Island and Coastal Aquifers - Hydrogeology: Objectives, Methods, Applications - page 66

continents lowers the global sea level. During the Würm, the last
glaciation, sea level dropped approximately 120 m from present-day
levels. Rises and drops in sea level occur in successive phases, with
stages whose existence is attested to by the presence of coastlines
and notches (Collina-Girard, 1996). The post-glacial rise began 18,000
years ago, with several arrests. The last pause lasted approximately
2,000 years, as indicated by the relative homogeneity in the elevation
of ancient ports in zones of low tectonic activity;
• fi nally, tectonic activity can sometimes isolate portions of the sea for
which infl ows from precipitation and rivers are less than losses to
evaporation, which causes a drop in their level. The Dead Sea is now
located at an elevation of 450 m b.s.l. Scenarios of this type are sometimes
proposed to explain the paleoreliefs and the evaporite deposits at the
bottom of the Mediterranean and in the Gulf of Mexico.
These great variations in base level of course had consequences on
the functioning of surface and subterranean aquatic environments, but
also, especially, on the spatial organisation of aquifers, for example, by
defi ning successive phases in the downcutting of valleys and their infi lling
by alluvium, or by allowing the emplacement of deep karst systems. The
Rhone valley, a typical example, was highly incised during the Messinian,
to more than 600 m deep in Arles, and was then fl ooded by the Pliocene
sea up to Lyon.
Hydrogeology in coastal regions cannot do without the study of these
variations. Hydrogeologic studies done during the construction of the
Toulon tunnel demonstrated that paleocirculations of groundwater had
existed between the Jurassic limestones of the Mont Faron to the north and
the coastal and submarine karst of the Muschelkalk in the bay of Toulon to
the south, through accumulations of highly soluble Keuper gypsum, when
sea level was lower (see chap. D2-3.3.2). In such a context, dewatering
pumping below sea level, over several years, can reactivate circulation,
causing, on the one hand, the leaching of the clay fi lling cavities, but also
the intrusion of sea water, which, by mixing with water in the karst aquifer,
allow gypsum dissolution due to common ion effects.
Currently, global warming, whose natural or anthropogenic origin
remains a subject of debate, has resulted in a noticeable rise in sea level,
of several millimeters over the last century. The predicted rise varies
depending on the scenario; it could accelerate and reach 1 meter in the next
century, which, aside from the problems facing the residents of the resulting
fl ooded areas, would have effects on the use of coastal aquifers.