Land subsidence occurs when large amounts of groundwater have been excessively withdrawn from an aquifer. The clay layers within the aquifer compact and settle, resulting in lowering the ground surface in the area from which the groundwater is being pumped.
Over time, as more water is removed from the area, the ground drops and creates a cone. Once the water has been removed from the sediment, it cannot be replaced. For example, only about 5.3 million acre-feet of the total rainfall “recharge” Texas aquifers each year. However, in 1996 approximately 9.9 million acre-feet of groundwater were pumped, resulting in a net loss of 4.6 million acre-feet of groundwater.
Land subsidence can lead to many problems, including changes in elevation; damage to structures such as storm drains, sanitary sewers, roads, railroads, canals, levees and bridges; structural damage to public and private buildings; and damage to wells. Most commonly, though, subsidence is known for causing an increase in the potential for flooding.
Because of the growing awareness and concern of subsidence-related problems, the 1975 Texas legislature created the Harris-Galveston Subsidence District. Their mission is “to control subsidence and manage groundwater resources in Harris and Galveston Counties through regulation of groundwater withdrawal, conservation, and cooperation with surface water suppliers to assure adequate future supplies of water for beneficial uses.” More recently, the Fort Bend Subsidence District was created in 1989 to monitor subsidence in inland areas west of Houston (Fort Bend County), where subsidence has accelerated because of extensive groundwater use.
Map showing the boundaries of HGSD and FBSD
Subsidence trends reflect patterns of resource development that shifted inland from coastal oil and gas extraction to ground-water extraction for municipal and industrial supplies.
USGS picture found at approximate location of maximum subsidence in the United States identified by research efforts of Dr. Joseph F. Poland (pictured). Signs on pole show approximate altitude of land surface in 1925, 1955, and 1977. The site is in the San Joaquin Valley southwest of Mendota, California