The photo at the left illustrates how much the land surface near Mendota, California was lowered because of groundwater pumping from 1925 to 1977. The maximum subsidence at this location is the most recorded anywhere on the planet and totaled more than 29 feet. The total land subsidence at this location in the western San Joaquin Valley was more than 29 feet. This was the result of a water level decline in the area from irrigation pumping which was as much as 600 feet in one area. The land surface decline is a reflection of the compaction of the major confining layer (the Corcoran Clay) and other clay/fine silt layers below the confining layer through which the water level declined.
The layers of unconsolidated deposits below this highly productive agricultural valley extend from the land surface to over 32,000 feet down. The deposits consist of gravels, sands, silt and clays, with over 50% being of the clay/silt compressible material. Over 5,200 square miles of the San Joaquin Valley had at least 1 foot of land subsidence from 1925 to 1977. What occurs when the water level is lowered so much is that the clay layers which are up to 70% water and have no real rock structure become subjected to the weight of the overlying earth above and are squeezed like a sponge full of water. The result is that the water is removed and the material is compacted to a very thin layer which cannot be refilled when the water level rises again. The land surface above these deposits then declines in an amount equal to the total of all the clay layers that are compacted.
The person standing alongside the telephone pole is Dr. Joseph Poland who has been often called the “father of land subsidence”. I will describe this person, who was my mentor on land subsidence, more fully in a future article within this blog.