Flow tracer

As a tracer (of English: . To trace = trace, track ) refers to substances that can be detected even in very low concentrations. With their help, processes in the natural and environmental sciences research process to track and quantify. To this end, they are introduced to the process under investigation (eg water cycle ). So-called environmental tracers are already in the system studied, as for example in the form of different isotopic concentrations or temperatures.

Are used tracer methods, among others, in hydrology (tracer method ), in meteorology and in hydrogeology.

Hydrological tracer

In hydrology ( hydrology ) tracers ( markers ) to study the flow of surface water and groundwater used. 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 an investigation.

Environmental tracer

Among the environmental tracers include:

• Natural isotopes (stable or radioactive)
• Chemical compounds (ionic or non- ionic)
• Civilizational substances ( chemical compounds or radioactive materials)

Artificial tracer

The artificial tracers include:

• Particle Tracer: spores, bacteria, phages, fluorescent microparticles
• Chemical Tracer: salts (anions and cations ), fluorescent tracers, food dyes
• Radioactive tracer: in the form of anions or cations

Fluorescent tracer

Special importance are the fluorescent tracer. For example, the green glowing sodium fluorescein ( uranine ) to very low concentrations by 0.001 milligrams per cubic meter of water is detectable. Sodium fluorescein is hardly adsorbed in the subsoil, thus spreading like the water and is therefore preferred for the study of groundwater flow. Other fluorescent tracers such as eosin and with restrictions sodium naphthionate have similarly good propagation characteristics, the detection limits, however, are worse than the fluorescein. In principle a multiplicity of organic fluorescent materials. A disadvantage of most fabrics but their bad Adsoprtionsverhalten and often also for field applications much too high price.

It analyzes the fluorescent dyes with a fluorescence spectrophotometer. The sample is excited with light of a suitable wavelength to emit light. This fluorescent luminaires are at a higher wavelength. For sodium fluorescein, the optimal excitation of 491 nm, the sample fluoresces then at 516 nanometers. With a fluorescence spectrophotometer, even extremely weak fluorescence up to ten thousand times the evidence under the limit of visibility yet. Thanks to the low detection limits of fluorescent tracers to get in groundwater investigations usually with tracer mass from a few grams to several kilograms from. Under the same circumstances would the use of salts, such as sodium chloride, can be entered up to several tons.

Salts and non-fluorescent food dyes are therefore rarely used for groundwater investigations. In contrast, common salt, potassium bromide and the food dye Brilliant Blue is often used in the laboratory and at very small laboratory-like test areas in the field.

Application of tracers

In surface water artificial tracers for discharge measurements are used. The greater the dilution of the tracer added to a flow, the greater the discharge. From the measured concentrations below the entry point can thus be directly calculate the outflow. This so-called tracer dilution method is especially suitable for turbulent waters. Preferred tracer is here saline, since this in place by the measurement of electrolytic conductivity in the simplest way can be measured. Moreover, it is absolutely harmless in the amounts normally entered for the environment. Wherein drains of more than a few cubic meters per second, but the mass of saline needed is too large and is therefore mostly used sodium fluorescein. For a discharge of one cubic meter per second one needs only a few grams of this tracer. Is analyzed on the spot with a small " Pocketfluorimeter " or with a Lichtleiterfluorimeter. Named second is the more expensive solution.

The propagation of the tracer in the groundwater is carried out in the simplest case, the laws of the hydro-mechanical dispersion. Here, the " tracer cloud" expands in the course of its flowing from more and more. This propagation is thus dependent on time, but also on the characteristics of the aquifer. With the so-called dispersion model, the flow rates and the aquifer can be calculated. If a tracer adsorbed partially or act during flow chemical processes on the tracer, then the analysis becomes more complex. Even in a heterogeneous aquifer with changing to the flow path characteristics, the evaluation can be difficult. Even in rivers the spread of the tracer in the mentioned dispersion model can be done, although here the causes of the spread are slightly different than in the groundwater.

Tracers can also do ocean currents are used for the investigation of marine or. But It is generally large the effort.

• Hydrology