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infi ltrated water) and an increase in sulfate concentration (marker for
the Triassic formation bordering the graben) (see chap. B3-6.1.4).
These observations nicely illustrate the mechanisms of the reservoir
being drawn upon, which is composed of two distinct parts (Figure 118 ):
on the one hand, the principal graben reservoir, which is highly
transmissive and makes up the drainage axis of the aquifer system;
on the other hand, the western slope, which, due to its increased extent
and its elevated position, makes up an annex reservoir primarily
serving as a storage site, but also highly regulating the subterranean
infl ows to the drainage axis.
The reaction limits of the annex reservoirs are revealed when the
graben's karst conduits are emptying the reservoir; which results in a
marked increase in drawdown, when the water taken from the graben
exceeds the compensating input from the annex reservoirs and from
the surrounding Triassic units (which enclose gypsum lenses), with an
accompanying increase in sulfate concentrations (Guglielmi
et al.
, 1997;
Reynaud
et al.
, 1999).
The characteristics of the site therefore lead to a perfectly understood
water resource, the exploitation of which must imperatively be regulated
by the quantitative and qualitative limits revealed.
2 EXPLOITATION OF A CONFINED AQUIFER (PAILLON
BASIN, ALPES-MARITIMES)
The SILCEN ( Association of municipalities of the counties of Levens,
Contes, l'Escarène and Nice) regroups some twenty towns, for which
potable water is taken directly from the Vésubie River and directed
through a 30 km long tunnel, which then feeds into three secondary
channels 25 km long.
Complementary inputs come from the Var alluvial aquifer, but the
Paillon sector, located at the eastern extremity of the service network,
needed an additional supply on the order of 100 L·s
-1
.
The studies done in 1989 led to the search for such a resource in the
Jurassic karst aquifer, which offers large infi ltration surfaces in the
Paillon basin, and for which the budget appeared to have a large
defi cit.
The Jurassic ranges in the area form an arc in the periphery of the Contes
syncline, and are organized in anticlinal folds, tilted over towards the south
and thrusting over narrow, pinched Cretaceous synclines until they reach
the coast, where they are submerged beneath the sea. In the Paillon valley,
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