Aquifer Compaction due to Groundwater Pumping

Science Center Objects

Although land subsidence caused by groundwater pumping has caused many negative effects on human civil works for centuries, especially in the highly developed urban or industrialized areas of Europe, the relation between subsidence and groundwater pumpage was not understood or recognized for a long time. Recognition began in 1928 when pioneer researcher O.E. Meinzer of the U.S. Geological Survey realized that aquifers were compressible. At about that same time, Karl Terzaghi, working at Harvard University, developed the one-dimensional-consolidation theory. The theory states generally that compression of soils results from the slow release of pore water from stressed clay materials and the gradual transfer of stress from the pore water to the granular structure of the clay.

Cause

Fine-grained sediments (clays and silts) within an aquifer system are the main culprits in land subsidence due to groundwater pumping. Fine-grained sediments are special because they are composed of platy grains (imagine the shape of dinner plates). When fine-grained sediments are originally deposited, they tend to be deposited in random orientations (imagine haphazardly placing your dinner plates in the sink). These randomly oriented sediment grains have a lot of room between them to store water. However, when groundwater levels decline to historically low levels, those randomly oriented sediments are rearranged into stacks (imagine plates stacked in the cupboard). These stacks occupy less space and also have less space between them to store water.

Illustration of the aquifer compaction process.

Effect

The effects of compaction fall into two categories: those on manmade infrastructures and those on natural systems. The greatest effects occur to infrastructures that traverse a subsiding area. In the San Joaquin Valley, the main problems reported are related to water conveyance structures. Many water conveyance structures, including long stretches of the California Aqueduct, are gravity driven through the use of very small gradients; even minor changes in these gradients can cause reductions in designed flow capacity. Managers of the canals, such as the California Department of Water Resources, the San Luis Delta-Mendota Authority, the Bureau of Reclamation, and the Central California Irrigation District, have to repeatedly retrofit their canals to keep the water flowing...albeit at reduced amounts. While water conveyance structures tend to be the most sensitive to subsidence, damage to roads, railways, bridges, pipelines, buildings, and wells also can occur.

While more focus has been placed on the highly visible infrastructure damage from subsidence, which generally can be repaired, compaction of the aquifer system, sight unseen, may permanently decrease its capacity to store water; subsidence occurring today is a legacy for all tomorrows. Even if water levels rose, compacted sediments would remain as-is; most compaction that occurs as a result of historically low groundwater levels is irreversible. Additionally, as the topography of the land changes by varying amounts in different places, the low areas, such as wetlands, will change size and shape, migrate to lower elevations, or even disappear. Rivers may change course or erosion/deposition patterns to reach a new equilibrium.