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 The entire surface water flow of the Alapaha River near Jennings, Florida going into a sinkhole leading to the Floridan Aquifer groundwater
The entire surface water flow of the Alapaha River near Jennings, Florida going into a sinkhole leading to the Floridan Aquifer groundwater

Groundwater is the water present beneath Earth's surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock become completely saturated with water is called the water table. Groundwater is recharged from, and eventually flows to, the surface naturally; natural discharge often occurs at springs and seeps, and can form oases or wetlands. Groundwater is also often withdrawn for agricultural, municipal, and industrial use by constructing and operating extraction wells. The study of the distribution and movement of groundwater is hydrogeology, also called groundwater hydrology.

Typically, groundwater is thought of as water flowing through shallow aquifers, but, in the technical sense, it can also contain soil moisture, permafrost (frozen soil), immobile water in very low permeability bedrock, and deep geothermal or oil formation water. Groundwater is hypothesized to provide lubrication that can possibly influence the movement of faults. It is likely that much of Earth's subsurface contains some water, which may be mixed with other fluids in some instances. Groundwater may not be confined only to Earth. The formation of some of the landforms observed on Mars may have been influenced by groundwater. There is also evidence that liquid water may also exist in the subsurface of Jupiter's moon Europa.[1]

Groundwater is often cheaper, more convenient and less vulnerable to pollution than surface water. Therefore, it is commonly used for public water supplies. For example, groundwater provides the largest source of usable water storage in the United States, and California annually withdraws the largest amount of groundwater of all the states.[2] Underground reservoirs contain far more water than the capacity of all surface reservoirs and lakes in the US, including the Great Lakes. Many municipal water supplies are derived solely from groundwater.[3]

Polluted groundwater is less visible, but more difficult to clean up, than pollution in rivers and lakes. Groundwater pollution most often results from improper disposal of wastes on land. Major sources include industrial and household chemicals and garbage landfills, excessive fertilizers and pesticides used in agriculture, industrial waste lagoons, tailings and process wastewater from mines, industrial fracking, oil field brine pits, leaking underground oil storage tanks and pipelines, sewage sludge and septic systems.

YouTube Encyclopedic

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  • Porosity and Permeability
  • Groundwater Flow - Part 1
  • Confined Aquifer Model
  • What is an Aquifer?
  • How Do We Get Water From Deep Underground?


Groundwater represents about a quarter of the US freshwater supply. Most of which is used for domestic purposes or for agriculture. And there are two essential properties of groundwater that we really need to investigate to understand how it works and those are porosity and permeability. Today we are going to describe what we mean when we use the term groundwater And we are also going to examine those two properties, porosity and permeability, and how they effect how much groundwater is there and how easy it is to get it out of the ground. All the water on Earth is linked together by the hydrologic cycle. In brief, this cycle begins when water evaporates from the oceans. Water vapor rises into the atmosphere and condenses to form clouds. Clouds lose their moisture through precipitation. Rain falling on land can run off into streams and lakes or may infiltrate through the soil and into the rocks or sediment below. This is groundwater. We want to examine the properties of earth materials that allow this water to be present underground. When people think of groundwater they often imagine water flowing through a cave system or maybe an underground lake While groundwater does exist in these forms, it is not that common. Most usable groundwater is actually stored in the tiny spaces between grains of sand and gravel. Porosity and permeability control the distribution of this water. We will consider each of these separately, starting with porosity. We have filled this beaker with about 300 milliliters of relatively unsorted gravel. Notice that there are grains of different sizes that loosely fill the container, leaving several visible open spaces. These spaces represent the porosity of the sediment. Porosity is the proportion of the volume of an earth material that is composed of void spaces. We can do a brief experiment to determine the proportion of space in the gravel occupied by porosity. We have 200 milliliters of water in this smaller beaker. We dyed the water blue with food coloring to make it easier to see. When we pour the water into the beaker, it fills up the empty pore spaces from below and the water eventually rises to the top of the gravel. Now, let’s look to see how much water we used. Remember that we started with 200 milliliters and now we have about 80 ml left. So we added 120 milliliters of water to a beaker containing what appeared to be approximately 300 ml of gravel. That tells us that about 40% of the gravel mixture was composed of air spaces that we subsequently filled with water. We can try the same experiment with smaller, better sorted sand grains. In this case, it is more difficult to make out spaces between the smaller individual sand particles. What proportion of the sand do you think is composed of empty spaces? This time the sand mixture accommodates 100 ml of water, indicating that the estimated porosity of this sand is about 33%, a little less than that of the gravel. In both cases, the water lies in the spaces between the grains. There are a few big, visible spaces in the gravels, and lots of small, less visible spaces in the sand. Depending how well these materials are sorted, they can have similar porosities. These porosity values are not unreasonable for unconsolidated gravels and sands near Earth’s surface. About 80% of shallow groundwater systems in the US are composed of these materials, sand and gravel. In most cases we can extract this groundwater using wells, in much the same way that we could extract the water from the gravel mixture using a straw. However, before we make this seem too simple, we have to consider the role that permeability, plays in controlling how groundwater moves through rocks and sediment. Permeability represents the capacity of water to flow through earth materials. It is not sufficient that groundwater is present; it must also be able to flow into our well so that we can extract it. Many rocks have pretty good porosity but their permeability values will differ. For example, some igneous rocks contain preserved gas bubbles that are not connected. These rocks would have good porosity but low permeability. We have designed a little experiment to demonstrate how permeability varies among gravel, sand and clay, common sediments at or near Earth’s surface We have taken a funnel and filled it with each type of sediment. We added a tiny piece of filter paper to prevent the sediment flowing through the funnel. Then we poured a constant amount of water into each set up and watched to see how long it took to collect in the beaker below. The faster the flow of water, the higher the permeability. Let’s see what happened. Let's see what happened. As you can see, water quickly passes through the gravel and almost all of the original water collects in the beaker below. Water pools on top of the clay, and is unable to flow downward between the tiny clay particles, making it essentially impermeable at the scale of this experiment. Finally, water passes through the sand more slowly than the gravel and only about 75% of the original water makes it to the beaker during the time of the demonstration. The permeability of these three materials decreases as we move from the gravel on the left to the clay on the right. Sand and gravel make for excellent groundwater sources because of their combination of good porosity and permeability. Other materials, such as sandstone, some limestones or fractured igneous rocks may also have high porosity and permeability values and serve as good groundwater reservoirs under specific circumstances. Material like clay or fine grained sedimentary rocks like shale, or unfractured metamorphic or igneous rocks such as granite have such low permeability values that they often act as barriers to groundwater flow. We had three learning objectives for today’s lesson, how confident are you that you could complete these tasks?



 Groundwater withdrawal rates from the Ogallala Aquifer in the Central United States
Groundwater withdrawal rates from the Ogallala Aquifer in the Central United States

An aquifer is a layer of porous substrate that contains and transmits groundwater. When water can flow directly between the surface and the saturated zone of an aquifer, the aquifer is unconfined. The deeper parts of unconfined aquifers are usually more saturated since gravity causes water to flow downward.

The upper level of this saturated layer of an unconfined aquifer is called the water table or phreatic surface. Below the water table, where in general all pore spaces are saturated with water, is the phreatic zone.

Substrate with low porosity that permits limited transmission of groundwater is known as an aquitard. An aquiclude is a substrate with porosity that is so low it is virtually impermeable to groundwater.

A confined aquifer is an aquifer that is overlain by a relatively impermeable layer of rock or substrate such as an aquiclude or aquitard. If a confined aquifer follows a downward grade from its recharge zone, groundwater can become pressurized as it flows. This can create artesian wells that flow freely without the need of a pump and rise to a higher elevation than the static water table at the above, unconfined, aquifer.

The characteristics of aquifers vary with the geology and structure of the substrate and topography in which they occur. In general, the more productive aquifers occur in sedimentary geologic formations. By comparison, weathered and fractured crystalline rocks yield smaller quantities of groundwater in many environments. Unconsolidated to poorly cemented alluvial materials that have accumulated as valley-filling sediments in major river valleys and geologically subsiding structural basins are included among the most productive sources of groundwater.

The high specific heat capacity of water and the insulating effect of soil and rock can mitigate the effects of climate and maintain groundwater at a relatively steady temperature. In some places where groundwater temperatures are maintained by this effect at about 10 °C (50 °F), groundwater can be used for controlling the temperature inside structures at the surface. For example, during hot weather relatively cool groundwater can be pumped through radiators in a home and then returned to the ground in another well. During cold seasons, because it is relatively warm, the water can be used in the same way as a source of heat for heat pumps that is much more efficient than using air.

The volume of groundwater in an aquifer can be estimated by measuring water levels in local wells and by examining geologic records from well-drilling to determine the extent, depth and thickness of water-bearing sediments and rocks. Before an investment is made in production wells, test wells may be drilled to measure the depths at which water is encountered and collect samples of soils, rock and water for laboratory analyses. Pumping tests can be performed in test wells to determine flow characteristics of the aquifer.[3]

Water cycle

 Relative groundwater travel times
Relative groundwater travel times
 Dzherelo, a common source of drinking water in a Ukrainian village
Dzherelo, a common source of drinking water in a Ukrainian village

Groundwater makes up about twenty percent of the world's fresh water supply, which is about 0.61% of the entire world's water, including oceans and permanent ice. Global groundwater storage is roughly equal to the total amount of freshwater stored in the snow and ice pack, including the north and south poles. This makes it an important resource that can act as a natural storage that can buffer against shortages of surface water, as in during times of drought.[4]

Groundwater is naturally replenished by surface water from precipitation, streams, and rivers when this recharge reaches the water table.[5]

Groundwater can be a long-term 'reservoir' of the natural water cycle (with residence times from days to millennia), as opposed to short-term water reservoirs like the atmosphere and fresh surface water (which have residence times from minutes to years). The figure[6] shows how deep groundwater (which is quite distant from the surface recharge) can take a very long time to complete its natural cycle.

The Great Artesian Basin in central and eastern Australia is one of the largest confined aquifer systems in the world, extending for almost 2 million km2. By analysing the trace elements in water sourced from deep underground, hydrogeologists have been able to determine that water extracted from these aquifers can be more than 1 million years old.

By comparing the age of groundwater obtained from different parts of the Great Artesian Basin, hydrogeologists have found it increases in age across the basin. Where water recharges the aquifers along the Eastern Divide, ages are young. As groundwater flows westward across the continent, it increases in age, with the oldest groundwater occurring in the western parts. This means that in order to have travelled almost 1000 km from the source of recharge in 1 million years, the groundwater flowing through the Great Artesian Basin travels at an average rate of about 1 metre per year.

 Reflective carpet trapping soil water vapor
Reflective carpet trapping soil water vapor

Recent research has demonstrated that evaporation of groundwater can play a significant role in the local water cycle, especially in arid regions.[7] Scientists in Saudi Arabia have proposed plans to recapture and recycle this evaporative moisture for crop irrigation. In the opposite photo, a 50-centimeter-square reflective carpet, made of small adjacent plastic cones, was placed in a plant-free dry desert area for five months, without rain or irrigation. It managed to capture and condense enough ground vapor to bring to life naturally buried seeds underneath it, with a green area of about 10% of the carpet area. It is expected that, if seeds were put down before placing this carpet, a much wider area would become green.[8]



Certain problems have beset the use of groundwater around the world. Just as river waters have been over-used and polluted in many parts of the world, so too have aquifers. The big difference is that aquifers are out of sight. The other major problem is that water management agencies, when calculating the "sustainable yield" of aquifer and river water, have often counted the same water twice, once in the aquifer, and once in its connected river. This problem, although understood for centuries, has persisted, partly through inertia within government agencies. In Australia, for example, prior to the statutory reforms initiated by the Council of Australian Governments water reform framework in the 1990s, many Australian states managed groundwater and surface water through separate government agencies, an approach beset by rivalry and poor communication.

In general, the time lags inherent in the dynamic response of groundwater to development have been ignored by water management agencies, decades after scientific understanding of the issue was consolidated. In brief, the effects of groundwater overdraft (although undeniably real) may take decades or centuries to manifest themselves. In a classic study in 1982, Bredehoeft and colleagues[9] modeled a situation where groundwater extraction in an intermontane basin withdrew the entire annual recharge, leaving ‘nothing’ for the natural groundwater-dependent vegetation community. Even when the borefield was situated close to the vegetation, 30% of the original vegetation demand could still be met by the lag inherent in the system after 100 years. By year 500, this had reduced to 0%, signalling complete death of the groundwater-dependent vegetation. The science has been available to make these calculations for decades; however, in general water management agencies have ignored effects that will appear outside the rough timeframe of political elections (3 to 5 years). Marios Sophocleous[9] argued strongly that management agencies must define and use appropriate timeframes in groundwater planning. This will mean calculating groundwater withdrawal permits based on predicted effects decades, sometimes centuries in the future.

As water moves through the landscape, it collects soluble salts, mainly sodium chloride. Where such water enters the atmosphere through evapotranspiration, these salts are left behind. In irrigation districts, poor drainage of soils and surface aquifers can result in water tables' coming to the surface in low-lying areas. Major land degradation problems of soil salinity and waterlogging result,[10] combined with increasing levels of salt in surface waters. As a consequence, major damage has occurred to local economies and environments.[11]

Four important effects are worthy of brief mention. First, flood mitigation schemes, intended to protect infrastructure built on floodplains, have had the unintended consequence of reducing aquifer recharge associated with natural flooding. Second, prolonged depletion of groundwater in extensive aquifers can result in land subsidence, with associated infrastructure damage – as well as, third, saline intrusion.[12] Fourth, draining acid sulphate soils, often found in low-lying coastal plains, can result in acidification and pollution of formerly freshwater and estuarine streams.[13]

Another cause for concern is that groundwater drawdown from over-allocated aquifers has the potential to cause severe damage to both terrestrial and aquatic ecosystems – in some cases very conspicuously but in others quite imperceptibly because of the extended period over which the damage occurs.[14]


 Wetlands contrast the arid landscape around Middle Spring, Fish Springs National Wildlife Refuge, Utah
Wetlands contrast the arid landscape around Middle Spring, Fish Springs National Wildlife Refuge, Utah

Groundwater is a highly useful and often abundant resource. However, over-use, over-abstraction or overdraft, can cause major problems to human users and to the environment. The most evident problem (as far as human groundwater use is concerned) is a lowering of the water table beyond the reach of existing wells. As a consequence, wells must be drilled deeper to reach the groundwater; in some places (e.g., California, Texas, and India) the water table has dropped hundreds of feet because of extensive well pumping. In the Punjab region of India, for example, groundwater levels have dropped 10 meters since 1979, and the rate of depletion is accelerating.[15] A lowered water table may, in turn, cause other problems such as groundwater-related subsidence and saltwater intrusion.

Groundwater is also ecologically important. The importance of groundwater to ecosystems is often overlooked, even by freshwater biologists and ecologists. Groundwaters sustain rivers, wetlands, and lakes, as well as subterranean ecosystems within karst or alluvial aquifers.

Not all ecosystems need groundwater, of course. Some terrestrial ecosystems – for example, those of the open deserts and similar arid environments – exist on irregular rainfall and the moisture it delivers to the soil, supplemented by moisture in the air. While there are other terrestrial ecosystems in more hospitable environments where groundwater plays no central role, groundwater is in fact fundamental to many of the world’s major ecosystems. Water flows between groundwaters and surface waters. Most rivers, lakes, and wetlands are fed by, and (at other places or times) feed groundwater, to varying degrees. Groundwater feeds soil moisture through percolation, and many terrestrial vegetation communities depend directly on either groundwater or the percolated soil moisture above the aquifer for at least part of each year. Hyporheic zones (the mixing zone of streamwater and groundwater) and riparian zones are examples of ecotones largely or totally dependent on groundwater.


Subsidence occurs when too much water is pumped out from underground, deflating the space below the above-surface, and thus causing the ground to collapse. The result can look like craters on plots of land. This occurs because, in its natural equilibrium state, the hydraulic pressure of groundwater in the pore spaces of the aquifer and the aquitard supports some of the weight of the overlying sediments. When groundwater is removed from aquifers by excessive pumping, pore pressures in the aquifer drop and compression of the aquifer may occur. This compression may be partially recoverable if pressures rebound, but much of it is not. When the aquifer gets compressed, it may cause land subsidence, a drop in the ground surface. The city of New Orleans, Louisiana is actually below sea level today, and its subsidence is partly caused by removal of groundwater from the various aquifer/aquitard systems beneath it.[16] In the first half of the 20th century, the San Joaquin Valley experienced significant subsidence, in some places up to 8.5 metres (28 feet)[17] due to groundwater removal. Cities on river deltas, including Venice in Italy,[18] and Bangkok in Thailand,[19] have experienced surface subsidence; Mexico City, built on a former lake bed, has experienced rates of subsidence of up to 40 cm (1'3") per year.[20]

Seawater intrusion

In general, in very humid or undeveloped regions, the shape of the water table mimics the slope of the surface. The recharge zone of an aquifer near the seacoast is likely to be inland, often at considerable distance. In these coastal areas, a lowered water table may induce sea water to reverse the flow toward the land. Sea water moving inland is called a saltwater intrusion. In alternative fashion, salt from mineral beds may leach into the groundwater of its own accord.


 Iron oxide staining caused by reticulation from an unconfined aquifer in karst topography. Perth, Western Australia.
Iron oxide staining caused by reticulation from an unconfined aquifer in karst topography. Perth, Western Australia.

Polluted groundwater is less visible, but more difficult to clean up, than pollution in rivers and lakes. Groundwater pollution most often results from improper disposal of wastes on land. Major sources include industrial and household chemicals and garbage landfills, industrial waste lagoons, tailings and process wastewater from mines, oil field brine pits, leaking underground oil storage tanks and pipelines, sewage sludge and septic systems. Polluted groundwater is mapped by sampling soils and groundwater near suspected or known sources of pollution, to determine the extent of the pollution, and to aid in the design of groundwater remediation systems. Preventing groundwater pollution near potential sources such as landfills requires lining the bottom of a landfill with watertight materials, collecting any leachate with drains, and keeping rainwater off any potential contaminants, along with regular monitoring of nearby groundwater to verify that contaminants have not leaked into the groundwater.[3]

Groundwater pollution, from pollutants released to the ground that can work their way down into groundwater, can create a contaminant plume within an aquifer. Pollution can occur from landfills, naturally occurring arsenic, on-site sanitation systems or other point sources, such as petrol stations or leaking sewers.

Movement of water and dispersion within the aquifer spreads the pollutant over a wider area, its advancing boundary often called a plume edge, which can then intersect with groundwater wells or daylight into surface water such as seeps and springs, making the water supplies unsafe for humans and wildlife. Different mechanism have influence on the transport of pollutants, e.g. diffusion, adsorption, precipitation, decay, in the groundwater. The interaction of groundwater contamination with surface waters is analyzed by use of hydrology transport models.

The danger of pollution of municipal supplies is minimized by locating wells in areas of deep groundwater and impermeable soils, and careful testing and monitoring of the aquifer and nearby potential pollution sources.[3]

Arsenic and fluoride

Around one-third of the world’s population drinks water from groundwater resources. Of this, about 10 percent, approximately 300 million people, obtains water from groundwater resources that are heavily polluted with arsenic or fluoride.[21] These trace elements derive mainly from natural sources by leaching from rock and sediments.

New method of identifying substances that are hazardous to health

In 2008, the Swiss Aquatic Research Institute, Eawag, presented a new method by which hazard maps could be produced for geogenic toxic substances in groundwater.[22][23][24][25] This provides an efficient way of determining which wells should be tested.

In 2016, the research group made its knowledge freely available on the Groundwater Assessment Platform GAP. This offers specialists worldwide the possibility of uploading their own measurement data, visually displaying them and producing risk maps for areas of their choice. GAP also serves as a knowledge-sharing forum for enabling further development of methods for removing toxic substances from water.


United States

In the United States, laws regarding ownership and use of groundwater are generally state laws; however, regulation of groundwater to minimize pollution of groundwater is by both states and the federal-level Environmental Protection Agency. Ownership and use rights to groundwater typically follow one of three main systems:[26]

  • The Rule of Capture provides each landowner the ability to capture as much groundwater as they can put to a beneficial use, but they are not guaranteed any set amount of water. As a result, well-owners are not liable to other landowners for taking water from beneath their land. State laws or regulations will often define "beneficial use", and sometimes place other limits, such as disallowing groundwater extraction which causes subsidence on neighboring property.
  • Limited private ownership rights similar to riparian rights in a surface stream. The amount of groundwater right is based on the size of the surface area where each landowner gets a corresponding amount of the available water. Once adjudicated, the maximum amount of the water right is set, but the right can be decreased if the total amount of available water decreases as is likely during a drought. Landowners may sue others for encroaching upon their groundwater rights, and water pumped for use on the overlying land takes preference over water pumped for use off the land.
  • In November 2006, the Environmental Protection Agency published the groundwater Rule in the United States Federal Register. The EPA was worried that the groundwater system would be vulnerable to contamination from fecal matter. The point of the rule was to keep microbial pathogens out of public water sources.[27] The 2006 groundwater Rule was an amendment of the 1996 Safe Drinking Water Act.

Other rules in the United States include:

  • Reasonable Use Rule (American Rule): This rule does not guarantee the landowner a set amount of water, but allows unlimited extraction as long as the result does not unreasonably damage other wells or the aquifer system. Usually this rule gives great weight to historical uses and prevents new uses that interfere with the prior use.
  • Groundwater scrutiny upon real estate property transactions in the US: In the US, upon commercial real estate property transactions both groundwater and soil are the subjects of scrutiny. For brownfields sites (formerly contaminated sites that have been remediated), Phase I Environmental Site Assessments are typically prepared, to investigate and disclose potential pollution issues.[28] In the San Fernando Valley of California, real estate contracts for property transfer below the Santa Susana Field Laboratory (SSFL) and eastward have clauses releasing the seller from liability for groundwater contamination consequences from existing or future pollution of the Valley Aquifer.

See also


  1. ^ Richard Greenburg (2005). The Ocean Moon: Search for an Alien Biosphere. Springer Praxis Books. 
  2. ^ National Geographic Almanac of Geography, 2005, ISBN 0-7922-3877-X, p. 148.
  3. ^ a b c d "What is hydrology and what do hydrologists do?". The USGS Water Science School. United States Geological Survey. 23 May 2013. Retrieved 21 Jan 2014. 
  4. ^ "Learn More: Groundwater". Columbia Water Center. Retrieved 15 September 2009. 
  5. ^ United States Department of the Interior (1977). Ground Water Manual (First ed.). United States Government Printing Office. p. 4. 
  6. ^ File:Groundwater flow.svg
  7. ^ Hassan, SM Tanvir (March 2008). Assessment of groundwater evaporation through groundwater model with spatio-temporally variable fluxes (PDF) (MSc). Enschede, Netherlands: International Institute for Geo-Information Science and Earth Observation. 
  8. ^ Al-Kasimi, S. M. (2002). Existence of Ground Vapor-Flux Up-Flow: Proof & Utilization in Planting The Desert Using Reflective Carpet. 3. Dahran. pp. 105–19. 
  9. ^ a b Sophocleous, Marios (2002). "Interactions between groundwater and surface water: the state of the science". Hydrogeology Journal. 10: 52–67. Bibcode:2002HydJ...10...52S. doi:10.1007/s10040-001-0170-8. 
  10. ^ "Free articles and software on drainage of waterlogged land and soil salinity control". Retrieved 2010-07-28. 
  11. ^ Ludwig, D.; Hilborn, R.; Walters, C. (1993). "Uncertainty, Resource Exploitation, and Conservation: Lessons from History" (PDF). Science. 260 (5104): 17–36. Bibcode:1993Sci...260...17L. doi:10.1126/science.260.5104.17. JSTOR 1942074. PMID 17793516. 
  12. ^ Zektser et al.
  13. ^ Sommer, Bea; Horwitz, Pierre; Sommer, Bea; Horwitz, Pierre (2001). "Water quality and macroinvertebrate response to acidification following intensified summer droughts in a Western Australian wetland". Marine and Freshwater Research. 52 (7): 1015. doi:10.1071/MF00021. 
  14. ^ Zektser, S.; LoaIciga, H. A.; Wolf, J. T. (2004). "Environmental impacts of groundwater overdraft: selected case studies in the southwestern United States". Environmental Geology. 47 (3): 396–404. doi:10.1007/s00254-004-1164-3. 
  15. ^ Upmanu Lall. "Punjab: A tale of prosperity and decline". Columbia Water Center. Retrieved 2009-09-11. 
  16. ^ Dokka, Roy K. (2011). "The role of deep processes in late 20th century subsidence of New Orleans and coastal areas of southern Louisiana and Mississippi". Journal of Geophysical Research. 116 (B6). doi:10.1029/2010JB008008. ISSN 0148-0227. 
  17. ^ Sneed, M; Brandt, J; Solt, M (2013). "Land Subsidence along the Delta-Mendota Canal in the Northern Part of the San Joaquin Valley, California, 2003–10" (PDF). USGS Scientific Investigations Report 2013-5142. Retrieved 22 June 2015. 
  18. ^ Tosi, Luigi; Teatini, Pietro; Strozzi, Tazio; Da Lio, Cristina (2014). "Relative Land Subsidence of the Venice Coastland, Italy": 171–73. doi:10.1007/978-3-319-08660-6_32. 
  19. ^ Aobpaet, Anuphao; Cuenca, Miguel Caro; Hooper, Andrew; Trisirisatayawong, Itthi (2013). "InSAR time-series analysis of land subsidence in Bangkok, Thailand". International Journal of Remote Sensing. 34 (8): 2969–82. doi:10.1080/01431161.2012.756596. ISSN 0143-1161. 
  20. ^ Arroyo, Danny; Ordaz, Mario; Ovando-Shelley, Efrain; Guasch, Juan C.; Lermo, Javier; Perez, Citlali; Alcantara, Leonardo; Ramírez-Centeno, Mario S. (2013). "Evaluation of the change in dominant periods in the lake-bed zone of Mexico City produced by ground subsidence through the use of site amplification factors". Soil Dynamics and Earthquake Engineering. 44: 54–66. doi:10.1016/j.soildyn.2012.08.009. ISSN 0267-7261. 
  21. ^ Eawag (2015) Geogenic Contamination Handbook – Addressing Arsenic and Fluoride in Drinking Water. C.A. Johnson, A. Bretzler (Eds.), Swiss Federal Institute of Aquatic Science and Technology (Eawag), Duebendorf, Switzerland. (download:
  22. ^ Amini, M.; Mueller, K.; Abbaspour, K.C.; Rosenberg, T.; Afyuni, M.; Møller, M.; Sarr, M.; Johnson, C.A. (2008) Statistical modeling of global geogenic fluoride contamination in groundwaters. Environmental Science and Technology, 42(10), 3662–68, doi:10.1021/es071958y
  23. ^ Amini, M.; Abbaspour, K.C.; Berg, M.; Winkel, L.; Hug, S.J.; Hoehn, E.; Yang, H.; Johnson, C.A. (2008). “Statistical modeling of global geogenic arsenic contamination in groundwater”. Environmental Science and Technology 42 (10), 3669–75. doi:10.1021/es702859e
  24. ^ Winkel, L.; Berg, M.; Amini, M.; Hug, S.J.; Johnson, C.A. Predicting groundwater arsenic contamination in Southeast Asia from surface parameters. Nature Geoscience, 1, 536–42 (2008). doi:10.1038/ngeo254
  25. ^ Rodríguez-Lado, L.; Sun, G.; Berg, M.; Zhang, Q.; Xue, H.; Zheng, Q.; Johnson, C.A. (2013) Groundwater arsenic contamination throughout China. Science, 341(6148), 866–68, doi:10.1126/science.1237484
  26. ^ "Appendix H, Groundwater Law and Regulated Riparianism", Final Report: Restoring Great Lakes Basin Water thorough the Use of Conservation Credits and Integrated Water Balance Analysis System, The Great Lakes Protection Fund Project # 763 (PDF), archived from the original (pdf) on 20 July 2011, retrieved 16 January 2014 
  27. ^ Ground Water Rule (GWR) | Ground Water Rule | US EPA. Retrieved on 2011-06-09.
  28. ^ EPA; "Archived copy". Archived from the original on 2013-04-26. Retrieved 2011-09-19. 

External links

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