Characterizing Aquifers - Hydrogeology: Objectives, Methods, Applications - page 152

7.5 Tracer tests
They are essential to karst hydrogeology. A substance, not naturally present
in the environment, is injected at a certain point, and is then looked for
downstream, in the waters of a spring, a river, a chasm, or a well. Depending
on the goals of the test and the substances used, detection is done by using
the naked eye, by taking samples for analysis, by continuous recording
of fl uorescence in the fi eld, or by placing packets of activated charcoal in
the water to capture certain tracers. Given the cost and the diffi culties in
execution of such operations, multitracer tests are common, in which several
different products are simultaneously injected in different locations.
7.5.1 Qualitative approach
This is a binary test. A tracer injected at a certain point emerges or does not
emerge at a spring. This is how the French caver Norbert Casteret proved, in
1931, that the Garonne originated in Spain and disappeared into the Trou du
Toro, only to reemerge at the Goueil de Jouéou before fl owing into France.
A tracer test can be accidental, as was the case in 1901, when, as the
Pernod distillery in Pontarlier caught fi re, all the reserves of alcohol and
liquor were emptied into the Doubs to prevent the entire town from going up
in fl ames. A few days later, the Loue spring was contaminated by absinthe,
proving a link with losses from the Doubs.
7.5.2 Quantitative approach
The restitution of the tracer(s) is examined, based on the discharge of the
spring and the precipitation over the catchment area. The arrival time, the
shape of the breakthrough curve, and the recovery rate provide information
about the aquifer:
￿ rapid and concentrated restitution indicates an evolved degree of
karstifi cation and small reserves. The tracer may have been injected
into a conduit in direct communication with the emergence (Figure
84);
￿ slow, diluted restitution indicates that the tracer passed through a
capacitative area, or that karstifi cation is poorly evolved (Figure 84);
￿ restitution in successive waves correlated with rainy periods indicates
that the tracer remains blocked in the epikarst or in an annex
compartment;
￿ the presence of peaks in the absence of discontinuous rainfalls can
indicate splitting off and compartmentalization of the aquifer.