Land beneath India’s five largest cities is sinking due to over-extraction of groundwater

Why in News ?

• A new Nature Sustainability (2025) study revealed that India’s five megacities — Delhi, Mumbai, Kolkata, Bengaluru, and Chennai — are sinking due to excessive groundwater extraction.
• The study used satellite radar data (2015–2023) to map urban land subsidence, impacting over 13 million buildings and 80 million residents.

Relevance

• GS 1: Urbanisation, geomorphological processes (subsidence).
• GS 3: Environmental degradation, water resource management, disaster risk reduction.
• GS 2: Governance — institutional response through CGWA, Jal Shakti Mission, and Smart Cities.

Key Findings

• Total subsiding area: 878 sq. km of urban land.
• Population exposed: 1.9 million people at >4 mm/year subsidence rate.
• Max subsidence rates:
  • Delhi – 51.0 mm/yr
  • Chennai – 31.7 mm/yr
  • Mumbai – 26.1 mm/yr
  • Kolkata – 16.4 mm/yr
  • Bengaluru – 6.7 mm/yr

City-wise Analysis

Delhi (NCT)

• Hotspots: Bijwasan, Faridabad, Ghaziabad.
• Cause: Compaction of alluvial deposits due to unregulated groundwater withdrawal.
• Local uplift: Detected near Dwarka (+15.1 mm/yr) due to rainwater harvesting and aquifer recharge policies post-2011.

Chennai

• Hotspots: Adyar floodplains (K K Nagar, Tondiarpet, Valasaravakkam, Kodambakkam).
• Cause: Compaction of Holocene alluvium (sandy clay, silt, sand) and intensive groundwater extraction.
• Impact: Highest projected structural risk among cities by 2075.

Mumbai

• Subsidence low overall, except in high-density informal settlements (e.g., Dharavi).
• Cause: Uneven land compaction due to localized extraction and structural load.

Kolkata

• Cause: Compaction of Pleistocene–Holocene sediments; subsidence along riverine and deltaic areas.
• Risk: Increasing vulnerability due to soft sediment structure.

Bengaluru

• Lowest subsidence due to igneous and metamorphic rock base (gneiss, granite).
• Warning: Recent surge in groundwater extraction (2022–2023) may increase risk.

Structural Risk Projections

Year	Delhi	Chennai	Mumbai	Bengaluru	Kolkata	Total (very high risk buildings)
2025	2,264	32	110	—	—	—
2055	3,169	958	255	—	—	—
2075	11,457	8,284	3,477	112	199	23,529

Underlying Causes

• Over-extraction of groundwater via millions of unregulated borewells.
• Urban load stress from vertical construction increasing soil compaction.
• Lack of recharge infrastructure and inefficient stormwater management.
• Climate variability: Declining rainfall recharge and increasing urban heat.

Broader Implications

• Infrastructure risk: Cracking foundations, damaged pipelines, and transport networks.
• Hydrological risk: Land sinking worsens flooding, especially during monsoons.
• Seismic risk: Uneven compaction increases earthquake vulnerability.
• Economic cost: Rising insurance risk and maintenance expenditure in megacities.

Positive Example

• Dwarka (Delhi):
  • Local uplift due to aquifer recharge and rainwater harvesting between 2012–2015.
  • Demonstrates success of policy-driven groundwater restoration.

Way Forward – Mitigation & Adaptation

1. Regulatory measures:
   1. Enforce groundwater extraction caps under CGWA guidelines.
   2. Mandate recharge pits and rainwater harvesting for large buildings.
2. Urban hydrology reform:
   1. Integrate surface water management with stormwater networks.
   2. Develop artificial recharge zones near floodplains.
3. Nature-based solutions:
   1. Promote re-vegetation and soil conservation to stabilise land.
4. Monitoring & Technology:
   1. Expand InSAR satellite surveillance for urban subsidence tracking.
5. Public awareness:
   1. Integrate groundwater literacy in urban planning and civic education.

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