A study published in Nature Sustainability, warns that land beneath India’s five largest cities – Delhi, Mumbai, Kolkata, Chennai, and Bengaluru is sinking.

Key Findings of the Study

• Extent of Subsidence: 878 sq km of urban land has sunk, exposing 1.9 million people to subsidence rates exceeding 4 mm per year
• Overall Impact: Over 13 million buildings and nearly 80 million residents live in subsiding areas.
• City-wise Observations:
  • Delhi: Caused by compaction of alluvial deposits due to massive groundwater withdrawal.
  • Chennai: Driven by groundwater extraction and compaction of Holocene alluvium (sandy clay and silt).
  • Kolkata: Subsidence linked to compaction of Pleistocene and Holocene sediments.
  • Bengaluru: Minimal subsidence due to igneous and metamorphic bedrock, but increasing groundwater extraction since 2022 poses risk.
  • Mumbai: Relatively stable overall, though Dharavi and high-density informal settlements face localised subsidence.
• Causes of Subsidence: 
  • Primary: Over-extraction of groundwater through millions of unregulated borewells.
  • Secondary: Cumulative load of urban structures exerting downward pressure on subsurface layers; Compaction of alluvial and fluvial sediments.
• Localised Uplift: Dwarka (Delhi) recorded an uplift rate of 15.1 mm/yr due to aquifer recharge measures, including rainwater harvesting and groundwater regulation (2005–2015).

What is Land Subsidence

• Land subsidence is the gradual sinking or lowering of the Earth’s surface due to the removal or displacement of underground materials such as water, oil, gas, or minerals, or due to natural geological processes.
• Causes: 
• • Groundwater extraction: Excessive withdrawal of groundwater (e.g., in cities like Delhi, Kolkata, Mexico City).
  • Mining activities: Removal of underground minerals or coal.
  • Oil and gas extraction.
  • Natural compaction of sediments.
  • Karst processes: Dissolution of limestone leading to sinkholes.
  • Earthquakes or tectonic activity.

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Impacts of Land Subsidence 

• Infrastructure Damage: Causes structural deformation such as cracks, tilting, and collapse in buildings, roads, bridges, and underground pipelines.
  • Example: Around 2,200 buildings in Delhi face high risk due to uneven sinking.
• Flooding and Waterlogging: The lowering of land elevation disrupts natural drainage patterns, heightening the risk of flooding and prolonged waterlogging during heavy rains.
• Groundwater and Environmental Degradation: Results in irreversible loss of aquifer storage capacity and promotes saline water intrusion, particularly in coastal regions.
• Economic and Social Impacts: Leads to declining land values, reduced agricultural productivity, and forced displacement of affected communities.
• Disaster Vulnerability: Increases susceptibility to secondary hazards such as landslides, sinkholes, and urban infrastructure failures — as seen in regions like Joshimath (India) and Jakarta (Indonesia).

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