An aquifer , formerly also the groundwater horizon or groundwater carrier , is a rock body with cavities that is suitable for the conduction of groundwater .
Definition of terms aquifer and groundwater aquifer (in Germany)
The term aquifer ( lat. Aquifer "water- carrying " or "water carrier" from aqua water and ferre carry), which was adopted from the English-speaking area, has meanwhile also experienced a wide spread , but it has not been included in the hydrogeological definition of the German DIN 4049- 3 taken over. Although in parts of the German professional world mostly used as a synonym for the aquifer, an aquifer originally referred to a sequence of layers or parts thereof suitable for the discharge of significant amounts of water. This includes water in the unsaturated zone , which is not the case in the definition of groundwater valid for Germany. In contrast to an aquifer, an aquifer therefore only extends over the saturated zone. It follows that both terms can only be used as synonyms in individual cases. However, it can be assumed that when the term aquifer is used in German-speaking countries, an aquifer is meant.
Types of aquifers and properties
There are three types of aquifers:
- Pore aquifers consist of loose or solid rock, the pore space of which is traversed by groundwater
- Fissured aquifers consist of solid rock, they contain fissures and rock joints that affect the flow
- Karst aquifers consist of karstified carbonate rocks with flow-effective karstings
An aquifer is geologically bounded by impermeable layers (e.g. clays) called aquifuge .
Knowing them and exploring them is important for the production of drinking water (see also dewatering ) and the influence of mining on the groundwater (open pit drainage).
The most important property of an aquifer is its transmissivity .
Terms and types
A groundwater non- conductor is a body of rock that does not conduct groundwater . A low conductor, on the other hand, has a very low permeability coefficient , whereby the transition to a non-conductor is defined fluently. An area that delimits the aquifer with poor permeability is an aquifer. Common to all of these are mostly small grain sizes and low porosity . The English terms are Aquiclude for the non-conductor and Aquitarde for the low conductor.
A groundwater body is a groundwater resource that can be clearly defined spatially, whereas the groundwater space denotes the rock body filled with groundwater. The groundwater thickness is defined as the perpendicular distance between the groundwater surface and the groundwater bed, i.e. between the upper and lower limit of the groundwater body.
Furthermore, a distinction is made between constrained and unconstrained aquifers, which are defined by the relative position of the groundwater surface and the groundwater pressure surface. The groundwater pressure area is the area up to which the water would rise according to its hydrostatic pressure in a free well. In so far as this area corresponds to the groundwater surface, it is an unconstrained or free aquifer. However, if the rise in the groundwater is prevented by a non-conductive or poorly conductive layer before the pressure surface is reached, then there is a confined aquifer. This distinction has a significant effect on the behavior of an aquifer when installing groundwater measuring points as well as carrying out pumping tests and the ultimate groundwater extraction.
Artesian aquifers form a special case of the confined aquifer, where the pressure surface is above the ground and the groundwater would thus form a source under unconstrained conditions (e.g. through a borehole).
Aquifers can be used to store thermal energy in the long term and thus make it available for heating or cooling buildings. For this purpose, warm water from an aquifer z. B. used in winter to heat buildings and cools down. This cooled water is fed back into the aquifer and can then be used to cool the building in summer. When cooling the building, the water can also be used afterwards, e.g. B. heated by solar panels and stored again in the aquifer. For this process, at least two wells, an intake and an extraction well, are required, which change their function depending on the season.
In non-volcanic areas, the underground temperatures can be very different. To obtain heat from an aquifer, deeper boreholes are usually necessary: temperatures above 100 ° C are required for economical electricity generation. If these are present, water can be pumped, cooled and reinjected. One then speaks of hydrothermal geothermal energy .
Storage in aquifers far below a usable groundwater depth (from a depth of 900 meters) is seen as an intermediate technology for avoiding CO 2 emissions during energy generation due to the limited storage potential.
In contrast to the sequestration period in oceans (up to 10,000 years), a storage period of over 1 million years can be expected. Storage in the oceans, however, harbors enormous ecological dangers if a storage bubble does penetrate the surface.
Extraction of raw materials
The solubility of a gas in a liquid increases with increasing pressure and decreases with increasing temperature. The solubility decreases due to the increasing temperature in increasing depth. In the sum of both effects, the water held in rock pores (formation water ) can bind larger amounts of gas with increasing depth. When the pressure is relieved, part of the gas is released and either escapes into the atmosphere as free gas or, if the geological-structural conditions are suitable, is trapped in storage facilities. The part remaining in the formation water is called aquifer gas. The main problem with mining is land subsidence , as observed in Japan and Italy . However, this can be countered by reinjection of the degassed water.
The share of 2.5% fresh water in global water resources is 68.7% as "bound water" in the ice layers of the polar ice caps , the remaining 31.3% is distributed in aquifers or is stored as soil moisture .
Across the world, aquifers are used to a large extent for the production of drinking water . If the extraction exceeds the inflow, or if non-renewable fossil water is used, sustainable and permanent use is not given.
Approx. 25% of the world's population get their drinking water from karst aquifers. The Institute for Applied Geosciences at the Karlsruhe Institute of Technology (KIT) published as a project of IAH Karst Commission ( International Association of Hydrogeologists ) in September 2017 at the 44th annual congress of the IAH in Dubrovnik in addition to the 2000 published groundwater - World Map (WHYMAP, World-wide Hydrogeological Mapping and Assessment Program ) together with the Federal Institute for Geosciences and Natural Resources (BGR) and UNESCO a " World Karst Aquifer Map ".
In drinking water treatment , methods for enriching the groundwater are used in some cases: surface water is often re-infiltrated into the aquifer in order to use the cleaning effect (demanganisation, iron removal, biological degradation) of the subsoil. However, pollutants can also enter the soil as a result. These pollutants then have to be used again in the further process of drinking water treatment. B. be eliminated by activated carbon filters . In order to keep the pollution as low as possible, the water infiltrated into the aquifer is often pretreated.
The use of non-renewable water for agricultural irrigation is very controversial from an economic and ecological point of view.
The Ogallala aquifer in Central North America was used for agricultural irrigation from 1911. As the amount of water withdrawn soon exceeded the amount of water flowing in, the water level began to drop rapidly. According to current estimates, the ratio of withdrawn to supplied water is around 25, which means that for every 25 liters of withdrawn water, only one liter of new water flows in through infiltration, and in some places the water level has dropped by up to 1.50 meters per year measured. Some parts of the aquifer are therefore already waterless today; if this drying up continues, agriculture in the area could become impossible in the medium term. Some rivers in the region are also sometimes deeper than the groundwater level, which also removes water from the aquifer. The magnitude of such a drop in the water level becomes even clearer if one takes into account the extent of this aquifer (see table below).
The use of fossil groundwater is extremely problematic. In western Egypt, after the artesian springs had dried up, the Nubian aquifer was tapped in some oases ( Bahariya , Farafra , Abu Minkar , Dahkla and Kharga ) . In this region, due to the prevailing hyperarid climate, there is almost no new groundwater formation rate. Due to the heavy use of groundwater resources, the groundwater level of the Nubian aquifer fell by around 60 m by 2009.
In the north-west Indian aquifers in the New Delhi area , in Punjab and in the Indian states of Haryana and Rajasthan , the water level has fallen by more than 30 centimeters per year over the past six years (2009); the loss is over 100 km³. Across the country, the area of irrigated arable land tripled between 1970 and 1999. According to a study by the nature conservation organization WWF , Indian farmers use 400 km³ of water annually to irrigate their fields. Only 150 km³ of this comes from precipitation, the rest comes from aquifers.
In California's Central Valley (California Central Valley) approximately 20 cubic kilometers of groundwater have been lost due to intensive agricultural use.
Many groundwater bodies worldwide are subject to enormous pollution from industry and agriculture. B. from direct discharges of sewage or cooling water or from indirect inputs such as the seepage of spray and fertilizers. Responsible use of the environment (for example, organic farming can significantly reduce nitrate pollution) is reflected in the reduction in pollution of the groundwater. The main problem is that contaminants can be stored for decades or longer and will therefore be effective for a long time to come.
In many regions of the USA, South America and Asia, very high concentrations of arsenic occur in the groundwater. Excessive values also occur in Europe, especially in Great Britain. In Germany, the groundwater in the Black Forest is contaminated with arsenic in places.
A study presented in the winter of 2014 by the American Cary Institute for Ecosystem Studies in Milbrook shows that the salt pollution from road salt in rivers and lakes in southeast New York state is almost the same and unexpectedly high in summer and winter. It assumes that de-icing salt loads that have entered the aquifers over decades are reflected in the increase in salt concentrations in surface waters and drinking water wells . In the cool and snowy Michigan , changes in the specific water densities were found in two of the lakes investigated, with a resulting hindrance to the circulation between the water layers . In one of the lakes there was no longer any mixing at all: there was an oxygen-free and very salty, constant layer of water close to the ground with corresponding changes in flora and fauna .
Potassium chloride (potash salt)
In the southern German-French Upper Rhine aquifer, there are in some cases considerable loads from salt input from spoil heaps and old storage basins from the mining of potash that has been abandoned here.
Salt water intrusion
In the course of the “ salt water intrusion ”, salt water naturally penetrates into the sea near fresh water sources due to the different density of fresh and salt water. Changes in the water level, e.g. B. as a result of constant overuse of freshwater resources by humans can seriously disturb the natural balance that has grown. An example of this is the Chicot Aquifer on the Gulf Coast of the United States of America, part of the Gulf Coast Aquifer .
In people with atypical intestinal flora and in infants in the intestine, nitrate can be converted to nitrite , which is toxic. In addition, nitrate is viewed as an indicator of undesirable nitrogen-containing organic pollution.
Anthropogenic increased nitrate content in groundwater is a phenomenon known worldwide, which usually occurs in densely populated areas and is caused by intensive agriculture ( nitrogen or manure fertilization ). Certain soils allow more nitrate to seep through, and dry periods associated with climate change (in which plants absorb less nitrate) increase the entry through the connoted heavy rain with increased leaching. In order to determine nitrate inputs from the soil-plant system into the groundwater, a deterministic nitrate displacement model is sometimes used, which calculates the mean nitrogen input from the area. The limit value for drinking water in the European Union is currently (2010) 25 mg / l.
Even in unpopulated, semi-arid areas, increased nitrate levels in groundwater can occur that are not due to anthropogenic influences and are therefore caused by natural processes. In the countries of southern Africa ( Republic of South Africa , Namibia , Botswana ) local nitrate concentrations of up to 600 mg / l are found, which are considerably above the limit value of the World Health Organization (WHO) of 50 mg / l. It has not yet been clearly clarified where the causes of these increased nitrate levels lie.
Sustainable use strategies
Limitation of water consumption
Some public water suppliers have started to support water-saving technologies and installations also for end users in the form of advisory services and financial subsidies in order to reduce water consumption. This can make a lasting contribution to the stabilization of groundwater resources and water quality.
Keeping things clean through ecological forestry and agriculture
With its waterworks, the city of Munich has been supporting farmers in the area on a large scale who are converting to organic farming . In this way, the nitrate pollution, which had risen sharply at the end of the 1960s, was stabilized at a medium level and the groundwater obtained could be sent to Munich without further treatment.
Transparency about material flows
As part of a “ farm gate balance ”, industrial fattening farms are supposed to provide a balance sheet for the nutrients they use. B. residues emitted via slurry application become possible.
Comparison of large aquifers worldwide
To the table, for comparison: Researchers last estimated (March 2017) the volume of all lakes worldwide at almost 200,000 cubic kilometers.
|Name of the aquifer||Expansion / km²||Length / km||Width / km||Volume / km³||Max. Depth / m||approximate thickness / m||Age / years||geology||geography|
|Aquífero Alter do Chão||86,000||Brazil : states of Amazonas , Pará and Amapá|
|Acuífero Guaraní||1,200,000||1,500||1,500||South America : Argentina , Brazil , Paraguay , Uruguay|
|Great Artesian Basin (Great Artesian Basin)||1,711,000||64,900||3,000||50-250 million||Australia|
|Upper Rhine aquifer||45||on average 70, up to 260||Debris / debris filling||Germany: Upper Rhine Graben , South Baden , France ( Alsace )|
|Nubian Sandstone Aquifer (Nubian Sandstone Aquifer)||2,000,000||373,000||up to 4,500||90||4,500-5,000||Fossil water||Middle East : Egypt , Libya , Sudan , Chad|
|Ogallala aquifer||450,000||122||160||approx. 5 million||North America : Great Plains|
|for comparison: Lake Constance||536||63||14th||48||250||3 million||fluvioglacial eroded tongue basin or glacier edge lake from the Würm Ice Age in the course of the Rhine river.||Germany: South Baden|
|for comparison: Hornberg basin||0.17||0.7||0.3||0.44||65||1974||artificially created pumped storage power plant - upper basin||Germany: South Baden|
|for comparison: Lake Baikal (20% of the free fresh water)||31,500||636||80||23,000||1,642||25-30 million||River basin of the Angara between the Baikal Mountains||Russia ( Asia ): Siberia|
|for comparison: Three Gorges Reservoir||1,085||660||39||about 110||Completion 2008||artificial reservoir for energy over the course of the Yangtze -Flusses||China|
Other large aquifers worldwide
- Gulf Coast , United States of America : Gulf Coast Aquifer , consisting of the Chicot - Evangeline - Jasper - and Catahoula - aquifer . It stretches from Florida through Texas into Mexico and is mostly over 100 miles (160 km) wide. The largest Chicot aquifer alone has a larger area than the German state of Hesse .
Further cross-national examples
- Iullemeden Aquifer , Africa : Mali , Niger , Nigeria
- Koyna River Basin , India
- Lotipiki Basin : Kenya , South Sudan (discovered in 2013, extension 4,164 km², volume 200,000 km³)
- Systems d'Aquiferes du Sahara Septentrional (SASS), Africa : Algeria , Libya , Tunisia
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