Geologic Hazards: All About Land Subsidence Due to Groundwater Pumping
Land subsidence, or the gradual settling and lowering of the Earth's surface, is growing problem around the world which has been documented in 45 states in America as well as in India, China, and the Middle East. Although many things have been known to cause land subsidence, the anthropogenic impacts of groundwater pumping on the landscape are noteworthy. One report by the United States Geological Survey asserts that more than 80 percent of the subsidence in the America is directly correlated to groundwater withdrawal . Figure 1 below shows the areas within the United States where subsidence has been attributed to the pumping of groundwater.
It has been estimated that the world's thirst for groundwater has reached an all time high with global extraction rates topping 982 km3/yr . For many regions of the globe, including portions of the United States, the rate of groundwater extraction exceeds the rate at which the water is replenished via natural processes. This has resulted in a measurable decline of the water table as well as significant subsidence of the overlying soil layers. For example, in the desert southwest near Tucson, Arizona the pumping of groundwater has resulted in water-level declines of between 300 and 500 feet in much of the area. Since the 1940s, as much as 12.5 feet of subsidence has been measured  with some researchers noting that its likely even more subsidence has occurred in the area.
Land subsidence is more than just a consequence of groundwater pumping, it's a cause for concern for engineers, urban planners, and water resource managers. The variety of problems associated with land subsidence is well documented with impacts ranging from changing drainage patterns and increased flooding, to the destruction of critical infrastructure and even the creation of earth fissures. Obviously this has the potential to affect many aspects of our increasingly industrialized lifestyle.
However, we now have more tools than ever before to measure, quantify, and even predict land subsidence which can help us mitigate its impact and plan for more resilient infrastructure and a more sustainable society. In addition to this, these tools can help water resource managers control, prevent, or even remediate land subsidence through the judicious use of groundwater management practices.
Characterization of Land Subsidence
The relationship between a change in groundwater levels and the compression of the corresponding aquifer system is based on the principle of effective stress . When water is removed from the ground the pore water pressure is subsequently reduced. Without the water to hold up the weight of the soil above it, the land surface subsides and the aquifer layers become more compact resulting in an overall reduction in the pore space of the soils. Some aquifer systems can "rebound" if water is pumped back into it, however, more often than not, this vertical deformation results in permanent changes to the aquifer system. This is especially true when the compressed layers soils consist of very fine grained clays. In many aquifer systems around the country subsidence has lead to the loss of groundwater storage capacity as well as other changes to the aquifer's hydraulic properties  including its ability to transmit water. Most current research suggests that the majority of aquifers experience only a small amount of reversible deformation, especially when subsidence has occurred over a long period of time.
Infrastructure Damage Caused Land Subsidence
In 1991, the National Research Council estimated that the annual cost of damage in the United States resulting from land subsidence exceeded $125 million . This figure was later revised by the USGS to $400 million dollars when they accounted for residual economic impacts such as property devaluation and increased operational costs for farmers. In today's dollars this equates to more than $685 million dollars annually. A more recent figure for annual damages could not be found however it's very likely that annual damages have increased.
One of the most obvious implications of land subsidence is the potential damage that it can do to cities and their infrastructure. When the ground surface is lowered, the entire city will sink with it ultimately impacting the stability of buildings and the functionality of the infrastructure that supports it.
One such location where significant subsidence has occurred is Mexico City, Mexico. In the 20th century alone the city sank almost 30 feet (averaging 3.6 inches per year) . With this much subsidence, the problems are many. As of 1998 the city resided nearly 6 feet below nearby Lake Texcoco. Many historic buildings have either collapsed or been condemned due to the instability of the structures. In addition to this, $870 million dollars was spent to construct massive pumping stations and 124 miles of pipe to carry sewage and storm water out of the city because the existing infrastructure could no longer function properly . While subsidence has lessened in recent years, many parts of the City are still sinking. In 2014 the European Space Agency created a subsidence map that shows which areas are still being impacted subsidence due to groundwater pumping (Figure 2 at right).
The United States isn't safe from land subsidence related damage either. In west Phoenix, Arizona in 1992 officials at Luke Air Force Base had to close the Base for 3 days to deal with unexpected flooding of the runways, offices, and more than 100 homes. Scientists with the Arizona Department of Water Resources as well as the Arizona Geological Survey concluded that land subsidence due to nearby groundwater pumping was the cause. They discovered that the ground surface (and underlying soil) had lowered so much that the storm sewer lines servicing the Base had started to flow in reverse. When a large storm dumped several inches of rain over the based, the storm sewer conveyed runoff towards the base instead of away from it . The problem was ultimately fixed at a cost of more than $3 million dollars  however constant monitoring of subsidence in the area is still needed to ensure the long term functionality of the rebuilt storm sewer system.
In Scottsdale, Arizona, the Central Arizona Project (CAP) canal crosses the City in an area of known land subsidence. The area experienced ground lowering on the order of 1.5ft over a twenty year period which resulted in the expenditure of $350,000 to raise the canal. In another part of the City, an additional $820,000 was spent to counteract the effects of subsidence  when the canal was found to be damaged there as well.
Other structures that are especially at risk from land subsidence include dams, levees, and other above ground features. These structures are typically constructed to control and direct the flow of surface runoff, to prevent flooding, and/or store water for future use. When the ground surface is lowered the storage capacity (and in the case of levees their freeboard) could be comprised. In a worst case scenario these structures could even fail resulting in the loss of life and property.
One reason that hurricane Katrina was so devastating to New Orleans was that land subsidence (attributed partly to the pumping of groundwater) had lowered the City to such an extent that it now resides below sea level . In addition to this, levees protecting the City were also lowered as well reducing the level of protection that they could provide . Figure 3 below obtained from the NASA Earth Observatory shows measured subsidence rates for a portion of New Orleans from April 2002 to July 2005. On the average, New Orleans subsided 0.31 inches per year relative to global mean sea level during this period  leading up to the hurricane. This combination of comprising events lead to one of the most expensive natural disasters of the 21st century.
Changing Drainage Patterns: Flooding Attributed to Land Subsidence
Another obvious implication of land subsidence is its effect on surface runoff patterns. The lowering of the ground surface can cause places to experience flooding that might not have otherwise seen it. This has the consequence of causing even more damage to a City that is already dealing with subsidence.
There have been many documented cases of flooding resulting from land subsidence, however, one notable example is the January 2010 flood of the Town of Wenden in Arizona. This had been the second time the town flooded in ten years. Scientists with the Arizona Department of Water Resources as well as the Arizona Geological Survey determined that land subsidence due to groundwater withdrawal in the nearby fields was making the flooding problem significantly worse . Subsidence upwards of 2.7 feet was measured for the Town for the twenty year period leading up to the 2010 flood. Since the Town abuts the nearby Centennial Wash, this subsidence caused more runoff to leave the channel and flow into the Town than had occurred in previous years. Figure 4 below shows the Town of Wenden during the flood as well as a three-dimensional subsidence map for the region.
In the image above you can see the subsidence bowl that has formed northwest of the Town. The bowl clearly shows how the topography has changed and how the new ground surface appears to "draw" water away from Centennial Wash towards the Town.
Another area that has experienced flooding caused by land subsidence are the towns residing in Harris, Galveston, and Fort Bend Counties in Texas . Near the coast land subsidence has been measured to exceed 10 feet in some areas. This has placed many homes and buildings at risk from coastal flooding. In the City of Baytown the land subsidence and resulting flooding had gotten so bad that a 400 home subdivision was eventually converted to a nature center consisting of open fields, wetlands, and plenty of trees .
Earth Fissures: The Result of Differential Subsidence
If subsidence weren't bad enough, it some cases this phenomenon can cause the formation of earth fissures. An earth fissure is characterized by an open crack or ravine that can occur when subsiding soil layers are situated over uneven bedrock or other subsurface features. Fissures can also form at the edges of subsidence bowls as well (such as at the interface of subsiding and non-subsiding strata). The leading research on the subject suggests that over time, differential subsidence causes the development of internal stresses within the soil layers near the surface. When the stress becomes large enough, a fissure forms which manifests itself as a visible crack on the ground's surface. The schematic at right shows how a fissure may form near the edges of a subsidence bowl where differential settlement is often at the highest:
Earth fissures are another hazard that can damage infrastructure and even threaten the lives of wandering cattle, horses, and humans. In fact, in 2011 a horse was killed when it fell into fissure that had opened up after a rainstorm in Queen Creek, Arizona . Aside from horses and other livestock, earth fissures have been documented as causing significant damage to roadways and other underground infrastructure and making land much harder to develop.
Measuring/Monitoring Land Subsidence
Historically speaking, measuring land subsidence has not always been an easy task. With most everything in a given area subsiding together at an imperceptible rate, finding a reference point to see or measure the ground's deformation was often difficult. Fortunately today we have a number of technologies that can be used to accurately measure and monitor land subsidence.
An extensometer is a device that consists of a pipe or cable that is anchored to the surface below an aquifer. As the land subsides over time, the machine's recorder makes note of the change in relative distance between the bottom of the hole (preferably at the bedrock surface) and the ground surface above . Extensometers are easy to set up (if you can find a good location) and require little oversight to operate. Similar to traditional surveying techniques, extensometers usually have an accuracy of about 1/100th of a foot. Figure 6 at right shows a schematic of a typical subsidence monitoring extensometer.
A leveling survey is a simple method of measuring subsidence that uses traditional surveyor's tools to complete. To complete the survey accurately the surveyors needs to measure the ground elevations of a subsiding area and reference those measurements to areas where no subsidence has occurred. Usually this means finding a benchmark that is connected to the underlying bedrock to ensure that it hasn't subsided. To improve accuracy surveyors typically increase the density of their measurements and also attempt to tie their survey to more than one benchmark. However accurate they may be, leveling surveys are typically very expensive and require a lot of time to complete for a relatively small area.
More recently, the advent of Global Position Systems (GPS) has augmented the abilities of surveyors to accurately measure ground subsidence. Though not as accurate as a tradition elevation surveying methods, GPS has allowed for the quick measurement of subsidence over larger scales than what was previously possible. Because of this, GPS is a great way to observe trends in elevation changes over time.
Interferometric Synthetic Aperture RADAR (InSAR) Measurement
InSAR is a relatively new technique that utilizes RADAR equipped satellites to measure changes in the earth's surface over time. This remote sensing technique can measure changes within a fraction of an inch and works best for areas of land that are not usually disturbed on a regular basis (such as farming operations) or where little vegetation that can skew the measurements exists. InSAR measurement can be expensive but when you consider the level of accuracy and amount of data that can be obtained, it usually turns out to be a bargain.. An entire metropolitan area can usually be measured in a single satellite pass producing with millions of data points that can be analyzed. Because InSAR can measure such large areas, the technology has often helped scientist discover previously unknown subsidence features.
The cover of this report (as well as figure 7 below) contains an interferogram (an image derived from InSAR data) for the Hawk Rock formation near Apache Junction, Arizona. The image shows a repeating color scale that illustrates relative ground displacement between the first and the second times that the area was scanned by the satellite.
The image shows relative subsidence for a 3.5 year period between 10/20/2004 and 04/02/2008. One cycle of colors represents approximately 2.8cm of subsidence. The area near Signal Butte Rd and Guadalupe Rd experienced the most subsidence coming in at 9cm of deformation for this time period. In Arizona, InSAR is being used to monitor more than 25 individual land subsidence features that cover more than 1,100 square miles of land . Other states, such as California, have invested heavily in this technology because of the valuable information that it can provide.
Preventing and Controlling Land Subsidence
The only real way to prevent land subsidence is stop or minimize the use of groundwater all together. However, this is not always practical since there is often not many alternatives for obtaining water for a community who is dependent on the groundwater. Unfortunately in the United States, the agricultural community, especially in the desert southwest, is heavily dependent on groundwater. Finding alternative sources of water to irrigate croplands has proven to be a significant challenge.
To combat land subsidence, government agencies across the country have created land subsidence monitoring programs which are used to supplement groundwater management policies. In areas that are affected by significant subsidence, the local governments have enacted regulations to limit groundwater withdrawals and even require the use of alternative sources of water when pumping limits are met. For example, in 1975 the Texas Legislature created the Harris-Galveston Subsidence District. This District's sole purpose is to provide for the regulation of groundwater withdrawal throughout Harris and Galveston counties for the purpose of preventing land subsidence .
In 1980, Arizona adopted a new Groundwater Management Code which was to be administered by the Arizona Department of Water Resources. The code was created to combat the problems associated with the overuse of groundwater and had three primary goals: 1) Control severe overdraft occurring in many parts of the state, 2) Provide a means to allocate the state's limited groundwater resources to most effectively meet the changing needs of the state; and 3) Augment Arizona's groundwater through water supply development. In 1986, the Ford Foundation selected this code as one of the 10 most innovative government regulations of its time . More recently, other states, such as California have followed suit by passing groundwater regulations similar to the policies created in Texas and Arizona.
Scientists and government agencies have recognized the threat that land subsidence has on our infrastructure, our cities, and our society. These regulations, and others like it, all serve to protect our groundwater resources in order to limit subsidence (among other things) and to wean ourselves from our dependency on this precious resource.
Summary and Conclusions
Humankind's dependence on groundwater has not come without a price. Among the many concerns related to groundwater withdrawal is the manifestation land subsidence features around the Country as well as the world. With subsidence impacting more than 17,000 square miles of the continental US resulting from groundwater withdrawal, the consequences of this seemingly innocuous occurrence are far from harmless. As we've seen, land subsidence as the potential to destroy infrastructure, cause flooding, and even spawn the formation of an even more dangerous land disturbance known as earth fissures.
Land subsidence poses a unique challenge for engineers, urban planners, and local governments. The risks of pumping too much groundwater are evident to many however learning to control our desire for this finite resource has proven to be very difficult. As world populations increase and droughts become more prevalent, finding alternative sources of water will become a necessary challenge if we want to mitigate the effects of land subsidence. Furthermore, through the implementation of groundwater management policies charged to reduce or eliminate land subsidence, the damage to infrastructure, life, and property can be mitigated ultimately helping to push society towards a future of resiliency and long term sustainable prosperity.
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