Characterizing Aquifers - Hydrogeology: Objectives, Methods, Applications - page 155

Figure 85 Fluorescence measurements characteristic of naphtionate restitution.
The values preceding the april 8th peak are due to natural fl uorescence.
When fl uororeceptors are used, the activated charcoal they contain
offers a large specifi c surface and, when left for a continuous period of time
in running water, they adsorb the tracers that are present. The receptors
therefore function as integrators of the fl ux of tracer that reappeared, and
allow a semi-quantitative evaluation. The concentrations measured in
the eluates are generally 10 times higher than in water; the sensitivity of
the measurement is therefore improved. Unfortunately, the receptor also
accumulates natural background fl uorescence, making the information
more diffi cult to use. Fluorescence monitoring equipment now enables
continuous measurement of the passage of tracers, but requires qualitative
confi rmation.
7.6 Prolonged emptying tests
This method, developed in Algeria by Collignon (1986 and 1988), enables
the rapid, simple, and precise evaluation of the water reserves stored in
karst aquifers.
It relies on prolonged pumping, accompanied by rigorous monitoring
of the extracted discharge and of the piezometric surface in the pumping
well. It also must be done during a period without any infl uence from
precipitation, and requires a very signifi cant volume of extracted water
(a few hundreds of thousands or a few million m 3 ).
Based on a large number of tests, Collignon demonstrated that,
during high-discharge pumping in a well relying on a karst aquifer, the
curve showing the evolution of the piezometric level as a function of the
extracted discharge was reproducible, and marked by three distinct phases
(Figure 86):