Characterizing Aquifers - Hydrogeology: Objectives, Methods, Applications - page 124

the area, is drained by the Grandes Gillardes springs, in a valley cutting into
its heart. This relatively suspended aquifer also contributes to the aquifers
developed below, through contacts generated by the structural complexity
of the area (Barremian, Hauterivian, and Tithonian aquifers).
Groundwater transfers through exchanges between aquifers can occur
over great distances and involve several hydrologic basins.
￿ In the subalpine chains of the Arc of Nice, between Nice and Menton,
there are no fewer than fi fteen or so small Jurassic limestone units
dispersed over a surface on the order of 220 km 2 , in the middle of
poorly permeable Cretaceous deposits. The structural conditions allow
relationships at depth between these karst units, for which the principal
drainage occurs, for a total recharge area of around a hundred km 2 ,
through three submarine springs totalling an estimated discharge of
500 L·s -1 (Pissarelles, Cabbé, and Mortola), and through coastal springs
with a discharge of 100 L·s -1 around the borders of Monaco (Gilli, 1999
& 2003).
￿ To the east of Marseille, the submarine Port Miou spring, which emerges
in the calanques of Cassis, appears to have an average discharge on the
order of 7 m 3 ·s -1 , which, given the regional specifi c infi ltration modulus,
would require the inputs of a basin of over 1000 km 2 . The catchment
area of this major outlet should therefore extend over a good part of
Basse-Provence through various hydraulically interconnected units,
thanks to structural complications (Gilli, 2001) (see chap. A9-2.5).
4 PIEZOMETRY OF AQUIFERS
4.1 Defi nition and methods
Within water-bearing formations, the position of various aquifers can be
described by their piezometric surface, which represents the distribution
of the aquifer's hydraulic charge. In the case of unconfi ned aquifers, this
surface coincides with the surface of the aquifer. In confi ned aquifers, the
piezometric surface can, however, be located above the surface of the aquifer,
which is determined by its impermeable roof.
The piezometric study of an aquifer provides information of great
importance on the characteristics of the aquifer. In particular, it allows a
global understanding of the groundwater fl ow conditions, as well as the
conditions of infl ow and drainage, and the variations in storage.
The collection of piezometric data requires the availability of access
points to the water table, which, in some cases, can consist of preexisting
wells or boreholes in the study area. Frequently, however, private water