APPENDIX D: Explanation of Compaction and Subsidence

The diagram in Figure D-1 shows the steps in the process leading to subsidence of the land surface and related effects caused by groundwater overdraft. In stage 1, the initial state before pumping withdrawals begin, the quantity of water stored in the aquifer is V1. Associated with this equilibrium state is the depth of the water table position below the ground, H1. The onset of pumping activities in stage 2 causes discharge rates to exceed recharge rates in a localized area, setting up a condition of overdraft. Drawdown of the water level in the aquifer (or a reduction in pressure head in confined aquifers) results. The volume of water in storage is reduced to V2. In some aquifer materials, especially clays, and in some types of unconsolidated aggregate and sand formations, de-watering or reduced water pressures lead to compaction (of clays) or consolidation (of sand/aggregates) of the affected layers. The immediate physical consequences are reduced porosity and subsidence of the land surface over the overdraft area.

A lowered porosity in the water bearing aquifer material reduces its water volume storage capacity, and can reduce transmissivity as well. Stage 3 of Figure D-1 demonstrates what can happen if increased recharge volumes become available, through artificial recharge for example, to recover lost storage. Because the actual physical storage capacity of the aquifer may have been reduced through the compaction of clays and consolidation during severe drawdown/overdraft events, refilling the aquifer to hold the same volume of water, V1, as in the initial condition will force the water level to rise above the stage 1 elevation. The distance separating the water table from the ground surface, H3, is reduced from the initial depth H1. High water tables can interfere with crop production and increase natural groundwater discharges at springs, to streams, and as subsurface outflow at the boundaries of the region. Moreover, simply restoring groundwater levels to historical averages may give false indications of the actual quantities of water in storage, resulting in less actual water available for future pumping than anticipated.

Compaction and subsidence can occur bit by bit during each drawdown event. When groundwater levels are restored, after an overdraft episode, partial rebound can occur, but initial conditions of porosity and land elevation can never be restored. Thus, each repeated overdraft episode in subsidence-susceptible aquifer materials, even within the limits of historical drawdowns, will add adversely to the degree of compaction and subsidence.

Problems arising from overdraft-induced compaction and subsidence are: a) structural damage, drainage problems and increased flooding risk; b) changes in flow patterns brought about by the higher water table in stage 3, and possible surface infiltration problems and evapotranspiration losses from groundwater; and c) reduced well yields associated with reduced transmissivities.