The causes for the temperature rise are various, and are directly connected to the continuing structural developments and the existing uses at the earth’s surface. There, the distinction is made between direct and indirect influences on the groundwater temperature (see Fig. 1).
Direct influences on the groundwater temperature includes all heat inputs to the groundwater through the sewage network, district-heat pipes, power lines and such underground structures as auto and metro tunnels, underground garages etc.
- Other such sources are those connected with the use and storage of groundwater heat.
Indirect influences on groundwater temperature processes connected with urbanization connected with the change of in the heat balance in the near-surface atmosphere. According to Gross (1991), the most important factors are:
- The disturbance of the water balance due to a high degree of surface imperviousness
- The change of soil characteristics caused by an aggregation of structures (differences in the near surface heat input and heat capacity)
- Changes in the irradiance balance by changes in the atmospheric composition
- Anthropogenic heat generation (domestic heating, industry and transport).
The above differences cause changes in the heat balance by comparison with the areas surrounding the city. The city heats itself up slowly, stores more heat overall, and passes it on again slowly to the surrounding areas, i.e., it can generally be considered a huge heat storage unit. Over the long term, this process leads to an increase in the annual mean air and soil temperatures (cf. Map 04.02, SenStadt 2001).
The long-term warming of the near surface soil also leads to a heating of the groundwater. Since the temperature affects the physical qualities as well as the chemical and biological nature of the groundwater, a deterioration of groundwater quality and an impairment of the groundwater fauna may result.
One hundred percent of Berlin’s drinking water comes from groundwater, which is extracted almost exclusively from the territory of the State of Berlin. The groundwater also supplies a large share of the water for industrial use. Therefore, the protection of the groundwater from serious change, such as an increase in groundwater temperature, is of great importance — specifically in terms of sustainable water use.
Since 1978, temperature profiles have increasingly been recorded at deep groundwater measurement points distributed throughout the area of the city, and processed and evaluated to develop a chronological and spatial depiction of the groundwater temperature structure.
The purpose of the present map is:
- the extrapolation of the existing documentation on chronological change of the groundwater temperature beneath the municipal area, and
- to serve as a basis for the authorization of measures which could cause changes in the groundwater temperature.
In addition, it can, in combination with other topical maps, such as those of the geological or hydrological structures, be used as an aid for decision making and preliminary planning for energy management of the groundwater. The underground temperature is an important quantum for the installation of geothermal energy probe systems.
In recent years, a sharply growing demand for geothermal energy probes in combination with heat pumps for heating and other thermal uses of the subsoil, e.g. the air conditioning of buildings, has been observable. Especially in urban areas, a wide variety of thermal uses compete in very small areas. To monitor the effects of these uses, regular supervision of the groundwater temperature is of increasing significance.
Groundwater Temperature and Annual Temperature Curve
The significant heat source for the near-surface subsoil to a depth of approx. 20 m is solar irradiance which reaches the earth’s surface. This is substantially responsible for the surface temperature.
The near-surface soil is heated by irradiated solar energy, and passes the heat on to the atmosphere and the subsoil. The annual total of that part of the solar irradiance which impacts onto a horizontal surface, the so-called global irradiance, averages approx. 1000 kWh per sq. m and year in the State of Berlin. How much energy is ultimately passed into the subsoil from the surface is very strongly dependent on nature of the surface. Here, such factors as colour, humidity content and type and degree of ground cover are play important factors.
Basically, the temperatures of the earth’s surface, and hence, too, the heat entry or exit, are subject to periodic fluctuation, during a cycle of one year, corresponding to the progression of the seasons.
The surface temperature penetrates with decreasing intensity into the soil. The penetration depth and the speed with which the heat is transported depend on the heat transfer capacity of the soil.
Subsoil heat transfer can be distinguished as either conductive and convective heat transfer.
While convective heat transfer, the movement of the heat is accomplished by moving material, such as groundwater or seepage water, conductive transfer results from the transmission of energy through the successive impact of molecules.
Compared with solar irradiance, the geothermal flow caused by the decay of radioactive isotopes in the subsoil is of much less importance as a source of surface heating.
In the continental crust of the earth, the geothermal flow density, which is defined as the thermal flow per unit of area perpendicular to the standard area, varies regionally. According to Hurtig & Oelsner (1979), and Honarmand & Völker (1999), the mean thermal flow density is in the State of Berlin is between approx. 80 and 90 mW/sq. m. Based on this, an energy total of approx. 0.75 kWh per sq. m and year can be calculated, which is only approx. 1/1000 of the global irradiance.
The near surface groundwater temperature is therefore essentially determined by the exchange of heat between the sun, the earth’s surface and the atmosphere, with a much lesser degree by the geothermal flow towards the surface.
The regional average annual temperature at the surface in Berlin, given no anthropogenic effects, is approx. 8.0 to 8.5 °C.
While daily fluctuation affect the soil only to a depth of approx. 1.0 m, seasonal fluctuations reach depths of between 15 and a maximum of 25 m. Below this depth, in the so-called neutral zone, where seasonal effects can no longer be ascertained, the temperatures rises depending on the heat transfer capability of the rock and the regional geothermal flow density (Fig. 2).
In the Berlin area, the average temperature increase to a depth of approx. 300 m is 3 °C per 100 m.