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Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of lithologic 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 fully 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, streams 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 Groundwater is also often withdrawn for agricultural, municipal and industrial use by constructing and operating extraction wells.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 liquid water flowing through shallow aquifers, but technically it can also include 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 which can possibly aid faults to move. Nearly any point in the Earth's subsurface has water in it, to some degree (it may be mixed with other fluids). Groundwater is not confined only to the Earth, either; subsurface water on Mars is believed to have given rise to some of the landforms observed there. Liquid water is also believed to exist in the subsurface of Jupiter's moon Europa.
An aquifer is a geologic unit (or layer) of permeable material (like sand, gravel or fractured bedrock) that is capable of providing usable quantities of water to a well. Aquifers can be confined or unconfined. A confined aquifer has a low permeability confining layer (an aquitard or aquaclude), such as clay, above and below which restricts the upward and downward movement of water from the aquifer. 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 or rise to a higher elevation than the static water table at the above, unconfined aquifer. The depth at which the groundwater's surface is located in an unconfined aquifer is called the water table or phreatic surface, where water pressure is equal to atmospheric pressure. Below the water table, where generally all pore spaces are saturated with water is the phreatic zone.
The porous media in which groundwater occurs are the complex geologic materials near the earth surface; hence local details of porosity and permeability are as complex as those materials. Generally, the more productive and useful aquifers are in sedimentary geologic formations, though weathered and fractured crystalline rocks yield smaller volumes of groundwater in many environments. Among the most productive groundwater environments are unconsolidated to poorly cemented alluvial materials that have accumulated as valley-filling sediments in major river valleys and geologically subsiding structural basins.
The high specific heat capacity of water and the insulating effect of soil and rock averages out climatic fluctuations to maintain groundwater at a relatively steady temperature, approximately in the low 50's Fahrenheit. Increasingly this effect is used to heat and cool structures. During hot weather, groundwater is sometimes cool enough to be used as is, to be simply pumped through radiators in a home, then returned to the ground in another well. During cold seasons, the water, because it has a high specific heat capacity can be used as a source of heat for heat pumps that is much more efficient than using air. The relatively constant temperature of groundwater can also be used for heat pumps.
Groundwater in the water cycle
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 shows how deep groundwater (which is quite distant from the surface recharge) can take a very long time to complete its natural cycle. Groundwater is naturally replenished by surface water from precipitation, streams, and rivers when this recharge reaches the water table. It is estimated that the volume of groundwater comprises 30.1% of all freshwater resource on earth compared to 0.3% in surface freshwater; the icecaps and glaciers are the only larger sources of fresh water on earth at 68.7%.
Groundwater makes up about twenty percent of the world's fresh water supply, which is about 0.61 percent of the entire world's water, including oceans and permanent ice. (Environment Canada Website) tissemand
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.
The time lags inherent in the dynamic response of groundwater to development have generally 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 (discussed in Sophocleous 2002) modelled a situation where groundwater extraction in a intermontane basin withdrew the entire annual recharge, leaving ‘nothing’ for the natural groundwater-dependent vegetation community. Even when the borefield was situated relatively 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 water management agencies have generally ignored effects which will appear outside the rough timeframe of political elections (3 to 5 years). Sophocleous (2002) 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 salinity and waterlogging result, combined with increasing levels of salt in surface waters. As a consequence, major damage has occurred to local economies and environments. Often, lessons of the past have not been learned (Ludwig et al. 1993).
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 (thirdly) saline intrusion (Zektser et al. 2005). Fourth, draining acid sulphate soils, often found in low-lying coastal plains, can result in acidification and pollution of formerly freshwater and estuarine streams (Sommer & Horwitz 2001).
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 due to the extended period over which the damage occurs (Sophocleous 2002).
Ludwig, D, Hilborn, R & Walters, C (1993) 'Uncertainty, resource exploitation and conservation: lessons from history.' Science, vol. 260, no. 2, p. 17.
Sommer, B & Horwitz, P (2001) 'Water quality and macroinvertebrate response to acidification following intensified summer droughts in a Western Australian wetland', Marine and Freshwater Research, vol. 52, pp. 1015-21.
Sophocleous, M (2002) 'Interactions between groundwater and surface water: the state of the science', Hydrogeology Journal, vol. 10, pp. 52-67.
Zektser, S, Loaiciga, HA & Wolf, JT (2005) 'Environmental impacts of groundwater overdraft: selected case studies in the southwestern United States', Environmental Geology, vol. 47, pp. 396-404.
Groundwater is a highly useful and often abundant resource, however over-use 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. Wells must consequently be deepened to reach the groundwater; in some places (e.g., California, Texas and India) the water table has dropped hundreds of feet due to excessive well pumping. A lowered water table may, in turn, cause other problems such as 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. Hypoheic zones (the mixing zone of streamwater and groundwater) and riparian zones are examples of ecotones largely or totally dependent on groundwater.
When we extract groundwater linked to a river system, we extract water from that river, even if the result is not evident for some time. And of course vice versa. Water management agencies around the world are still struggling to come to terms with this simple fact. See Kansas Geological Survey.
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, due to 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. In the first half of the 20th century, the city of San Jose, California dropped 13 feet due to land subsidence caused by overpumping; this subsidence has been halted with improved groundwater management.
Generally, 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 sea. Sea water moving inland is called a saltwater intrusion. Alternatively, salt from mineral beds may leach into the groundwater of its own accord.
Sometimes the water movement from the recharge zone to the place where it is withdrawn may take centuries (see figure above). When the usage of water is greater than the recharge, it is referred to as mining water (the water is often called fossil water because of its geologic age). Under those circumstances it is not a renewable resource.
Not all groundwater problems are caused by over-extraction.Pollutants released to the ground can work their way down into groundwater. Movement of water and dispersion within the aquifer spreads the pollutant over a wider area, which can then intersect with groundwater wells or find their way back into surface water, making the water supplies unsafe. The interaction of groundwater contamination with surface waters is analyzed by use of hydrology transport models.
The stratigraphy of the area plays an important role in the transport of these pollutants. An area can have layers of sandy soil, fractured bedrock, clay, or hardpan. Areas of karst topography on limestone bedrock are sometimes vulnerable to surface pollution from groundwater. See environmental engineering and remediation. Water table conditions are of great importance for drinking water supplies, agricultural irrigation, waste disposal (including nuclear waste), and other ecological issues.
Upon commercial real estate property transactions both groundwater and soil are the subjects of scrutiny, with a Phase I Environmental Site Assessment normally being prepared to investigate and disclose potential pollution issues.
Love Canal was one of the most widely known examples of groundwater pollution. In 1978, residents of the Love Canal neighborhood in upstate New York noticed high rates of cancer, and an alarming number of birth defects. This was eventually traced to organic solvents and dioxins from an industrial landfill that the neighbourhood had been built over and around, which had then infiltrated into the water supply and evaporated in basements to further contaminate the air. 800 families were reimbursed for their homes and moved, after extensive legal battles and media coverage.
Another example of widespread groundwater pollution is in the Ganges Plain of northern India and Bangladesh where severe contamination of groundwater by naturally occurring arsenic affects 25% of water wells in the shallower of two regional aquifers. The pollution occurs because aquifer sediments contain organic matter (dead plant material) that generates anaerobic (an environment without oxygen) conditions in the aquifer. These conditions result in the microbial dissolution of iron oxides in the sediment and thus the release of the arsenic, normally strongly bound to iron oxides, into the water. As a consequence, arsenic-rich groundwater is often iron-rich, although secondary processes often obscure the association of dissolved arsenic and dissolved iron.
Solutions in the Netherlands
In the Netherlands the high groundwater levels often causes problems for the construction industry. However, long experience with these problems have brought many new solutions: gravity-drilling, sonic drilling, water-injection, pulsing, etc. Dropping the groundwater level, so that construction-workers can build a foundation, is also often used.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Groundwater". A list of authors is available in Wikipedia.|