Characterizing Aquifers - Hydrogeology: Objectives, Methods, Applications - page 138

point (piezometer, cave), natural exit (spring), or artifi cial outlet (catchment,
gallery, well, borehole), groundwater carries a message indicating its origins
and the source of its mineralization, the global hydrologic balance of its
recharge zone and the different components of the hydrosystem, its residence
time in the reservoir, and its natural or artifi cial sources of contamination.
This message can be interpreted if the identifi able chemical components
present suffi cient contrasts in concentration. Unlike mandatory analyses, the
nature and frequency of which are legally mandated, prospecting analyses
can consider any relevant tracer, whether natural or anthropogenic (see
chap. 11), and any range of time and space.
6.1 Physical and chemical analyses
These analyses, done by various organizations, allow the determination, for
example, of sources of mineralization, the evaluation of global hydrologic
budgets, and the inputs from different components of the hydrosystem. We
will illustrate the value of natural tracers with a few examples.
6.1.1 The role of soil and superfi cial reservoirs. Example of
the Montagne Noire (Guyot, 1983)
Poussarou and Malibert are two karst springs, issuing from Cambrian
limestone and dolomite. Their respective contiguous recharge areas have
the same surface (19 km 2 ) and the same altitude; subjected to the same
climate, they differ only in their soil and vegetation cover.
The catchment basin with a poor soil cover, Poussarou, has a more
abrupt hydrologic response, briefer and more pointed, but the subsequent
falling discharge is less sustained (Figure 74). The chloride content tends to
decrease during the peak. In Malibert, on the other hand, the soil and the
epikarst dampen the peak (delay, attenuation) and sustain the subsequent
decrease; the chlorides present a peak followed by a decrease, before
returning to the level of background noise. During infi ltration episodes, the
soil and superfi cial reservoirs therefore undergo leaching of the solutions
that evaporation had concentrated there.