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What is land subsidence?

Land subsidence is a gradual settling or sudden sinking of the Earth's surface owing to subsurface movement of earth materials caused by groundwater or oil extraction.

Potential Effects of Land Subsidence:

   Differential changes in elevation and gradient of stream channels, drains, and water transport structures
   Failure of water well casings due to compressive stresses generated by compaction of aquifer systems
   Tidal encroachment in lowland coastal areas
   Tensional or compressional strain in engineering structures and houses

Factors Leading to Subsidence:

Land subsidence in the Sacramento River, San Joaquin River, and Tulare Lake Basins is most likely to occur as a result of aquitard consolidation. As the head declines in response to groundwater withdrawals, aquitards between production zones are exposed to increased vertical loads. These loads can cause aquitards to consolidate leading to land subsidence. Factors that influence the rate and magnitude of consolidation in aquitards include mineral composition, the amount of prior consolidation, cementation, the degree of aquifer confinement, and aquitard thickness. Pleistocence-Holocene age aquitards in confined aquifer systems are most susceptible to subsidence in these Basins.

Subsidence has elastic and inelastic deformation components. As the head lowers in the aquifer, the load that was supported by the hydrostatic pressure is transferred to the granular skeletal framework of the formation. As long as the increased load on the formation does not exceed the pre-consolidation pressure, the formation will remain elastic. Under elastic condition, the formation will rebound to its original volume as hydrostatic pressure is restored. However, when the head of the formation is lowered to a point where the load exceeds pre-consolidation pressure, inelastic deformation may occur. Under inelastic consolidation, the formation will undergo a permanent volumetric reduction as water is expelled from aquitards.

Areas Most Prone to Subsidence:

Land subsidence is most prone to occur, and will occur most rapidly, in areas where the following geologic and hydrologic conditions exist:

  Strongly confined aquifer system.
  Coarse-grained aquifers with multiple thin aquitard interbeds.
  Large head declines from groundwater extraction.
  Aquitard interbeds subject to low natural pre-consolidation pressures.

Monitoring Land Subsidence

Currently, DWR monitors land subsidence using various methods, which include Geodetic Stations, Cable Extensometers, and Pipe Extensometers. Click here for more discussion on these and other methods.