What is the temperature of the groundwater
Bavarian State Office for
Aim of the temperature measurements
The temperature measurements serve primarily to expand the hydrogeological fundamentals and are intended to provide information on temperature anomalies, the natural temperature stratification of the groundwater and anthropogenic changes in the groundwater temperatures. With temperature profile measurements it is also possible to determine in a simple way vertical flows in groundwater measuring points originating from floor connections.
Since 1990 Bayer. Water management administration at approx. 350 groundwater measuring points in quarterly rotation, depth profile measurements of the groundwater temperature in the observation tube.
The measurements were made according to the specifications of the Bayer. State Office for Water Management (today: Bayer. State Office for the Environment) by the Bayer. Water management offices carried out. The results are summarized in a report.
Basics of temperature depth profile measurement
The heat balance in the subsurface results from the individual proportions of the heat balance variables, namely the natural influences (such as global warming from the sun, heat input from percolating precipitation, rising geothermally heated water) and the anthropogenic influences (e.g. heat input from pipes and buildings) .
As the measurement depth increases, the temperature curves usually show a phase shift compared to the seasonal course of the air temperature.
Figure 1 shows this phenomenon using the example of a measuring point in the Quaternary river valley fillings of the Isar. It can be clearly seen that the course of the temperature curve on the groundwater surface is subject to considerably greater fluctuations and, in contrast, the amplitude of the temperature fluctuations becomes significantly flatter with increasing depth. Among other things, this is relevant for the temperature stability of measuring devices, for example in the case of temperature-sensitive pressure probes.
Result of the temperature-depth profile measurements at the groundwater measuring point Englischer Garten-E2, No. 16146 with an average corridor distance of 3.6 m and a distance of approx. 800 m from the banks of the Isar and approx. 200 m from the Eisbach
The heat transfer underground takes place almost exclusively via the heat transfer of the flowing water.
The technical literature states that the temperature distribution in the subsurface is:
- The temperature fluctuations in the groundwater near the earth's surface are relatively high due to the seasonal changes in heat exchange.
- At a depth of 20 m, an annual temperature change can hardly be determined.
- The groundwater temperature in the subsurface increases with depth (in relation to the surface of the earth) by about 3 ° C per 100 m.
These three facts are illustrated schematically in Figure 2.
Since the heat has a tracer-like property and the water temperature is the easiest of all parameters to measure, in addition to the geophysical conditions mentioned above, the following statements can be made with the help of the temperature distribution:
Temperature profiles in the subsurface at different times of the year
- The temperature measurements enable evidence of local groundwater flow processes, for example the rise of warm groundwater from the depths.
- In the vicinity of construction work, possible effects on the groundwater can be determined with temperature measurements (for example in the case of thermal power plants).
- For the functional control of groundwater measuring points, conclusions can be drawn from the temperature distribution, for example whether there are leaks in the full pipe.
When evaluating the temperature measurement data obtained, it will be determined to what extent the relatively extensive measurement results in Bavaria support these statements, or whether new findings can be derived if necessary.
Until 1990, the groundwater temperature was primarily measured directly below the water level. The measurement results were difficult to compare and were also influenced by the temperature exchange with the air temperature in the observation tube.
In 1987, the state working group on water implemented the findings from geophysical research in the "Guideline for the observation and evaluation of groundwater temperature". The guideline primarily provides for the measurement of the groundwater temperature at defined depth measuring points (see Fig. 3), namely at each measuring point from the water level to the bottom or to the lower edge of the installed filter pipes.
The depth measuring points are precisely specified in the guideline and are defined from the top of the terrain (see Fig. 3). Defining these specific depth measurement points has the advantage that the measurement results at the various groundwater measurement points can be compared with one another.
The groundwater guideline 2/87 recommends the following graduation of the measuring points:
- up to 10m below the ground, distance 1m
- up to 40m below the ground, distance 2m
- over 40m below ground distance 10m
Schematic sketch for the arrangement of the temperature measuring points in the subsurface
In the measurement program for Bayer. Water management offices were planned:
- There are four measuring dates per year per measuring point, namely in February, May, August and November. This means that temperature changes can be determined with sufficient accuracy (see Fig. 1).
- Approx. 350 measuring points were selected that have been observed for more than 30 years or are deeper than 100 m.
- The measurement results were entered in special measurement lists in which the depth measurement points had already been converted based on the relevant measurement point height.
The results of the temperature measurements and the most important technical statements are from materials no. 103 from Bayer. State Office for Water Management (today Bavarian State Office for the Environment), edition Nov. 2001, summarized.
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