Seawater intrusion is the subsurface landward movement of saline ocean water into freshwater coastal aquifers. Along India’s extensive 7,500 km coastline, this process contaminates drinking water sources, salinizes agricultural lands, and undermines freshwater security.

Underlying Hydrogeological Mechanism (Ghyben-Herzberg Principle)

Freshwater has a slightly lower density ($1.000text{ g/cm}^3$) than seawater ($1.025text{ g/cm}^3$). In an undisturbed coastal aquifer, freshwater floats over denser saline water, with every metre of freshwater head above sea level supporting approximately 40 metres of freshwater depth below sea level. When freshwater extraction lowers the hydraulic head, seawater advances inland, forming an invasive cone of ascension.

Key Causes of Seawater Intrusion in India

1. Anthropogenic Drivers

• Over-Extraction of Groundwater: Unregulated pumping for urban water supply and irrigation lowers freshwater tables below sea level, reversing natural hydraulic gradients (e.g., groundwater salinization across coastal Saurashtra and Kutch in Gujarat).
• Intensive Coastal Aquaculture: Unmonitored brackish-water shrimp farming and commercial ponds allow saline water to seep into shallow coastal aquifers.
• Industrial Demands: Concentrated extraction by coastal chemical hubs, thermal power plants, and refineries creates deep cones of depression in surrounding aquifers.
• Human-Induced Land Subsidence: Excessive withdrawal of subsurface fluids causes aquifer compaction, lowering ground elevation relative to sea level.

2. Natural and Climatic Drivers

• Sea Level Rise: Global climate change and rising sea levels increase hydrostatic pressure along shorelines, pushing saline water into freshwater aquifers (e.g., in low-lying deltas like the Sundarbans and coastal Odisha).
• Cyclonic Storm Surges and Tsunami Inundation: Extreme maritime events submerge coastal lands, allowing saltwater to infiltrate directly through permeable soil layers (as observed during the 2004 Indian Ocean tsunami in Tamil Nadu).
• Porous Coastal Geomorphology: Sandy, highly permeable sedimentary strata and fractured coastal rocks facilitate rapid lateral seawater infiltration.

Remedial Measures to Combat the Hazard

• Artificial Aquifer Recharge: Construct percolation tanks, check-dams, and recharge shafts to direct monsoon runoff underground, raising freshwater hydraulic heads to hold back seawater.
• Constructing Subsurface Physical Barriers: Install underground impermeable cutoff walls or slurry trenches parallel to the coast to physically block lateral seawater migration.
• Controlled and Regulated Pumping: Enforce Central Ground Water Authority (CGWA) norms, restrict deep borewells near the coast, and transition to metered groundwater usage.
• Conjunctive Water Management: Pair groundwater extraction with canal irrigation and treated surface water to reduce stress on vulnerable coastal aquifers.
• Municipal Desalination and Wastewater Recycling: Deploy seawater reverse-osmosis (SWRO) desalination plants and recycled industrial water for urban demand (e.g., Chennai’s municipal desalination plants) to avoid tapping groundwater.
• Coastal Green Buffers: Restore coastal mangrove forests to attenuate tidal surges and reduce saltwater infiltration during cyclones.

Preventing seawater intrusion requires balancing sustainable groundwater extraction with artificial aquifer recharge, ensuring drinking water security and agricultural resilience along India’s coastlines.