This post will follow along the lines of my “Geography vs. Geology” one, but this time I will be comparing land subsidence and sinkholes.
Despite similarities the two are not the same. In simplest terms, land subsidence is a lowering of the ground while sinkholes involve a collapsing of the land and usually creating a giant hole. You need certain conditions for each to happen.
I wrote a paper and created a poster specifically on land subsidence back during my undergrad, so get ready for information overload. I’ll begin with defining land subsidence and sinkholes then present them with case studies: California and Florida.
Occurs when more fluid (water or oil and even gas) is taken from an aquifer than is replaced. When fluids that were filling the pores spaces (space between particles) of the aquifer are removed, compaction of the aquifer occurs. This can become permanent because of the weight of the land, which through intense pressure keeps the aquifer from reflating even when water is pumped back into it.
According to the United States Geological Survey (USGS), sinkholes occur when the rock below the land is limestone, carbonate rock, salt beds or any kind of rock that can be easily dissolved by (usually acidic) groundwater that flows through them. This dissolving of rock thus creates spaces underground, which can be small or become so large that sinkholes are created because there is no longer enough rock below to hold up the land surface. Thus a dramatic collapse can occur, which is especially dangerous if a building or person is above it at the time. Sinkholes are common in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania.
California Land Subsidence:
Many places in California suffer from significant land subsidence, mainly because of two types of fluid extraction: groundwater and oil. In fact, California has the most known subsidence locations in the United States.
The areas of greatest subsidence are:
- Santa Clara Valley: 12 ft., which includes San Jose: 12.8 ft.
- San Joaquin Valley: 28.9 ft., which includes Mendota: 29 ft.
- Long Beach/Los Angeles: 29.5 ft.
(Galloway and Sneed 2013; Leake 2013; Nelson 2012)
Groundwater extraction for agricultural and municipal/industrial use is the leading cause of land subsidence in the country. California with its large population and agricultural industry as well as dry climate has led to substantial overpumping of groundwater in excess of its natural recharge rate. Oil extraction in the Long Beach area has also caused subsidence. Subsidence causes damage to structures on the surface as well as in the subsurface (wells, pipes, etc.) and creates hazardous conditions, such as being prone to flooding and sea level rise. Mitigation efforts have been quite costly, but the cost of not mitigating would be far worse (Galloway and Sneed 2013; Shoemaker and Thorley 1955).
The San Joaquin Valley has seen significant subsidence throughout the region mainly due to irrigation for agriculture and was first noted as early as 1935. The sediments there are fine-grained, stream and lake deposits, allowing compaction to easily occur. The dry climate of the area, averaging 5-16 inches/year of rainfall and the ephemeral nature of some of the streams combined with great variability each year contribute to the subsidence problem (Galloway and Riley nd).
Santa Clara Valley is about 90 miles southeast of San Francisco and used groundwater for agriculture in the form of fruit and vegetable orchards, which dominated the land pre-WWII. Afterwards the population increase led to urban and industrial water use. Subsidence became known in 1933 because of a 4 ft. subsidence of bench marks set in 1912 in San Jose. Rainfall ranging from 40 inches in some places and 14 in others, was not enough for specialty crops, thus the increase in production led to using groundwater (Ingebritsen and Jones nd).
Subsidence in the Long Beach/Los Angeles area is mainly due to oil extraction. This area has many oil fields and the Wilmington one is the second largest in California and the third largest in the United States with 1,428 producing wells as of 2008. It was discovered in 1932 and about 87 percent of the oil here has been pumped as of 2002 with only about 385 million barrels in reserve (Los Angeles Almanac: Great Oil Fields 2013).
The differences and similarities in land subsidence in these areas highlights the importance of early detection, action and the need for more advance methods of detection. Mitigation efforts range from structure improvements (elevate buildings) to fluid injection to offset groundwater and oil losses.
The areas providing the water though also need to protect themselves from becoming prone to land subsidence, which means that mitigation of subsidence is a complex and interrelated issue that requires corporation and understanding among all affected parties (Ingebritsen and Jones, nd).
Land subsidence in California has been a big problem. The early to mid-20th century saw significant subsidence in several areas usually due to groundwater extraction for irrigation or oil. Knowledge of subsidence was recognized fairly early on in some places but the damage was already done and continued to occur even after subsidence was known.
Mitigation efforts were swift in some places like the Santa Clara Valley, preventing further subsidence while in places like San Joaquin Valley, the heavy groundwater use with a slower response allowed the land to subside significantly in a short time. The same goes for Long Beach, which also had a high level of subsidence that was noticed early on but not effectively mitigated till many years later.
The mitigation efforts implemented back in the late 1960s and early 1970s though has helped to slow down and even arrest subsidence and the continuation of these efforts along with advanced monitoring efforts helps keep these subsidence prone areas from slipping further and detecting new areas that might become susceptible to subsidence. The dependence on imported surface water to alleviate the strain on groundwater though can pose other problems, such as relying on corporation between the owners of the imported water and droughts that can affect water quantity for the subsidence areas as well as the water origin areas.
Note: The above section was adapted from my paper/poster for my undergraduate geography class.
According to the Florida Department of Environmental Protection, sinkholes are only one of the many types of karst landforms that occur in Florida. Karst refers to the type of terrain created by erosional processes associated with chemical weathering. Chemical weathering in this case would be the dissolution of carbonate rock, such as limestone and dolomite, which are very common in Florida. Other karst landforms include underground caves, disappearing streams and springs.
The limestone in Florida is porous, allowing acidic water to flow through and slowly dissolve the rock. This process is so slow that any change is hardly noticeable. In Florida, the process has mostly occurred over eons, forming intensive underground voids and drainage systems.
The USGS even uses Florida as an example when discussing sinkholes. They mention several types of sinkholes in relation to Florida. The first is the dissolution type, which I have already discussed. The next type is suffosion, which is when cavities form below the land surface.
Cover-subsidence is another type. This involves sinkholes that gradually develop due to the covering sediments being both permeable and containing sand.
Cover-collapse sinkholes involve the sudden collapse of the surface due to clay sediments that lead to drainage and erosion and a shallow bowl-shaped depression.
Like land subsidence, sinkholes can also be man-made through groundwater pumping, construction, when water drainage patterns are changed and the creation of run-off and industrial storage ponds.