Types - compaction of aquifer systems, dewatering of organic soils, and mass wasting through dissolution and collapse of susceptible earth materials

Mining Ground Water - long-term ground-water-level declines can result in a vast one-time release of water from compacting aquitards (silts and clays), which manifests itself as land subsidence.

Almost all permanent subsidence occurs due to the irreversible compression or consolidation of aquitards during the typically slow process of aquitard drainage.

Relation between changes in ground-water levels and compression of the aquifer system is based on the principle of effective stress. As fluid pressure decreases the effective stress (stress on the mineral framework) increases.

For small changes in ground-water levels the aquifer/aquitard undergoes compression or expansion that is recoverable (elastic deformation).

Interbedded layers of silts and clays comprise the bulk of ground-water storage capacity of a confined aquifer system because of their higher porosity and compressibility.

When the preconsolidation stress (maximum past level of effective stress) is exceeded the aquitard may undergo significant, permanent rearrangement resulting in irreversible pore collapse (compaction).

Vertical drainage of aquitards into adjacent pumped aquifers may proceed very slowly, and thus lag far behind the changing water levels in adjacent aquifers. Thin aquitards drain first. Thick aquitards may take decades to fully drain. Thus, land subsidence may continue long after pumping stops.

Differential compaction has produced extensive faulting.

Increased surface water usage has slowed or stopped subsidence near the coast but subsidence is now occurring in the rapidly expanding northern part of Houston.

The Chicot-Evangeline aquifer system used for ground-water in Houston extends across south Louisiana.