Tracer (Earth Sciences)
A tracer is a substance that can be detected in very low concentrations. With their help, processes in geological and environmental process research can be tracked and quantified. For this purpose, they are introduced into the process to be examined (e.g. water cycle ). So-called environmental tracers are already in the system under investigation, for example in the form of different isotope concentrations or temperatures.
Tracer methods are used in hydrology , meteorology and hydrogeology, among others .
Hydrological tracers
In hydrology (water science) and hydrogeology, tracers are used to investigate the flow of surface water and groundwater . The aim is to obtain information about the origin of the water, its flow paths and forms of movement, as well as the properties of the affected aquifer or body of water. A distinction is made between environmental tracers and artificial tracers . Environmental tracers are already present in the water, artificial tracers are added to the water for analysis.
Environmental tracer
The environmental tracers include:
- Natural isotopes (stable or radioactive )
- Chemical compounds ( ionic or non-ionic)
- Civilizing substances (chemical compounds or radioactive substances)
Artificial tracers
Artificial tracers include:
- Particle tracers: spores , bacteria , phages , fluorescent microparticles
- Chemical tracers: salts (anions and cations), fluorescent tracers, food colors
- Radioactive tracers: In the form of anions or cations
Fluorescence tracer
The fluorescence tracers have achieved particular importance . For example, the green glowing sodium fluorescein ( uranine ) can be detected down to the smallest concentrations of 0.001 milligrams per cubic meter of water.
Sodium fluorescein is hardly adsorbed in the subsurface and thus spreads like water and is therefore preferred for investigating groundwater currents. Other fluorescence tracers such as eosin and, with restrictions, sodium naphthionate have similarly good propagation properties, but the detection limits are worse than for fluorescein . In principle, a large number of organic substances fluoresce. The disadvantage of most substances, however, is their poor adsorption behavior and often their price, which is far too high for field applications .
The fluorescent dyes are analyzed with a fluorescence spectrophotometer . The sample is excited to glow with light of a suitable wavelength . This fluorescent glow takes place at a higher wavelength. For sodium fluorescein, the optimal excitation is 491 nanometers, the sample then fluoresces at 516 nanometers. With a fluorescence spectrophotometer , extremely weak fluorescences up to ten thousand times below the visibility limit can still be detected. Thanks to the low detection limits of fluorescence tracers, one can usually get by with tracer masses of a few grams to a few kilograms for groundwater investigations. Under the same conditions , up to several tons would have to be entered when using salts such as table salt .
Salts and non-fluorescent food colors are therefore only rarely used for groundwater investigations. However, are sodium chloride, potassium bromide and food dye Brilliant Blue in the laboratory and laboratory-like with very small experimental areas used in the field often.
Use of tracers
The following questions are dealt with in tracer technology:
- Where does the groundwater flow to?
- Where does the groundwater come from?
- Is there a connection between two points?
- What is the course of the water between two points?
- How much groundwater moves between two points?
Artificial tracers are used to measure runoff in surface waters . The greater the dilution of a tracer added to a river, the greater the runoff. The discharge can thus be calculated directly from the concentrations measured below the input point. This so-called tracer dilution method is especially suitable for turbulent waters. The preferred tracer here is table salt, as this can be measured in the simplest way on the spot by measuring the electrolytic conductivity. In addition, it is absolutely harmless to the environment in the quantities usually entered. With outflows of more than a few cubic meters per second, however, the required mass of common salt becomes too large and therefore sodium fluorescein is usually used. It only takes a few grams of this tracer to drain one cubic meter per second. Analysis is carried out on the spot with a small "pocket fluorimeter" or with a fiber optic fluorimeter. The latter is the more expensive solution.
In the simplest case, the spread of a tracer in the groundwater takes place according to the laws of hydromechanical dispersion . The "tracer cloud" expands more and more in the course of its flow. This spread is therefore dependent on time, but also on the properties of the aquifer . With the so-called dispersion model, the flow velocities and the aquifer properties can be calculated. If a tracer is partially adsorbed or if chemical processes act on the tracer while it is flowing, the evaluation becomes more complex. The evaluation can also be difficult in the case of a heterogeneous aquifer with changing properties along the flow path. The tracer can also spread in running waters in the above-mentioned dispersion model, although the causes of the spread are somewhat different here than in groundwater.
Tracers can also be used to study sea or even ocean currents . But the effort is generally great.
literature
- Werner Käß : Geohydrological marking technology. (= Textbook of Hydrogeology. Volume 9). Borntraeger brothers, Berlin / Stuttgart 1992, ISBN 3-443-01013-X .
Individual evidence
- ↑ Werner Käß : Geohydrological marking technology. (= Textbook of Hydrogeology. Volume 9). 1992, ISBN 3-443-01013-X , p. 13.
- ↑ Werner Käß : Geohydrological marking technology. (= Textbook of Hydrogeology. Volume 9). 1992, ISBN 3-443-01013-X , p. 15.
- ↑ Werner Käß : Geohydrological marking technology. (= Textbook of Hydrogeology. Volume 9). 1992, ISBN 3-443-01013-X , p. 16.
- ↑ Werner Käß : Geohydrological marking technology. (= Textbook of Hydrogeology. Volume 9). 1992, ISBN 3-443-01013-X , p. 14.