Geography
Groundwater Extraction and Land Subsidence in Coastal Cities
Quick fact
Some coastal cities, like Jakarta, are sinking up to 10 cm per year due to groundwater extraction, outpacing sea-level rise by over a decade.
Why this is interesting
Have you ever noticed sidewalks cracking or buildings tilting in a coastal city? What could cause the very ground beneath your feet to sink?
Read the full explanation
Understanding Groundwater Extraction and Land Subsidence in Coastal Cities
To understand land subsidence, picture an aquifier as a saturated sponge. When you squeeze water out of it, the sponge compresses and shrinks. Similarly, groundwater extraction removes water from the void spaces in sediment layers, reducing the water pressure that supports the overlying soil. This causes the sediments to compact, and the ground surface sinks. In coastal cities, this subsidence can be rapid and dramatic, especially when extraction is intense.
A deeper explanation
Groundwater is stored in pore spaces between particles of sand, silt, and clay, and the pressure of that water supports the weight of the overlying layers. When we pump water out faster than it can be replenished, this pressure drops. The sediment skeleton must then bear more of the load, causing the grains to rearrange and pack tighter together, a process called compaction. Because this compression is often irreversible, the land permanently sinks. In coastal cities, subsidence is hazardous because it reduces the elevation relative to sea level, dramatically increasing flood risk—often more than climate-driven sea-level rise itself.