The extraction of groundwater, oil, gas, or other fluids from underground reservoirs can cause significant subsidence of the land's surface. In some cases the removal of underground water is natural. During times of severe drought, soil moisture may decrease dramatically and drought-resistant plants with deep root systems can draw water from great depths, reaching a hundred feet or more (many tens of meters) in some cases. In most cases, however, subsidence caused by deep fluid extraction is caused by human activity.
This deep subsidence mechanism operates because the fluids that are extracted served to help support the weight of the overlying regolith. The weight of the overlying material places the fluids under significant pressure, known as hydrostatic pressure, that keeps the pressure between individual grains in the regolith at a minimum. This in turns helps prevent the grains from becoming closely packed or compacted. If the fluids are removed, the pressure between individual grains increases and the grains become more closely packed and compacted, occupying less space than before the fluid was extracted. This can cause the surface to subside. A small amount of this subsidence may be temporary, or recoverable, but generally once surface subsidence related to fluid extraction occurs, it is non-recoverable. When this process occurs on a regional scale, the effect can be subsidence of a relatively large area. Subsidence associated with underground fluid extraction is usually gradual but still costs millions of dollars in damage every year in the United States.
The amount of surface subsidence is related to the amount of fluid withdrawn from the ground and also to the compressibility of the layer that the fluid has been removed from. If water is removed from cracks in a solid igneous, metamorphic, or sedimentary rock, then the strength of the rock around the cracks will be great enough to support the overlying material and no surface subsidence is likely to occur. In contrast, if fluids are removed from a compressible layer such as sand, shale, or clay, then significant surface subsidence may result from fluid extraction. Clay and shale have a greater porosity and compressibility than sand, so extraction of water from clay-rich sediments results in greater subsidence than the same amount of fluid withdrawn from a sandy layer.
One of the most common causes of fluid extraction-related subsidence is the over-pumping of groundwater from aquifers. If many wells are pumping water from the same aquifer the cones of depression surrounding each well begin to merge, lowering the regional groundwater level. Lowering of the groundwater table can lead to gradual, irreversible subsidence.
Surface subsidence associated with groundwater extraction is a serious problem in many parts of the southwestern united States and in coastal cities such as New Orleans. Many cities such as Tucson, Phoenix, Los Angeles, Salt Lake City, Las Vegas, and San Diego, rely heavily on groundwater pumped from compressible layers in underground aquifers.
The San Joaquin Valley of California offers a dramatic example of the effects of groundwater extraction. Extraction of groundwater for irrigation over a period of 50 years has resulted in nearly 30 feet (9 m) of surface subsidence. Parts of the Tucson Basin in Arizona are presently subsiding at an accelerating rate, and many investigators fear that the increasing rate of subsidence reflects a transition from temporary recoverable subsidence to a permanent compaction of the water-bearing layers at depth.
The world's most-famous subsiding city is Venice, Italy. Venice is sinking at a rate of about one foot per century. The city has subsided more than 10 feet since it was founded near sea level. Much of the city is below sea level or just above sea level and prone to floods from storm surges and astronomical high tides in the Adriatic Sea. These acqua altas (meaning high water in Italian) flood streets as far as the famous Piazza San Marco. Venice has been subsiding for a combination of reasons, including compaction of the coastal mud that the city was built on. One of the main causes of the sinking of Venice has been groundwater extraction. Nearly 20,000 groundwa-ter wells pumped water from compressible sediment beneath the city, with the result being the city sank into the empty space created by the withdrawal of water. The Italian government has now built an aqueduct system to bring drinking water to residents, and has closed most of the 20,000 wells. This action has slowed the subsidence of the city, but it is still sinking, and this action may be too little too late to spare Venice from the future effects of storm surges and astronomical high tides.
Mexico City is also plagued with subsidence problems caused by groundwater extraction. Mexico City is built on a several-thousand-foot-thick sequence of sedimentary and volcanic rocks, including a large dried lake bed on the surface. Most of the groundwater is extracted from the upper 200 feet (60 m) of these sediments. Parts of Mexico City have subsided dramatically, whereas others have not. The northeast part of the city has subsided about 20 feet (6 m). Many of the subsidence patterns in Mexico City can be related to the underlying geology. In places like the northeast part of the city that are underlain by loose compressible sediments, the subsidence has been large. In other places underlain by volcanic rocks, the subsidence has been minor.
The extraction of oil, natural gas, and other fluids from the Earth also may result in surface subsidence. In the united States, subsidence related to petroleum extraction is a large problem in Texas, Louisiana, and parts of California. One of the worst cases of oil field subsidence is that of Long Beach, California, where the ground surface has subsided 30 feet (9 m) in response to extraction of underground oil. There are approximately 2,000 oil wells in Long Beach, pumping oil from beneath the city. Much of Long Beach's coastal area subsided below sea level, forcing the city to construct a series of dikes to keep the water out. When the subsidence problem was recognized and understood, the city began a program of reinjecting water into the oil field to replace the extracted fluids and to prevent further subsidence. This reinjection program was initiated in 1958, and since then the subsidence has stopped, but the land surface can not be pumped up again to its former levels.
Pumping of oil from an oil field west of Marina del Rey along the Newport-Inglewood fault resulted in subsidence beneath the Baldwin Hills Dam and Reservoir, leading to the dam's catastrophic failure on December 14, 1963. Oil extraction from the Inglewood oil field resulted in subsidence-related slip on a fault beneath the dam and reservoir, which was enough to initiate a crack in the dam foundation. The crack was quickly expanded by pressure from the water in the reservoir, which led to the dam's catastrophic failure at 3:38 p.m. on December 14, 1963. sixty-five million gallons of water were suddenly released, destroying dozens of homes, killing five people, and causing $12 million in damage.
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