Case Study: Land subsistence in the American Southwest

Land subsidence is the lowering of the land-surface elevation from changes that take place underground. Human activity pumping water, oil, and gas from underground reservoirs; dissolution of limestone aquifers (sinkholes); collapse of underground mines; drainage of organic soils; and initial wetting of dry soils (hydrocompaction) are all common causes of land subsidence. Overdrafting of aquifers is the major cause of subsidence in the southwestern United States. As ground water pumping increases, land subsidence increases as well. In many aquifers, ground water is pumped from pore spaces between grains of sand and gravel. If an aquifer has beds of clay or silt within or next to it, the lowered water pressure in the sand and gravel causes slow drainage of water from the clay  and silt beds. Land subsidence has been getting worse over the passed few years because large amount of ground water has been excessively withdrawn from aquifers. Reduced water pressure is a loss of support for the clay and silt beds, since these beds are compressible they compact and it results with a lowering of the land surface.

Land subsidence has many impacts including changes in elevation and slope of streams, canals, and drains; damage to bridges, roads, railroads, storm drains, sanitary sewers, canals, and levees; damage to private and public buildings; and failure of well casings from forces generated by compaction of fine-grained materials in aquifer systems. Subsidence has resulted in tides moving into low-lying areas that were previously above high-tide levels, in some coastal areas.

In some areas where ground water pumping has causes subsidence, switching from ground water to surface water supplies has stopped the subsidence. If surface water is not available, then other means must be taken to reduce subsidence. Possibilities include reducing water use and determining locations for pumping and artificial recharge that will minimize subsidence.