table can cause a 40-meter upward rise of the saltwater interface, leading to rapid salinization.
• Reduced Freshwater Recharge: Haphazard coastal urbanization and concretization reduce the natural infiltration of rainwater into underground aquifers, lowering the hydrostatic pressure of freshwater.
• Upstream Damming and Hydrological Alterations: Construction of dams and barrages on peninsular rivers reduces the downstream flow of freshwater into estuaries and coastal plains (e.g., reduced flow in the Cauvery delta), weakening the natural pushback against seawater.
• Unregulated Aquaculture: The proliferation of intensive shrimp and brackish-water aquaculture in coastal states (like Andhra Pradesh and Odisha) often involves pumping saline water inland, which leaches into underlying freshwater aquifers.
• Sand Mining: Excessive coastal and riverbed sand mining destroys the natural physical barriers and reduces the freshwater retention capacity of the coastal ecosystem.

2. Natural and Climate Change-Induced Causes:

• Sea-Level Rise (SLR): Global warming is causing the sea level to rise, physically pushing the marine saltwater wedge further inland into coastal aquifers.
• Cyclones and Storm Surges: Increased frequency of extreme weather events (e.g., Cyclones Amphan, Biparjoy) causes sudden marine inundation. The stagnant seawater percolates and contaminates shallow aquifers.
• Coastal Topography: Flat coastal plains and highly permeable sandy aquifers (such as those along the Malabar and Coromandel coasts) naturally facilitate easier inland migration of seawater.

Remedial Measures to Combat Seawater Intrusion

Addressing SWI requires a multi-pronged approach combining engineering solutions, regulatory frameworks, and ecological restoration.

1. Engineering and Structural Interventions:

• Artificial Recharge: Injecting freshwater into depleted aquifers via injection wells, percolation tanks, and check dams to increase the freshwater "head" and push back the saltwater wedge.
• Subsurface Barriers: Constructing impermeable subsurface structures (like slurry walls, sheet piles, or clay cutoff walls) parallel to the coast to physically block saltwater ingress. Example: Gujarat has successfully built Tidal Regulators and Salinity Ingress Prevention Structures along the Saurashtra coast.
• Coastal Reservoirs: Storing excess floodwater in coastal reservoirs to increase local freshwater availability and enhance continuous groundwater recharge.

2. Policy and Regulatory Measures:

• Stringent Groundwater Regulation: Enforcing strict zoning and restricting the installation of deep borewells in 'Critical' and 'Over-exploited' coastal blocks under the Central Ground Water Authority (CGWA) guidelines.
• Aquifer Mapping and Monitoring: Expanding the National Aquifer Mapping and Management Programme (NAQUIM) using advanced heliborne surveys to accurately map the freshwater-saline water interface and install continuous monitoring piezometers.
• Mandatory Rainwater Harvesting (RWH): Ensuring strict compliance with RWH structures in coastal urban planning to boost natural recharge. Example: Tamil Nadu’s RWH policy significantly slowed seawater intrusion in Chennai.

3. Agronomic and Demand-Side Management:

• Crop Diversification: Shifting from water-intensive crops like paddy to low-water requiring or salt-tolerant varieties (e.g., Pokkali rice in Kerala).
• Micro-Irrigation: Promoting drip and sprinkler irrigation in coastal agriculture to drastically reduce the volume of groundwater extracted.
• Alternative Water Sources: Expanding the use of treated wastewater and setting up Seawater Reverse Osmosis (SWRO) desalination plants (e.g., Minjur and Nemmeli plants in Chennai) for industrial and urban consumption to reduce reliance on groundwater.

4. Ecological Restoration (Nature-Based Solutions):

• Mangrove Afforestation: Restoring mangroves and coastal wetlands. These "bio-shields" not only buffer against storm surges and tidal inundation but also enhance localized soil moisture and freshwater retention.

Seawater intrusion poses a severe threat to India’s coastal water security, agriculture, and the broader Blue Economy. Combating it requires transitioning from mere resource exploitation to sustainable aquifer management. Integrating ground-level hydrological management with the Integrated Coastal Zone Management (ICZM) framework is imperative to ensure climate resilience and the achievement of SDG 6 (Clean Water and Sanitation).