PROBLEMS - Hydrogeology: Objectives, Methods, Applications - page 344

occur each day during the daytime. A more careful examination shows
different variations on Sundays and holidays. Anthropogenic origins are
therefore indubitable. These variations, visible only during dry periods, are
therefore tied to the overexploitation system, which, by lowering the karst
aquifer, provokes an intrusion of surface water in the catchment system.
The results of the tracer test show that the eosin, injected into the
effl uent of a defective water treatment plant located on the marl of the
graben upstream of the spring, is restored only a few hours after injection.
The water loaded with effl uent is therefore lost into the Jurassic limestone
and contaminates the spring.
The same remark can be made for the restitution of fl uorescein, which
proves a rapid connection between the ponor E and the spring. The curve
shows two peaks, the second of which is due to the rainfall episode at the
end of February that prompted a fl ushing of the coloring agent.
The absence of naphthionate restitution enables the exclusion of the
eastern belts from the drainage basin.
This spring therefore presents a high vulnerability tied, on the one
hand, to overexploitation, and on the other hand to the developments on
its recharge basin.
Answer 2: It will be necessary to limit the lowering of the water table during
dry periods, or to guarantee an irreproachable surface water quality. The
collective sanitation system (water treatment plant) will then have to be
improved, for example by including oxidation lagoons.
Problem #12—Search for the origin of a saline contamination
(after Blavoux et al., 2004)
The submarine Cassis springs (Bouches-du- Rhône) could supplement the
water resources of the region east of Marseille, but the water is brackish
(see chap. 9).
The origin of the salinity of the brackish Port Miou river has given rise
to two hypotheses:
• the leaching of continental evaporites, whose presence has been
observed in certain geologic formations in the drainage basin
(Oligocene or Muschelkalk);
• a contamination of marine origin.
Contamination by evaporites, acquired at the level of the recharge area
and therefore affecting the entire aquifer, as the concerned geologic series