Tracing tests

Tracing test represents an indirect method of hydrogeological research in karst, which helps to determine the groundwater flow direction and velocity, the location of hydrogeological water divide, and thereby to evaluate the genesis of karst aquifer (Käss, 1998; Goldscheider & Drew, 2007). Marking process can be performed on surface, when the ponor-spring connection is to be determined, or underground, when tracers are used to determine the connection between specific speleological objects.

• Photo on the left side: Marking of Nekudovo ponor (photo by I. Jemcov) (In: Milanović S., 2012)
• Photo on the right side: Marking of waters which leak below Višegrad dam (Milanović S., 2009)

Tracing test is performed with the aid of tracers (markers) which are inserted in the water flow (or ponor) in order to determine underground water distribution in space and time. As tracers, some soluble materials (dye, radioactive isotopes, inorganic salts) or insoluble materials (bran, sawdust, chaff, bacteria) can be used, while lately, some noble gasses and bromides have also been introduced (White, 2007).

In the choice of tracers, it is necessary to bear in mind the concentration sensibility, i.e. the lowest tracer concentration which can be asserted for sure. Furthermore, the quantity of the tracer is the parameter which plays an important role in the tracer choice. Tracer quantity depends of various factors, some of which can be hard to detect. Hence, parameters that are easier to detect can be used for determining approximately the quantity of tracer dye: the capacity of swallow hole, springflow, the distance between springs and possibly the concentration sensibility of the tracer. Also, particular care should be taken to ensure that the tracer is easily detectable when in small amounts, that it does not change the smell and the colour of water, nor the chemical characteristic of water. In addition, tracer must not be harmful for persons who handle it and it also must be easy to find on the market (Milanović P., 1997, Ford & Williams, 2007). Of course, one of the most important factors is the price of the tracer, because some tests may require dozens kilos of it. One of the most used tracers is sodium fluorescein.

The process of marking of karstic water flows starts with preparation of the ponor for dyeing and determining the place where tracer is expected, after which the tracer is injected into the ponor. After that, a way of sampling is organized depending of the ponor-spring distance. When it comes to sampling, it should be noted that any member of the team which injects the tracer, must not be present at observation points, because the tracer can remain on clothes, hair or hands while the flow is marked. 

- The first “scientific” tracing test was carried out on a part of Danube river which flows through karstic field of Schwarzwald, between Immendingen & Hettingen (Germany). People in this area have always believed that the “lost“ waters of the Danube river were directed to the strongest spring in Germany, Aach spring. This fact was proved already in 1877 by professor Knopp from the University of Karlsruhe, who performed a tracing test of groundwater flow with his team, at first with 10 tons of salt, and later, for the first time in hydrogeology, he performed tracing test with today widely used uranine (sodiume fluorescein) (Stevanović, 2013).

- In Carpatho-Balkanides of Eastern Serbia, to date, more than 30 tracing tests have been performed. Stevanović & Dragišić give synthetic review of tracing tests in Carpatho-Balkanides of Serbia, and ascertain relatively high velocities of aquifer flow, whose virtual velocity varies between 0.0045 and 0.222 m/s, with an average value of 0.063 m/s. Also, the existence of multidirectional circulation within a complex hydraulical karstic systems and redistribution of water of neighboring watershed phenomenon is noted. Aquifer flow velocities are significantly higher at high-level waters than at low-level waters period.

- Milanović P. (1981) remarks that using more than 100 tracing tests the main directions of circulation in Dinarides karst were determined. He notes a wide range of determined flow velocities which vary from 0.002 to 55.2 cm/s. Some of the results indicate that the flow velocities at a low-level water period might be two to five times slower than at a high-level waters period.

• Sketch: Determined directions of karstic aquifer waters circulation in karst of Southern Kučaj mt (Eastern Serbia) (In: Stevanović & Dragišić, 1992)

 

• Sketch above: Determined flow directions of karstic aquifer water in Eastern Herzegovina (Milanović P., 1981)
• Histogram below: Distribution of aquifer waters flow velocities based on performed tracing tests in Dinarides karst (from Milanović P., 1981)