Cracks due to water pumping
Introduction
subsidence due to overexploitation of aquifers is the sinking of land as a result of the unsustainable extraction of groundwater. It is a growing problem as cities increase in population and water use without adequate regulation of pumping, or without this regulation being enforced.
It is estimated that 80% of serious land subsidence problems in the United States are associated with excessive groundwater extraction.[1].
Groundwater is a natural resource with relatively easy access, since anyone who can pay for drilling to the water table can usually extract it (depending on local regulations). Even if regulations do not allow water extraction, or limit the amount of water that can be extracted, it is difficult to close illegal wells or guarantee that legal wells extract only the permitted volume.[2] So, as seen in the figure, extraction causes a drop in the water table around the well. Ultimately, if there are many wells, this can affect a large area, making it increasingly expensive to extract the water and causing various problems, such as drying up of springs, drying up of lagoons or the sinking of the land itself. Thus, the extraction of groundwater becomes a tragedy of communal goods, with the consequent economic externalities.
The word "subsidence" is a specialized term for geological use, it is included in the RAE[3] and occurs not only with the extraction of water, but also oil and other underground resources. There is a UNESCO working group on subsidence.
Mechanism
The cause of long-term surface changes associated with overexploitation of aquifers is well known.[4] As shown in the United States Geological Survey figure, aquifers are typically found in compressible layers (strata of the ground) of silt or clay.
As water is extracted from these strata, the effective stress exerted on them by the upper layers of the soil changes, precipitating consolidation, which is often irreversible. Thus, the total volume of silt and clay is reduced, and the land surface decreases. The phenomenon is similar to that of a cake that dries out: if it initially had a thickness of, say, 10 centimeters, dry it can be reduced to 8 or less. Surface damage is much greater if there are differential settlements or large ground features such as sinkholes and fissures.
Along with land subsidence, the compaction of aquifers is a serious problem, because the water storage potential of many aquifers can be significantly reduced when unsustainable extraction and the consequent lowering of the water table cause the permanent compaction of layers of fine sediments (silts and clays). A study conducted in an arid agricultural region of Arizona[5] showed that, even with a water level recovery of 100 feet (about 30 meters) after groundwater extraction stopped, the land surface continued to sink for decades. This is a result of the continuous dehydration of the aquitards (fine-grained layers that slow down the movement of groundwater) due to the stresses mentioned in the previous paragraph.