Characterizing Aquifers - Hydrogeology: Objectives, Methods, Applications - page 118

Table 10 Examples of Water Budgets for the Aquifers of Provence.

System Average

annual
discharge
L·s -1

Geologic units Geologic

basin
in km 2

Modulus in
L·s -1 ·km -2

Calculated
basin
in km 2

Budget over
the known
basin

Vaucluse 19,000 Lure Mountain,

Mt. Ventoux and
Northern Lubéron

1200 15 1,260 correct

Port Miou 7,000 Beausset and

Sainte Baume

300 10 700 too small

basin

Argens 800 East Ste Victoire

and Bois de
Pourrières

165 12 65 too large

basin

Agnis 450 Agnis 50 10 45 equilibrium

2 RESURGENCE OF WATER

The resurgence of water infi ltrated into a rock unit is also highly variable,
and can correspond to a number of situations determined by the particular
characteristics of the environment.

2.1 Nature and location of springs

The positioning of an aquifer's outlets is determined by the relative
geometries of the aquifer, the neighboring impermeable layers, and the
surface topography.
In most cases, emergences will be located at the contact between the
water-bearning formation and the underlying and adjoining impermeable
layers, at the lowest possible elevation (Figure 63). These are discharge
springs draining an unconfined aquifer. They can be located at the
bottom of a hillside or mid-slope, depending on the local geologic context
(Figure 64a).
When the basal impermeable units remain below the ground and do
not outcrop at the surface, the aquifer is drained at the points which are
geographically the lowest ones, generally along the hydrographic network,
which forms the deepest incisions. These are called depression springs, in
the unconfi ned water table developed in bordering units (Figure 64b).
Occasionally, springs will be located at the roof of the aquifer, at its
lowest contact with the overlying impermeable unit. These are overfl ow
springs for unconfi ned aquifers, which become confi ned immediately
downslope when they pass under the impermeable cover (Figure 64c).