1 Introduction

Land subsidence is a sudden or gradual settling of the land surface caused by changes in the stress regime of the structure of subsurface. Subsidence driven by groundwater extraction most commonly occurs when aquifer materials are composed of lenses or layers of fine‑grained compressible sediments in which pore‑water heads are lowered causing sediment compaction. This reduction in volume results in a lowering of the land surface. Injecting water into aquifer systems has sometimes mitigated this anthropogenic subsidence; however, compaction of fine‑grained sediments is not completely reversible in most cases. Land subsidence can be a major geomechanical consequence of groundwater withdrawal and results in:

• lowering of the elevation of the ground surface, that is, land subsidence, as a result of aquifer overdraft;
• formation of earth fissures caused by groundwater pumping in subsiding basins, especially in arid and semiarid regions;
• activation of pre‑existing shallow faults, creating a failure of the land surface; and,
• induction or triggering of micro‑seismic and seismic events because of changes in the natural stress regimes.

Additional impacts can also occur when aquifers experiencing subsidence are naturally recharged or attempts to stabilize or reverse groundwater related land subsidence by injecting water into the aquifers occurs. Upheaval of the ground surface has been observed in a number of areas as shown later in this book.