The lowering of the groundwater level can have natural causes, for example if the water-bearing layers are changed by earthquakes. However, humans are responsible for most of the changes in the water table. If, for example, more water is withdrawn from a well than is supplied by the groundwater flow, a lowering of the groundwater is inevitable.
A basic distinction is made between closed and open groundwater lowering. With open groundwater lowering, the water is visible when lowering. In the case of closed groundwater lowering, however, the lowering takes place in the closed earth body, i.e. H. you can't see the water. The groundwater lowering , i.e. the size of the lowering funnel , can be measured by means of measuring wells , e.g. B. "well-points" (needle wells, see also: Wellpoint method ), can be determined or calculated (e.g. according to LUCKNER).
Deliberate lowering of the groundwater
In lignite mining, it is necessary to lower the groundwater around the excavation pit, because otherwise it would fill with water. The groundwater has to be lowered below the coal to be mined in order to ensure the stability of the large equipment weighing thousands of tons (e.g. conveyor bridge). The depth of the subsidence in Lausitz open-cast mines is between 50 and 100 meters below ground. As a result of this depth and the slightly permeable layers of sand, it also affects the area around the open pit for kilometers. The Vattenfall mining company is increasingly building underground cut-off walls to limit this effect by using special equipment to create a vertical, water-impermeable layer in the earth. (Already built: Cottbus-Nord opencast mine to the north, Jänschwalde opencast mine to the west. Planned: Welzow-Süd opencast mine to the south, Reichwalde opencast mine.) Where this has not yet been done, it is required as a protective measure by landowners, waterworks or environmental associations. (Nochten opencast mine to the north, Jänschwalde opencast mine to the north and west)
The Garzweiler opencast mine probably led to the groundwater dropping 75 centimeters in the Happelter Heide near Brüggen - it is located in the Schwalm-Nette Nature Park . Rheinbraun is investigating the phenomenon with test boreholes up to 250 meters deep.
“Experts suspect that a brown coal seam has a gap at a depth of about 250 meters under the Happelter Heide. Underneath is water, which usually builds up pressure. This water column also supports the wetlands on the Schwalm . Due to the swamps in the open pit, the pressure is reversed, and water flows off in the depths. According to the principle of communicating pipes, the water level sinks where there is a connection to higher layers. If the assumption is true, Rheinbraun swamp water from the opencast mine will also have to infiltrate the Happelter Heide in order to establish and support the natural water level. "
In order to have a dry construction pit , a groundwater lowering is necessary for construction pits that reach into the groundwater. In the case of smaller construction pits that reach into an inadequate groundwater flow, this can be achieved with a pump sump in the construction pit (open drainage).
In the case of larger construction pits, which are located in an abundant groundwater flow, extensive groundwater lowering is necessary, which is carried out by means of wells around the construction pit. This lowering of the groundwater in construction pits is called closed drainage . It is generally necessary when the highest groundwater level is more than 50 cm above the base of the excavation. It must be operated until the sealing work on the structure is effective and the completed parts of the structure are no longer endangered by static buoyancy .
An alternative to lowering the groundwater level is to seal off the construction pit with diaphragm or sheet pile walls . These should be integrated into a water-impermeable layer, otherwise there is a risk of hydraulic ground failure .
During tunnel construction work in the groundwater flow is usually not lowered today, but the groundwater flow is diverted using freezing technology.
The lowering of the groundwater level serves to improve the workability of the arable land or to gain arable land through amelioration measures . The groundwater level is permanently lowered by a network of drainage pipes and ditches. Artificial irrigation may be required in order to guarantee the safeguarding of agricultural yields . Well-known examples of such improvement measures are the Rhinluch and the Oderbruch .
Unwanted lowering of the groundwater
Coniferous instead of deciduous forests
The Lower Saxony Chamber of Agriculture examined what influence the different silviculture variants have. The dominant evergreen coniferous forests evaporate significantly more water over the course of the year than deciduous trees. In coniferous forests, with predominantly pines, 23% less groundwater forms than in mixed forests with oaks and beeches.
Regulation (straightening) of flowing waters
River regulation and straightening are interventions that certainly have the most dramatic impact on the water table. From an ecological point of view, such measures often represent serious interventions in the environment (destruction of biotopes such as wet meadows ).
Civil engineering work
Incorrect planning or execution can lead to B. as a result of linear civil engineering work, the groundwater flow and thus the groundwater level is changed. This does not always have to result in a lowering, but can also lead to exactly the opposite, to an increase in groundwater. Such construction measures are z. B. tunnels, canal routes or other large underground pipeline routes. There are two main reasons for the change in the groundwater flow: On the one hand, water-impermeable layers can be penetrated and there is no longer any separation between the different groundwater levels . On the other hand, the groundwater can preferentially flow along the route in the sand or gravel bed over large distances and thereby change the groundwater level.
Drinking water in wells
The water that is taken from a well must flow in so that the well does not dry up. This happens because a local groundwater gradient forms around the well shaft, the subsidence funnel . The greater the water withdrawal, the lower the groundwater level in the well itself falls. If more water is withdrawn than can flow in through the groundwater flow, the groundwater level continues to drop.
Consequences of the lowering of the groundwater
Large-scale and long-lasting lowering of the groundwater level can cause serious damage to the lowering funnel. Damage to buildings can occur in nearby towns.
When the groundwater is lowered, soil layers dry out and lose their buoyancy from the water and the soil is more compressed. Technically speaking, the lowering of the groundwater level increases the uplift-free area of the ground and increases the effective stresses that cause settlement in the grain structure below the original groundwater level. In the event of a lowering of the groundwater, fine soil particles can also be washed out of the grain structure of the soil, known as suffosion . This causes the coarser grains to sag. In both cases, depending on the nature of the soil, settlement occurs , the characteristics of which can also differ on a small scale. This can cause cracks in the ground, some of which extend to the surface of the earth. In buildings, uneven settlement amounts can lead to settlement damage, which usually shows up as cracks in the masonry.
The lowering of the groundwater level can have far-reaching consequences for vegetation . The capillary border , which is important for the plants , shifts to greater depths. Trees and crops lose their natural groundwater connection, forest dieback and extensive drought damage can occur. With the meliorative application of the groundwater lowering, succession occurs .
Due to its large heat capacity, water has a strong moderating effect on the local climate. This can be seen particularly well on the coast and in water. The sea ensures cooler temperatures on the coast in summer than inland and vice versa in winter.
The groundwater level also changes the heat capacity of the soils inland. According to Dr. Stefan J. Kollet, even at a depth of seven meters, small fluctuations in the water table can have a major impact on the energy flows on the land surface. A comparison with desert landscapes shows what extreme consequences this can have. Temperature fluctuations of more than 30 ° between day and night are only possible without clouds and soil moisture.
- Udo Quentin, Johannes G. Schwerdtle: Drainage in agriculture . 1st edition. DLG Verlag, Frankfurt am Main 2013, ISBN 978-3-7690-2029-8 .
- Example for applying for a lowering of the groundwater in Hamburg
- Website of an environmental association on problems of lignite mining
- Map (PDF; 3.4 MB).
- Rheinische Post from May 12, 2011 page C3 (Viersen) ( Memento from May 14, 2011 in the Internet Archive ).
- Effects of forest conversion on groundwater recharge, rural development projects: Lower Saxony Chamber of Agriculture. Retrieved February 29, 2020 .
- Groundwater as a climate moderator - University of Bonn. Retrieved February 29, 2020 .