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. When fine-grained sediments are originally deposited, they tend to be deposited in random orientations. 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. These stacks occupy less space and also have less space between them to store water.
Land subsidence in the United States
This report explores the role of science in defining and understanding subsidence problems, and shows that the optimal use of our land and water resources may depend on improved scientific understanding to minimize subsidence. More than 80 percent of the identified land subsidence in the Nation is a consequence of human impact on subsurface water, and is an often overlooked environmental consequence of our land- and water-use practices. Nine illustrative case studies demonstrate the role of subsurface water in human-induced land subsidence. These studies represent three distinct processes that account for most of the water-related subsidence--compaction of aquifer systems, drainage and subsequent oxidation of organic soils, and dissolution and collapse of susceptible rocks. The compaction of alluvial aquifer systems that can accompany excessive ground-water pumping and resulting ground-water level declines is, by far, the single largest cause of subsidence.