Regional land subsidence caused by the compaction of susceptible aquifer systems accompanying groundwater extraction

Land subsidence includes both gentle downwarping and sudden sinking of
segments of the land surface. Major anthropogenic causes of land subsidence
are extraction of fluids including water, oil, and gas. Measurement and detec-
tion of land subsidence include both ground-based and remotely sensed air-
borne and space-based methods. Methods for measurement of subsidence at
points include differential leveling, global positioning system surveys, and
extensometers. Satellite-borne differential interferometric synthetic aperture
radar and airborne LiDAR techniques can detect land-surface movement over
wide areas of interest. Aquifer-system compaction and subsidence owing to
groundwater extraction typically occurs in areas of unconsolidated alluvial or
basin-fill aquifer systems comprising aquifers and aquitards. Approaches to
analyzing and modeling deformation of aquifer systems follow from the basic
relations between head, stress, compressibility, and groundwater flow.
Analysis and simulation of aquifer-system compaction have been addressed
primarily using either an approach based on conventional groundwater flow
theory or an approach based on linear poroelasticity theory. Both approaches
rely on the principle of effective stress outlined by Karl Terzaghi in 1925. In
the approach based on conventional groundwater flow theory, an aquitard
drainage model explains the compaction of fine grained material using the
principle of effective stress and theory of hydrodynamic lag. Packages for the
widely-used MODFLOW groundwater model are available to simulate aqui-
fer-system compaction and land subsidence using the aquitard-drainage
approach. Poroelasticity theory describes the more fully coupled processes of
groundwater flow and three-dimensional deformation of aquifer systems.
The general theory accounts for compressible fluid, porous matrix and solid
grains. Simulation codes using the poroelastic theory include some commer-
cial software products and a few research codes.