Karst Aquifers - Hydrogeology: Objectives, Methods, Applications - page 59

4.1.3 Hydrogeology
a) Hydrometry
Discharge measurements on the spring over long time periods have allowed
the precise quantifi cation of the amount of water fl owing out. The discharge
ranges between 4 and 100 m 3 ·s -1 . Analysis of the fl ood curves shows rapid
reactions to precipitation, which indicates a well-evolved and very much
hierarchised karst. The slow recessions show the importance of the reserves
(Mangin, 1975). Tracer tests on the Monts du Vaucluse and hydrogeologic
budgets now allow the clear delineation of the fountain's watershed, which
has a surface area of approximately 1200 km 2 (Puig, 1987; Mudry and Puig,
1991). The Apt borehole, 605 m deep, found the same Urgonian limestone,
poorly connected to the Vaucluse aquifer. The study of temporary springs
and of a few drilling projects near the edges of the Montagne de Lure show
that the Aix fault is the Eastern limit of the aquifer (Rousset, 1997).
b) Hydrochemistry
The study of intrinsic or dissolved trace indicators in the water (carbonates,
magnesium, chlorides, oxygen-18) indicates the contributions of several
different sources:
• very rapid infi ltrated water (residence time: 2 days), with a high
concentration of soil-derived tracers and a very low discharge,
originating in the plateau's well-developed epikarst (sinks, karrenfi elds)
(Blavoux & Mudry, 1986);
• rapid infi ltrated water (residence time: a few days), responsible for
mighty fl oods and for overfl ow of the spring, as well as for its decreased
mineralization (Bakalowicz, 1979);
• water with a residence time of several weeks in the unsaturated zone
(slow infi ltration with chemical markers from gas phase in the soil),
which can be remobilised by subsequent infi ltration events (Lastennet
& Mudry, 1997; Emblanch, 1997);
• water with a residence time of several years in the open voids of the
saturated zone, which contribute to sustaining fl ow during dry periods
(Evin et al. , 1967; Michelot & Mudry, 1985).
4.2 Origins of the system
Two hypotheses are generally put forward to explain the existence of a
vertical conduit extending below sea level at Vaucluse: