The rock in which the groundwater resides and flows is known as the aquifer (from Latin also: aquifer , water-carrying or water-carrier ).
Basics and definition
According to DIN 4049, groundwater is defined as
- "Underground water that fills the cavities of the earth's crust continuously and whose movement is determined exclusively or almost exclusively by gravity and the friction forces triggered by the movement itself ".
The Water Resources Act defines groundwater as
- "The underground water in the saturation zone that is in direct contact with the ground or the subsoil".
The driving forces for the groundwater flow are the weight force and the pressure forces caused by it. Groundwater moves (flows, flows ) through the underground cavities due to differences in the piezometer height (= hydraulic potential ). According to this definition, backwater also counts as groundwater.
The underground water of the unsaturated soil zone which is hygroscopic , bound by surface tension and capillary effects ( soil moisture , retained water , see also boundary floor distance ) does not count as groundwater . The predominantly vertically moving seepage water in the unsaturated soil zone is also not part of the groundwater.
The cavities in the earth's crust mentioned in the definition are, depending on the geological nature of the subsoil: pores (clastic sediments and sedimentary rocks such as sand , gravel , silt ), fissures (solid rocks such as granite , quartzite , gneiss , sandstones ) or large ones created by solution Cavities (for example limestone ). Accordingly, a distinction is made between: pore groundwater (see also: pore water ), cleft groundwater and karst groundwater.
Groundwater takes part in the water cycle . However, the dwell time in the subsurface can vary widely and ranges from less than a year to many millions of years. Very old groundwater is also known as fossil water .
A groundwater resource or a delimitable part of a groundwater resource is referred to as a groundwater body. The upper boundary surface of a groundwater body is called the groundwater table , the lower boundary surface is called the groundwater bed , groundwater bed surface or groundwater subsurface. The vertical distance from the groundwater bed to the groundwater surface is known as the groundwater thickness.
Rocks that are able to absorb and conduct significant amounts of water are called aquifers . However, they do not necessarily always have to contain water. The part of an aquifer that is filled with water at a given point in time is called the aquifer . Aquifers are limited at the bottom by impermeable or impermeable rocks. Such a groundwater non-conductor is also known as an aquicludge . If there are several aquifers and aquifers in a vertical sequence, there may be several aquifers lying one above the other .
In the case of an unconstrained aquifer, the hydrostatic pressure is by definition equal to the air pressure; Conveniently, the air pressure in hydromechanics is often set to zero; the hydraulic pressure potential ( hydraulic head ) is equal to the sum of its geodetic height and the air pressure (or zero) on the free groundwater surface. The exposed groundwater surface in a groundwater measuring point is called the standpipe level . The distance between the terrain surface and the groundwater surface is referred to as the corridor distance or groundwater corridor distance . If the geological unit lying above the aquifer, the groundwater cover , is a water-permeable layer, unstressed conditions prevail. If the groundwater cover is impermeable to water, the groundwater conditions may be constrained, which means that the hydraulic potential is higher than the actual groundwater surface (confined, artesian groundwater when the earth's surface is exceeded ). Water layers is by water retention layers above the ground water at the percolation hindered, usually close to the surface, independent of the main aquifer groundwater. If there is a non-water-saturated zone underneath, one speaks of floating groundwater.
Like surface water , groundwater follows gravity and flows in the direction of the greatest (piezometric) gradient. For groundwater flow areas, this can be determined from maps on which standpipe levels are shown as hydroisohypses ( groundwater level plan ). The greatest gradient and thus the direction of the groundwater flow or the groundwater flow lines are always at right angles to the groundwater levels. The simplest method of creating a groundwater level plan is to use the hydrological triangle method .
Compared to surface water, groundwater mostly flows at a much slower rate. Also note the difference between filter speed and spacing speed . In gravel ( grain sizes 2–63 mm) the distance speed is 5–20 m / day (maximum values are 70–100 m / day), in fine-pored sediments such as sand (grain sizes 0.063–2 mm) only about 1 m / day, always also depending on the gradient. In deep aquifers, the speed can decrease to a few m / year.
Mathematical groundwater models are used to forecast or simulate groundwater flows , with which the inflow, extraction, subsidence and regeneration of groundwater can be represented well and in detail. Hazards (migration of environmental pollutants) can also be identified at an early stage and even historical conditions can be traced, for example when exploring contaminated sites.
Groundwater recharge and amount of groundwater
Groundwater arises from the fact that precipitates seep or water in the bottom and bank area of surface waters by migration or artificial enrichment (infiltration systems, for example Sickerbeete, slit trenches, infiltration wells) into the ground infiltrated . Of the 22.6 million km³ of groundwater in the upper two kilometers of the earth's crust , around 0.1–5.0 million km³ are less than 50 years old. This is also called conversion water, which is a recent component of the water cycle. In contrast, there is fossil groundwater , which in the deeper subsurface has been cut off from the water cycle for geological time periods (a few tens of thousands to many millions of years).
Influence of the soil passage
During the long underground passage, the groundwater is changed by physical, chemical and microbiological processes; a chemical and physical equilibrium is established between the solid and liquid phase of the soil or rock . For example, caused by absorption of carbon dioxide (the soil organisms from the respiration) and its reaction with the calcite and dolomite , the water hardness . If the residence time is long enough , pathogenic microorganisms (bacteria, viruses) can be eliminated to such an extent that they no longer pose a threat. From a water management point of view, these processes are predominantly positive for the quality of the groundwater and are therefore collectively referred to as self-cleaning .
However, when acidic waters seep away , for example acid rain or from open-cast lakes, significant amounts of aluminum can also be released from crystalline rock , including from soils in spruce and fir forests. Furthermore, acidic groundwater , especially acidified groundwater due to pyrite weathering , can have high levels of iron (II) compounds .
Threats to groundwater and groundwater protection
Human interventions can have negative qualitative and quantitative effects on the groundwater: For example, in China 60 to 80 percent of the groundwater is heavily polluted and no longer suitable for drinking . In Germany, quantitative bottlenecks due to excessive groundwater abstraction are only of local importance. In semi-arid or arid regions with little groundwater recharge, excessive abstraction of groundwater leads to a large-scale lowering of the groundwater surface and corresponding environmental damage. Criminal proceedings are often initiated against environmental polluters in the event of a gross violation of applicable laws .
Dangers to the quality of the groundwater are, for example, the deposition and soil passage of air pollutants , the excessive application of fertilizers and pesticides by agriculture or highly concentrated pollutant plumes from contaminated sites .
The nurturing (curative) and restoring (remedial) groundwater protection is therefore very important in environmental protection . The designation of water protection areas in the catchment area (extraction systems) of waterworks is part of the preventive groundwater protection . The remediation of groundwater damage is usually expensive and time-consuming.
Saltwater intrusion can be problematic for wells near the coast and the water supply on islands : Due to the sensitive hydrostatic balance between freshwater and saltwater in the subsurface, even a slight extraction of freshwater can lead to a rapid reduction in the thickness of the freshwater layer due to the rise of saltwater. As a result, the water at the point of use can become inedible for humans or unusable for irrigation.
World map on groundwater hazard
The "world map for groundwater hazards caused by flood and drought" ( Global Map of Groundwater Vulnerability to Floods and Droughts ) was created in cooperation of the project "Groundwater for Emergency Situations" ( Ground Water for Emergency Situations , GWES) of the "International Hydrological (program" International Hydrological Program , IHP) of UNESCO with the "International Association of Hydrogeologists" and the " World-wide Hydrogeological Mapping and Assessment Program " ( WHYMAP), coordinated by UNESCO and the German Federal Institute for Geosciences and Natural Resources (BGR) . The map, which actually consists of several maps, is essentially based on the map " Groundwater Resources of the World 1: 25,000,000 " ( Groundwater Resources of the World 1: 25,000,000) of the WHYMAP from 2011; It shows in three levels, "low", "medium", "high" how much the groundwater in the various regions of the world is at risk from certain natural disasters due to the respective natural conditions . The card was presented to the public at the Seventh World Water Forum , which took place April 12-17, 2015 in Daegu , South Korea .
Effects of climate change
Most of the world's groundwater resources are currently still roughly in equilibrium in terms of inflow and outflow / withdrawal quantities. On the other hand, the groundwater would sink and eventually dry up if the mean inflow, mean runoff and withdrawal quantities in an area could no longer balance out. Using a groundwater model created in international cooperation , it was shown that in the course of climate change in the next 100 years, only about half of the world's groundwater resources could be in equilibrium. In the other half, even extreme rainfall could no longer fill the reservoirs on average due to the accumulation of dry periods. Although this only became noticeable with a time lag, these groundwater resources ultimately dried up completely. The delayed occurrence of the effects of climate change on the formation of new groundwater is described as an “environmental time bomb”.
In metropolitan areas, a warming of the groundwater is observed primarily as a result of the heat island effect. This “thermal pollution” is viewed by hydrogeologists as a potential threat to the living things in the groundwater and therefore to the quality of the groundwater.
Protection in the European Union
The groundwater is classified as a protected asset in the EU's Water Framework Directive . Directive 2006/118 / EC of the European Parliament and of the Council of December 12, 2006 on the protection of groundwater against pollution and deterioration makes special requirements . She justifies this with the special importance of this protection for groundwater-dependent ecosystems and for the use of groundwater to supply water for human consumption. It obliges the member states to monitor, in particular with the help of groundwater measuring points, and defines criteria for assessing the water quality. According to this, a groundwater body is to be regarded as groundwater in a good chemical condition if, in particular, the threshold values to be set by the member states for various substances and the groundwater quality standards set out in the directive are observed at all measuring points. For nitrate, this quality standard is set at 50 mg / l and for pesticide active ingredients (including all relevant reaction or degradation products) at 0.1 μg / l or a total of 0.5 μg / l.
Groundwater spaces are among the largest and oldest (that is, most stable in the long term) continental habitats in the world; they are with constant relatively cool temperatures of z. B. 14 ° Celsius also thermally very stable; many of the species that live here are " living fossils, " e.g. B. Well crabs , probably many still undetected.
In the European Union (EU) according to Directive 2000/60 / EC (EU Water Framework Directive, WFD), the ecological status of rivers and surface waters as well as groundwater is analyzed according to various criteria and classified according to five grades: "very good", “Good”, “moderate”, “unsatisfactory”, “bad”. 2015 were z. In Lower Saxony, for example, 13 groundwater bodies are in a "poor chemical condition" .
First German state monitored Baden-Wuerttemberg in the course of monitoring the groundwater fauna . A relatively high number of species was found at a measuring point in Neuchâtel am Rhein, even in an international comparison, namely 21: on average, only two to three species are found at such a test point.
The state and development of the groundwater in Switzerland is determined by the National Groundwater Observation NAQUA of the Federal Office for the Environment . In 2014, residues of pesticides were detected in more than half of all groundwater measuring points . At around 20 percent of the measuring points, the concentrations of plant protection product metabolites were above 0.1 µg / l. From 2014 to 2017, atrazine , bentazone and metolachlor exceeded the limit value at several measuring points every year. In 2019, the use of chlorothalonil was particularly in the public eye , as the limit values could not be observed in many places. The drugs , the drugs could sulfamethoxazole (antibiotic), carbamazepine (antiepileptic), amidotrizoic and iopamidol (both X-ray contrast agent) are most commonly found in groundwater.
Despite the ecological importance of the groundwater areas, there is still some catching up to do in terms of legal recognition as a habitat, since groundwater, in contrast to z. B. surface water is primarily treated and seen as an "inanimate" resource .
Groundwater animals are mostly transparent or white and " blind ". To date, more than 2,000 animal species have been recorded for the fauna of the groundwater in Europe , and more than 500 in Germany; Crustaceans are predominant - probably worldwide ; are next to find " oligochaetes worms" ( earthworm -related), nematodes , some small snails . As a rule, these animals are quite small, the largest being cave shrimp with a length of up to four centimeters if there is enough space in the gap system. Many of the species living here are very small with a size of a millimeter or less - and are filtered out before any human use of the groundwater .
The animals living in the groundwater are promised a not insignificant function for the cleaning of the groundwater from organic constituents : They feed on bacteria films on the rock surfaces and sediment grains, whereby these bacteria have the predominant part in the cleaning of the groundwater, but the feeding activity of the groundwater animals Curbs the growth of bacteria and keeps the pores and crevices in the aquifers open. In this way the self-cleaning power of the ecosystem is maintained.
Since 2002, the GRACE satellites have been used to roughly measure the increase and decrease in groundwater.
Groundwater hazards to humans
Normally, groundwater does not pose a direct danger to humans (as for example in the case of directly adjacent igneous activity, see phreatomagmatic explosion ). However, there are occasional floods and undercuts from escaping groundwater. Groundwater entering the tunnel is a deadly danger. Groundwater can attack concrete and the steel reinforcement . Therefore, a groundwater sample must be taken wherever concrete parts can come into contact with water, which must be examined for concrete aggressiveness in accordance with DIN 4030 .
Around 300 million people around the world obtain their water from groundwater supplies. However, around 10 percent of the groundwater wells are contaminated with arsenic or fluoride . These trace substances are mostly of natural origin and are washed out of rocks and sediments by the water.
In 2008 the Swiss water research institute Eawag presented a new method with which hazard maps for geogenic toxins in groundwater can be created without having to check all wells and groundwater supplies in a region. In 2016, Eawag made its knowledge freely accessible on the Groundwater Assessment Platform (GAP). This internet portal offers members of authorities, employees of NGOs and other experts the possibility of uploading their own measurement data and creating risk maps for areas of their choice.
, Groundwater in the construction industry as a pressurized water called, especially up in the civil engineering is a problem when changing, reaching in the construction sector groundwater levels are not observed or if deliberately built into the groundwater, which then in the pit or pushes the building. A construction method of cellars and other structures based on waterproof concrete and thus groundwater-proof is known as a white tank . The buoyancy (buoyancy = weight of the displaced liquid) that pushes the tub upwards must always be taken into account.
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