The USGS Water Science School
This photo shows the approximate location of maximum subsidence in the United States, identified by research efforts of Dr. Joseph F. Poland (pictured). The site is in the San Joaquin Valley southwest of Mendota, California. Signs on pole show approximate altitude of land surface in 1925, 1955, and 1977.
In this case, excessive groundwater pumping allowed the upper soil layers to dry out and compress and compact, which is by far the single largest cause of subsidence. Soil compaction results in a reduction of the pore sizes between soil particles, resulting in essentially a permanent condition—rewetting of the underground soil and rock does not cause the land to go back up in altitude. This results in a lessening of the total storage capacity of the aquifer system. Here, the term "groundwater mining" is really true.
Back to: Land subsidence | Sinkholes | Groundwater decline
Credit: http://water.usgs.gov/ogw/pubs/fs00165/ - Land Subsidence in the United States, USGS USGS Fact Sheet-165-00