Examples of the Search for Drinking Water - Hydrogeology: Objectives, Methods, Applications - page 231

From a hydrogeologic point of view, the Jurassic aquifer is limited by
continuous impermeable formations: Upper Triassic clays at its base and
Cenomanian marls at its roof.
The intensity of the brittle and ductile deformation, however, enables
complex geometric relationships (and therefore undeniable exchanges) with
the surrounding anticlinal slices, with the alluvial valley fi ll, and with the
Turonian marl-limestone in the overlapping Cretaceous synclines.
The different karst units provide preferential drainages following fold
axes, and suffer signifi cant transversal leakage, particularly during high-
water periods, thanks to transfers between stair-step slices, from north
to south (Emily, 2000). Two outlets drain this aquifer, one, the Ste Thècle
spring, along the Paillon, at an elevation of 150 m a.s.l., and the other, the
Pissarelles spring, which is partially submerged by the sea, at an elevation
of 0 m. With an average annual discharge on the order of 130 L·s -1 , these
emergences indicate a distinct restitution defi cit, estimated to be around
250 L·s -1 , based on a catchment basin of 42 km 2 and a specifi c infi ltration
module of 8 to 10 L·s -1 ·km -2 .
The search focused on the confl uence of the two Paillons, on the Sagna
site, established at an elevation of 100 m a.s.l., where the karst aquifer
becomes confi ned under its cap of Cenomanian marl, while infi ltration
occurs laterally, on outcropping areas where the aquifer is unconfi ned,
between the elevations of 300 and 1,000 m. This area corresponds to a
magnifi cent hydraulic trap (cross-section 1 of Figure 120), and the water
present was revealed to be highly artesian, with a pressure head varying
from 2 to 5 bars depending on the season (aquifer between the elevations
of 120 and 140 m a.s.l.).
A prolonged emptying test was undertaken in 1994, using the aquifer's
artesian properties, and resulted in the extraction of 900,000 m 3 of water in
a little over 5 months (Figure 121).
This test enabled an estimate of the following characteristics for the
part of the aquifer hydraulically connected to the well:
￿ a specifi c volume Vs of 45,000 m 3 ·m -1 ;
￿ a permanent reserve of 2.7 million m 3 up until an elevation of 100 m
a.s.l. (corresponding t to the ground level on-site) and of 9 million m 3
up until an elevation of 30 m NGF a.s.l.;
￿ a renewable reserve of 1.6 million m 3 .
The results obtained went above the initial goals, and enabled the
discovery of a sizeable, well-protected resource.
Another well was drilled later on in the same area, in order to supplement
Drap's drinking water supply, and two others are being considered in order
to back up the water supply for nearby towns.