Land subsidence, the gradual sinking of the Earth’s surface, is a phenomenon impacting areas across the globe. While various factors can cause this sinking, the most widespread cause worldwide is the excessive removal of groundwater from underground storage areas known as aquifers. This process causes the ground to lower slowly over months or years, often by several centimeters annually. It is a problem of significant scale, with estimates suggesting that nearly one-fifth of the global population resides in susceptible areas. For example, the largest recorded subsidence due to this cause is approximately 14 meters in New Zealand, and parts of California’s San Joaquin Valley have subsided by as much as 10 meters.
The Physics of Compaction and Effective Stress
The mechanism causing the ground to sink is a change in the balance of underground forces supporting the overlying soil and rock layers. Water within an aquifer fills the small spaces, or pores, between the sediment grains. This water exerts an outward force known as pore pressure, which helps keep the sediment grains apart and supports a portion of the total weight of the material above.
The remaining weight is supported by the physical grain-to-grain contact between the soil particles, a force called effective stress. When groundwater is pumped out, the water level in the aquifer drops, resulting in a decline in pore pressure. This reduction in fluid support means the total weight of the overburden is transferred to the solid framework of the aquifer material.
As the pore pressure decreases, the effective stress on the sediment grains increases, forcing them closer together. This process is called compaction, and it causes a reduction in the volume of the porous material. The cumulative compaction of the underground layers manifests as the sinking of the ground surface.
Geological Formations Prone to Sinking
The composition of underground layers determines an area’s susceptibility to sinking, as not all geological materials compact equally when groundwater is withdrawn. Aquifer systems are composed of coarse-grained sediments, like sand and gravel, interbedded with fine-grained materials, such as silts and clays. These fine-grained layers, known as aquitards, are the most susceptible to compaction.
Clays and silts are highly compressible because their tiny particles are arranged loosely and hold a large amount of water. When pore pressure drops, the increased effective stress permanently rearranges the particles, squeezing water out and causing the layer to compact. This compaction is often irreversible; even if water levels recover, the ground surface will not return to its original elevation. Coarser materials like sand and gravel are much stiffer and tend to compress only elastically, meaning they can recover their volume if water pressure is restored.
Impacts on Infrastructure and Water Storage
The sinking of the land surface creates numerous problems, especially in densely populated or heavily farmed areas. The most immediate consequence is damage to the built environment, as uneven ground movement stresses rigid structures. Foundations can crack, roads and bridges can buckle, and underground utility networks, such as pipelines, can be fractured. In some areas, the sinking causes well casings that tap into the groundwater to protrude from the ground, rendering the wells unusable.
Subsidence also has severe long-term hydrological consequences beyond the visible damage. The compaction of fine-grained layers results in a permanent loss of the aquifer’s storage capacity, reducing the amount of water the reservoir can hold. In coastal regions, the lowered elevation increases the risk of flooding from storm surges and tides. Furthermore, it can disrupt the natural drainage of surface water and increase the threat of saltwater intrusion into freshwater aquifers.
Managing and Reversing Groundwater Depletion
Intervention strategies focus on either reducing the rate of groundwater pumping or actively restoring water levels in the depleted aquifers. Regulatory measures are implemented by local authorities to limit the volume of water extraction through permits, pumping fees, or strict enforcement controls. Such policies have been effective, as seen in the Bangkok basin in Thailand, where groundwater level declines were reversed following the introduction of pumping fees and licensing.
Artificial recharge is a direct method of intervention, which involves injecting or spreading water onto the surface so it can percolate back into the aquifer system. This can involve using injection wells to pump recycled water or stormwater directly into the ground. Raising the water level increases the pore pressure, which reduces the effective stress and can slow or halt the compaction process. While these methods can stabilize the ground, the permanent compaction that has already occurred in the clay layers cannot be undone, highlighting the importance of prevention before significant sinking takes place.