school energy&environment

Tracers and dyes

At times, the fact that a cave and a spring belong to the same system can be perceived immediately, especially in the case of the so-called hydrogeological tunnels, where the course of the underground waters can be followed physically by cavers from the sinkholes to the spring. Other times, on the contrary, the relationship between caves and karst springs is not obvious; it may happen that the closest springs, that rationally seem to be the most likely to be connected to a karst system, do not in fact belong to it. One must not forget that, while on the surface topographic morphology makes the identification of the divide between different hydrographic basins quite easy, underground, the dependence of karst systems on the geological structure can create dividers that are difficult to identify externally unless the geology of the area is well-known. The easiest and safest method to establish the connection between caves and springs is water tracing. The technique is very simple: a tracer, mostly a dye, is put in the water, in any part of the system, either at the entrance or at a deep internal point, and then its presence is verified at the spring. Finding the tracers at the spring is an unmistakable proof of the connection between the input point and the check point. Moreover: the analysis of the time the tracer takes to reach and the dilution it has undergone when correlated to the discharge at the spring and to a chemical analysis of the water, enable the extrapolation of important information regarding the karst aquifer, its water reserves, the water flow rate and also the depth of the saturated zone and the presence of big underground drainage conduits. The most commonly used tracers are dyes such as Fluorescein (that gives a green colouring) or optical bleachers such as Tinopal (the substance that makes our washing ‘whiter than white’). These substances not only have very low toxicity, even on the most delicate organisms, but have the advantage of being identified with simple methods even at low concentrations invisible to the naked eye, which makes it possible to use only modest quantities. In the past, different substances were used, among which some really curious ones that are now part of the literature on this subject, such as the legendary eels used to ‘trace’ the waters of the Timavo river, rather than straw, sawdust, spores, radioactive elements and kitchen salt. At times the colouring has been totally involuntary as in the case of the overturning of a road tanker full of Pernod in the South of France that brought about the discovery of the connection between a small stream on the side of the road and an important karst system close by, to the joy of the cavers present in the cave when the ‘tracer’ flowed past… However simple in theory, tracing operations need, as a matter of fact, a series of precautions so as to avoid pollution and incorrect results and they must be carried out by specialists…to avoid accidents that can be tragicomical at times. Some examples are the big green patch that mysteriously appeared in front of Nesso, on Lake Como, in the Eighties or the tens of km2 of fluorescent rice fields in the Philippines caused by an Italian expedition, whose members were subsequently forced to drink the water to prove to the infuriated inhabitants, lead by some old beheaders, that the substance was not toxic…the side effects connected to a glass of water from a rice field are certainly greater than the toxic effect of the fluorescein used…