At the same time, it is an important resource for industry, agriculture and ecosystems, it is subject to the challenges of climate change and it represents an important habitat. Groundwater is therefore an important resource that needs special protection.
Groundwater is not only important as a resource for drinking water production, but also for industry, for example as process water and raw material for food production. It feeds important ecosystems such as wetlands and forests and is used for agricultural irrigation. Groundwater is also increasingly seen as an economic good because it is the most used raw material worldwide. Unlike other raw materials such as oil, natural gas and ore deposits, groundwater is a resource that in many cases is renewed again.
On and below the earth's surface, water is in a constant cycle between the oceans, the atmosphere and the continents. A large part of the precipitation that hits the earth's surface in Germany evaporates again directly without reaching a surface water body or the groundwater. The remaining precipitation water - in Germany around 300 mm per square meter - runs off above ground in rivers and streams, especially during heavy rainfall. Germany's humid climate generally leads to groundwater recharge in the winter months and rather to a decrease in the summer months, when the evaporation rate is higher. In the cool season, a significant portion seeps into the ground, continues to flow below the surface and becomes groundwater.
Groundwater fills the pores and cracks in sands, gravels and other rocks, much like water fills a sponge. It moves slowly, usually at a rate of a few centimeters per day, and sometimes remains in an aquifer for centuries or longer. The earth's deep groundwater reservoirs do not participate in the water cycle, or only to a limited extent. Most of them were formed in the last thousand to ten thousand years and represent large underground freshwater reserves. Increasingly, these fossil groundwater reservoirs are being used in the water-scarce regions of the world. However, only that part of the groundwater that participates in the water cycle can be used permanently. At greater depths, the groundwater is often highly saline and is called brine; the salt content can be up to 10 times higher (> 300g/l) than in seawater.
Prolonged drought with a lack of precipitation, reduced seepage rates and groundwater recharge leads to a drop in the groundwater surface. For example, in 2018 and 2019, due to the prolonged drought, groundwater levels in the near-surface aquifers have dropped significantly in some regions of Germany. While there was no shortage of drinking water and there have been no area-wide negative impacts on the water supply from groundwater resources so far. However, in the summer of 2018, for example, self-supply with drinking water came to a partial standstill in the particularly affected regions because domestic wells went dry.
Groundwater is used directly as drinking water in many parts of the world. In Germany, around 70 per cent of drinking water is obtained from groundwater and spring water. On its way to our taps, various barriers protect drinking water in Germany from contamination. In many cases, the protective regulations in the designated drinking water protection areas (first barrier) already ensure that hardly any pollutants or pathogens enter the water. If necessary, the treatment of the raw water in the waterworks acts as a second barrier. The maintenance of the pipe network by the water suppliers and the professional design of the domestic distribution system represent further barriers. This principle of many barriers against pollution is called the "multi-barrier principle".
Groundwater is not only a valuable water resource for human use, but also interacts in many ways with surface waters, terrestrial and aquatic ecosystems. Groundwater-dependent terrestrial ecosystems describe the habitats (biotopes) that are in contact with groundwater and whose biotic communities, especially plants, depend on a sufficient amount of groundwater. In dry periods with rising temperatures, increased evaporation and prolonged vegetation phases, low groundwater levels are therefore not only problematic for water abstraction for drinking water production, but also for shallow-rooted trees and groundwater-dependent biotopes. Furthermore, rivers and lakes in our latitudes are fed by groundwater. When groundwater levels fall, the underground runoff into surface waters decreases, even to the point of reversing the direction of flow.
Germany is rich in groundwater. Depending on the regional hydrological and geological conditions, the groundwater is distributed spatially very differently. It can lie a few centimeters to several hundred meters below the earth's surface. The underground spaces that can store and transport the water are called aquifers. They are separated from each other by layers of non-conducting or poorly conducting rock. A distinction is made between pore, fissure and karst aquifers. Pore aquifers are characterized by loose rock in which the water fills the pore space between the individual rock grains. The pore size also determines the flow rate of the groundwater. While in clayey strata the flow rate is only a few centimeters per year, groundwater in a sandy subsoil can flow several meters per day. With a share of about 40 percent of Germany's land area, pore aquifers occur most frequently. The largest contiguous pore aquifer in Germany extends over the glacial sand and gravel layers in the North German Lowlands. But also in the foothills of the Alps in the so-called Molasse Basin, in the Upper Rhine Rift and on the Lower Rhine, large-scale and sometimes very productive pore aquifers are typical. A proportion of about 20 percent of Germany's surface area is covered by fissure or karst aquifers. Fissure aquifers are rock layers in which water flows through cracks, openings and fissures in solid rock. These layers usually store significantly less groundwater than pore aquifers, but have significantly higher flow velocities. Typical occurrences are found in the Vogelsberg region in Hesse, in the Black Forest and in the Spessart. Karst aquifers are a special form of fissured aquifers. These are calcareous and thus water-soluble solid rocks in which the cracks typical of fissure aquifers form, which the groundwater flowing through flushes out and enlarges over time. They are mainly found in the Swabian and Franconian Alb, but also in Thuringia and in a line south of the Main River to the Black Forest. A further 40 per cent of Germany's area comprises various aquifers that have very little or no groundwater resources.
Uses and Pressures
Groundwater is subject to a variety of uses and the resulting pressures. These can have a negative impact on its quality and quantity.
Groundwater resources under agricultural land are often exposed to pressures caused by intensive land use. The main problem is diffuse nutrient inputs from nitrogenous fertilization, which is often not applied in a way that is appropriate to the location and use and contributes significantly to nitrate pollution of groundwater. A yardstick for the level of pollution is balanced nitrogen surpluses. This is the amount of nitrogen that is not taken up by plants and then escapes as nitrate into ground and surface waters and as ammonia and nitrous oxide into the air. On average for Germany, this surplus is currently more than 80 kg per hectare and year. The German Sustainable Development Strategy sets a target of a maximum of 70 kg per hectare per year. It is therefore necessary to continue to work on noticeably reducing the surpluses in order to achieve this target and to sustainably protect the groundwater. In addition to the contamination of groundwater with nitrates, the contamination with plant protection products continues to be a cause for concern. The majority of pesticide contamination also comes from agricultural use. In addition, the cultivation of biomass for energy production has risen sharply in recent years. Large areas of energy maize are often cultivated for this purpose, which is intensively fertilized and treated with pesticides. This leads to increased inputs of nitrate and pesticides into the groundwater. In addition, biogas production produces nutrient-rich residues, the so-called fermentation residues, which are also used as fertilizer and can also contribute to groundwater pollution.
Groundwater abstractions for drinking water supply vary greatly in Germany depending on the federal state. While in Saarland, Bremen, Hamburg and Schleswig-Holstein almost all drinking water is extracted from groundwater resources, the share in Saxony and Thuringia is significantly lower, where in many cases reservoir water is also used there. In Germany as a whole, around 3.6 billion cubic meters of groundwater are extracted each year for drinking water. In addition to drinking water, groundwater is also abstracted by industry, agriculture and the energy sector for various purposes. Across all uses, around 6 billion cubic meters of groundwater are used in Germany per year. This corresponds to about 12 percent of the average newly formed groundwater.
Mineral and Medicinal Waters
Mineral waters are formed by long residence times and deep circulation of groundwater underground. The contact of the groundwater with easily soluble rocks and gases leads to a mineralisation of the waters with special chemical and physical properties. Higher temperatures in deeper layers and increased contents of dissolved free carbonic acid increase the solubility of the rock through which the groundwater flows. Under favourable conditions, mineral waters emerge naturally at the earth's surface. However, the majority of the deposits are tapped through boreholes, some of which are several hundred meters deep. The waters are used in a variety of ways, such as in spa operations and in the beverage industry. The sale of the water as medicinal or mineral water is linked to certain legal requirements. They are the only foodstuffs in Germany that require official approval. Similar legal guidelines apply to medicinal waters and mineral waters. For example, both must come from naturally or artificially tapped underground sources, which must not be polluted under any circumstances and must be free of microbacteria. Mineral contents of at least 1000 mg/l are prescribed for both. This mineral concentration is supposed to guarantee a healing effect in the case of medicinal water and a nutritional-physiological effect in the case of mineral water - under the additional condition of a concentration of 250 mg/l carbonic acid. If a water does not meet these limits, it can still be sold as medicinal water if it has a minimum content that relates to a specific mineral - for example fluorine.
Medicinal and mineral waters are not staple foods. The use of normal tap water as drinking water (table water) is considered harmless in Germany and sufficient for a healthy person. Mineral water consumption in Germany has increased more than tenfold since 1970 and was 133.8 liters per year and person in 2020; in the hot summer of 2018, per capita consumption was even 147.7 liters. Unlike tap water, mineral water and medicinal water are not subject to the strict requirements of the Drinking Water Ordinance.
Thermal Water and Geothermal Energy
In Germany, groundwater is called thermal water if its temperature at the source is more than 20 °C and warmer than the surrounding groundwater near the surface. Hot and warm springs were already popular with the Romans for therapeutic purposes (e.g. in the Imperial Baths in Trier), as thermal water is rich in dissolved minerals. To this day, thermal springs have a long tradition in Germany. In general, thermal springs occur in areas of increased volcanic activity and near deep flow systems. Energy stored in the form of heat below the surface of the solid earth is becoming increasingly important in the context of the energy transition. Deep geothermal energy is advancing into other dimensions compared to the use of geothermal energy near the surface. Not only are heat reservoirs being tapped at greater depths, with boreholes being drilled to depths of up to five kilometers. The plants operated with it are also much larger and more powerful. Geothermal energy from deep geothermal reservoirs is used to feed heating networks and supply entire city districts with heat. If the temperature level is high enough, a geothermal power plant can also generate electricity. Geothermal energy is not dependent on weather conditions and can supply environmentally friendly electricity almost continuously throughout the year. Near-surface heating and cooling with geothermal energy as well as underground heat storage can also make a significant contribution to a climate-neutral energy supply. However, as the number of interventions in the subsurface increases, so does the risk to the groundwater ecosystem and the use of groundwater for drinking water purposes. Microorganisms are adapted to certain temperature ranges. Significant changes in temperature lead to changes in biotic communities and, associated with this, to a possible deterioration in self-purification processes. For underground storage technologies, especially in urban areas, concepts for thermal management of groundwater will be required in the future in order to use underground heat and cold reservoirs where temperature changes do not have adverse effects on other water uses, such as the quality of groundwater as a drinking water resource or groundwater as a habitat. To this end, thermal and ecological quality targets must be developed for groundwater.
Groundwater Monitoring and Groundwater Quality
In Germany, the monitoring of groundwater quality is ensured by a close-meshed monitoring network operated by the authorities of the federal states. The legal framework for this is set by the European Water Framework Directive. This requires EU member states to assess groundwater status at the level of groundwater bodies. The aim of the Directive is to achieve a good chemical and quantitative status of groundwater bodies throughout Europe. To determine the chemical groundwater status, the concentration of pollutants is measured at the monitoring sites and it is checked whether these exceed the applicable limit values. There are currently uniform limit values for nitrate and pesticides throughout Europe. In addition, the member states must set their own limit values for other substances named in the EU Groundwater Directive. To determine the quantitative groundwater status, the abstraction quantities are compared with the groundwater recharge. Good quantitative status means that there is at least a balance between groundwater abstraction and groundwater recharge. In total, the federal states operate more than 7000 monitoring sites to determine the chemical groundwater status. About 6000 monitoring sites are used to determine the quantitative groundwater status. The results of the last monitoring period show that just under one third of German groundwater bodies are in poor chemical status. The main causes are exceedances of the limit values for nitrate and pesticides, which often have their origin in intensive land management. Current nitrate data are available in the Federal Environment Agency's Nitrate WebApp. Further pollution results from residues of pharmaceuticals, organic compounds, artificial sweeteners and degradation products of pesticide active substances. The latter group of substances was detected at more than half of the monitoring sites investigated, and thus particularly frequently. In order to improve the chemical status of groundwater, Member States are required to take comprehensive measures to reduce harmful substance inputs. Currently, there are only a few groundwater bodies in Germany that have water quantity problems. Only a few groundwater bodies fail to achieve good quantitative status, even though groundwater levels in shallow aquifers fell significantly in some regions during the dry years of 2018 and 2019. Large-scale problems occur in connection with mining activities. In these regions, the groundwater level has often been severely lowered over many decades. Even after mining has ended, it usually takes many decades before the natural groundwater level is restored.
The underground is not only an important usable water reservoir, but also a habitat populated by a diverse community of organisms. Probably the largest limnic, i.e. freshwater-determined, ecosystem extends worldwide in the groundwater and assumes important intermediary functions in the global water cycle. Numerous groundwater animals live in the groundwater, invisible to us, and move around in the water-filled gaps and fissures of the subsoil. The large spectrum of groundwater animals includes species that often have relatives in surface water. The relatively constant physico-chemical conditions in the subsurface, such as permanent darkness, low and constant temperatures and low nutrient and oxygen concentrations, allow for a high biodiversity within the groundwater fauna. The most important groups are crustaceans. In addition, there are isopods, snails, worms and occasionally even fish. As a rule, the animals cannot survive outside the underground habitats. Experts say they are "troglobiont" (living in caves). Many of them are on the red list of endangered species. The first groundwater animals were probably discovered more than 460 years ago. In 1541, people found the first blind cave fish in a cave in China. Only a few years ago, in the cave system of the Aachtopf in the Lake Constance region, a fish was also discovered in Germany, the cave loach (Barbatula barbatula), which lives exclusively in underground water. Another animal that has adapted to the groundwater habitat is the Husmann's well snail (Bythiospeum husmanni). This snail, which is only two millimeters in size and belongs to the dwarf lidded snail family, is only found in riverine groundwater streams in the Ruhr area and is extremely rare. It is known that this snail can only live in very clean and uniformly cool water, so its occurrence is a good indicator of excellent groundwater quality. To draw attention to the importance of this snail species for groundwater, it was named "Mollusc of the Year 2009". More common in groundwater are various species of cave or groundwater amphipods of the genus Niphargus.
In Germany alone, 15 different species of these blind and colourless animals have been identified. Depending on the species, they are between a few millimeters and up to three centimeters in size. They feed mainly on dead microorganisms, so-called detritus, but also on smaller animals and move around lying on their sides. To draw attention to the sensitive habitats of caves and groundwater with their great diversity of species, the groundwater flea crab was named "Cave Animal of the Year 2009" by the Association of German Cave and Karst Researchers. Of the more than 170 European freshwater isopod species, more than 60 percent live exclusively in groundwater. One widespread species is the so-called cave water isopod (Proasellus cavaticus). The animals, which are about 8 millimeters in size, are found throughout Central Europe and, like many other groundwater creatures, feed on dead plants. Their biggest natural enemies are the cave fleas of the genus Niphargus described above. The organisms adapted to the special conditions include groundwater animals as well as various microorganisms such as bacteria, aquatic fungi and protozoa, which make up the main part of the biomass.
The different types of bacteria have the ability to use different carbon and nutrient resources and, as a result of their metabolic activities, significantly regulate the material and energy cycles in the subterranean ecosystems. Some of the bacteria live freely in the groundwater, but more often they are attached to small sediment grains, stones or particles of organic material. By coating the small particles, they form the so-called biofilm, which serves as a food base for groundwater animals. The unicellular organisms (protozoa) in groundwater are only a few micrometers in size and are adapted to the limited supply of nutrients and the small pore spaces in which they can move. So far, various forms of flagellates, amoebae and ciliates have been found, whose main food is bacteria. By grazing on the bacterial biofilm or even filtering bacteria out of the water flow, they regulate bacterial growth and thus prevent the aquifer from clogging, for example. Among the protozoa themselves, there are also predatory specimens, such as the heliozoa. They are somewhat larger than the other species that serve them as food. This in turn prevents excessive mass development of the protozoa. Only a few types of aquatic fungi have been identified and less is known about their life forms compared to the other groups of organisms. Fungi also use organic carbon compounds as a basis for life and thus take on an important role in the microbial metabolism of the subsurface food web.
The interaction of the individual groups of organisms contributes decisively to the stability of the biological services in the system. Even though this habitat appears hidden and inaccessible, and many of its facets are still unexplored, we already know quite a bit about this unique ecosystem and its inhabitants, which are adapted in a special way to the meagre conditions in groundwater and require comprehensive protection.
The UBA’s motto, For our environment (“Für Mensch und Umwelt”), sums up our mission pretty well, we feel. In this video we give an insight into our work.
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