If you need to know the highest groundwater level for a specific location and the observation period is not long enough at any of its groundwater measuring points, an approximate value can be determined by interpolation from the highest groundwater levels of neighbouring measuring points. Such an interpolated value is also referred to as an HGL.
In many cases, knowing the highest groundwater level that occurred in the past is very useful but not always fully satisfactory or sufficient. For instance, if the HGL is to be used for measurements in waterproofing a building against water under pressure, this value observed in the past must of course also not be exceeded in the future, i.e. within the useful life of the building, and must occur only in extremely wet situations. If the observed history of the groundwater level is essentially determined by natural causes (seasonal differences in new groundwater formation, alternation of years with low and high precipitation), it can be assumed to behave similarly in the future. This also applies in the case of anthropogenic interventions with consequences for the groundwater surface, if these are permanent and will thus not change in the future.
In large parts of Berlin, groundwater conditions have not been natural for a long time. The level of the groundwater surface is subject to artificial influence due to both permanent and temporary interventions in the groundwater balance.
Permanent measures include:
- rainwater sewerage, which may have the effect of reducing new groundwater formation and thus lowering the groundwater level;
- decentralised rainwater disposal in percolation facilities, which may locally raise the groundwater surface, depending on precipitation events;
- drainages and ditches that were intentionally used to lower the groundwater level locally;
- hydrological construction measures (impoundments, shore revetments, straightening of watercourses), which may lead both to a rise and to a drop in the groundwater level;
- structures protruding into the groundwater, with the effect of impounding the groundwater in the direction of inflow and lowering the same in the direction of outflow.
Temporary measures and those that may vary considerably in their impact include:
- groundwater extractions for the public and private water supply and for keeping water out of construction pits or for remediation, which lower the groundwater surface;
- groundwater replenishments for increasing groundwater availability for the public water supply, which raise the groundwater level in the vicinity of the replenishment facilities;
- repercolation of extracted groundwater, e.g. in the context of groundwater preservation measures for construction purposes, which also raise the groundwater surface – usually only locally.
This multitude of potential artificial measures that affect the groundwater illustrates that in some cases it is difficult even for specialists to judge whether and to what extent an observed (= measured) highest groundwater level (HGL) is anthropogenically influenced and whether such a value can also be used for addressing future issues.
In order to further increase the quality of the HGL value and to make it more readily available to the user, a map has been developed that directly specifies the “expected highest groundwater level”, abbreviated “EHGL”. This is defined as follows:
The expected highest groundwater level (EHGL) is the maximum that can arise due to weather effects. It can occur after extremely wet periods if the groundwater level is neither lowered nor raised by artificial interventions in the vicinity.
According to this definition and current knowledge, this groundwater level will not be exceeded after very heavy precipitation events if the following geohydraulic conditions apply: the natural conditions (e.g. water permeability of the subsoil) on the one hand and the permanently artificially modified conditions (e.g. impoundments of the watercourses, see above) on the other hand.
Groundwater levels higher than the EHGL can theoretically occur, but only as a result of further artificial interventions. Of course, such interventions (e.g. discharges into the groundwater) are not predictable in the long term. However, they do not need to be taken into account for most questions, as they require permission or approval from the water authorities.
The definition of the expected highest groundwater level thus essentially corresponds to the definition of the “design groundwater level” (Bemessungsgrundwasserstand) for waterproofing buildings according to the BWK guidelines, Bulletin BWK-M8 (2009; BWK Bund der Ingenieure für Wasserwirtschaft, Abfallwirtschaft und Kulturbau e.V.).
The term “expected highest groundwater level” is used here instead of the term “design groundwater level” because the EHGL map is made available also for other questions besides that of the required waterproofing of buildings.
In this context, it should also be noted that the responsibility of determining design groundwater levels for construction measures generally lies with the constructor or their specialised planner or surveyor. In some cases, individuals are unable to determine these by themselves, or only with great effort, based on groundwater investigations at the construction site or in its immediate vicinity only. Berlin’s groundwater conditions are extremely complex and strongly influenced by humans; therefore, the State of Berlin is sharing groundwater level information with the public.
The Geological Survey working group (“Landesgeologie”) of the Senate Department for the Environment, Urban Mobility, Consumer Protection and Climate Action has been providing information on the groundwater for decades, including the highest groundwater level (HGL), which is determined by specialists on the basis of the available groundwater level data. As the HGL, according to its definition (see above), is not necessarily an uninfluenced groundwater level, the aim is to develop a city-wide map of the EHGL, which is more conclusive regarding future matters (e.g. waterproofing of buildings). Accessing the map through the Internet allows the user to obtain the EHGL for their site of interest. Thus, waiting periods previously caused by written inquiries can be avoided.
The EHGL map is available for four areas of Berlin (see Figure 2).
- In geological terms, these are the area of the Berlin glacial valley and the area of the Panke valley. Both are characterised by subsoil that is predominantly composed of sands with high water conductivity near the surface and their groundwater surface that is generally located just below ground (depths to groundwater of a few metres, sometimes even less than one metre) (SenStadtUm).
- Furthermore, the EHGL map was developed for the areas connecting to the glacial valley in the South, i.e. the Teltow Plateau and the Nauen Plate west of the Havel. In the eastern part, the plateau is covered by relatively thick boulder marl or boulder clay of the ground moraine, which are also partly responsible for confined groundwater conditions. The western part is predominantly characterised by thick sand sequences. Boulder marl and meltwater sands are equally represented in the Nauen Plate area. South of the glacial valley, the area is characterised by groundwater surface levels found at depths that are usually much greater than 10 m, in some parts of the Grunewald and on the Wannsee peninsula even greater than 20 m. By contrast, surface waters, such as the Havel and the Grunewald lakes, but also the Rudower Fließ area, the southern part of Lichtenrade and the former Karolinenhöhe sewage farms feature low
depths to groundwater.
- The section of the Barnim Plateau north of the glacial valley and south-east of the Panke valley was added to the EHGL map as part of this update. In this area, the extensive boulder clay formations of the ground moraines of the Weichselian and Saalian Glaciations, which are often interspersed with meltwater sands, mainly determine the hydrogeological conditions. The aquifer in this area is generally covered and largely confined. It is also artesian in some areas. Its hydraulic gradient is comparatively high. The depth to groundwater can measure up to several tens of metres. As cover sands are often deposited on top of ground moraine sediments, stratum water commonly occurs.
A map on groundwater levels titled “Expected highest groundwater level (EHGL)” is published here covering each area. The methodological approach was not the same across all areas, however.