A sinking foundation
The Pearl River Delta, one of the world's most densely populated urban centers, is built on a geologically young and unstable foundation. The entire region rests on thousands of years of unconsolidated river sediments, which are naturally prone to compaction. This natural process is now dangerously accelerated by human activity, causing the ground to sink in a phenomenon called land subsidence.
Scientists use a satellite-based technique called Interferometric Synthetic Aperture Radar (InSAR) to map this sinking with millimeter-scale precision. By comparing radar images taken at different times, they can create detailed deformation maps across the entire delta. These maps show that large areas are subsiding, with some parts of Guangzhou and the area around Humen Town sinking at rates greater than 25 mm per year. The process is not uniform; different areas sink at different rates, creating a slow, large-scale tilting of the delta towards the sea.
Causes and consequences
The primary driver of this accelerated subsidence is the over-extraction of groundwater. The massive urban population and industries of the delta pump enormous volumes of water from underground aquifers. As the water is removed, the pore pressure in the silts and clays of the aquifer system decreases, causing the sediment layers to compact under the immense weight of the cities above. The construction of high-rise buildings and extensive subway lines in cities like Guangzhou and Shenzhen further compresses the soft water-logged soil, contributing to the problem.
The consequences are severe. The low-lying delta, much of which is less than 2 meters above sea level, becomes increasingly vulnerable to coastal flooding and storm surges. This effect is amplified by global sea-level rise. Sinking ground damages infrastructure, including buildings, roads, and underground metro lines. It increases the risk of saltwater intrusion, where saline water from the sea pushes into and contaminates the region's freshwater aquifers, threatening the water supply for millions of inhabitants.