PROBLEMS - Hydrogeology: Objectives, Methods, Applications - page 330

Question 3: What phenomena can be sources of error in this calculation?

Answer 1:
Considering an average signal from the Durance (average of the
measurements) D = -10.85‰ and from local recharge L = -7.13‰, we can
then consider that the water in the aquifer is a perfect mixture of only these
two poles.
The percentage X of water from the Durance is then given by a fl ux
conservation equation (N: concentration in the aquifer; D: concentration
in the Durance; L: concentration in local aquifers)

N·100 = [D·x] + [L·(100 - X)], or

X = [N - L]/[D - L]
Answer 2:

Table 6 proportions of water originating from the Durance and from irrigation.

Location Type X Durance 100-X Local

Pey FOR aquifer 84% 16%

Puyv P 12 aquifer 60% 40%

Mal F 14 aquifer 29% 71%

Sen P 20 aquifer 77% 23%

Sen F 14 aquifer 34% 66%

Mer P 2 aquifer 44% 56%

Nov aquifer 88% 12%

Gre STP aquifer 45% 55%

Cav TAP aquifer 48% 52%

Answer 3:

Irrigation leads to evaporation conditions that can enrich infi ltrated water,
and therefore causes an underestimation of the infl ow from the Durance.
The simultaneous extraction of well water and surface water is legal
only in the case of highly transmissive aquifers, in which the water in the
aquifer is from the same season as that in the river.
This evaluation is possible only if there are no other components in the
hydrosystem (discharge from deep aquifers).
The calculation is valid only if the aquifer is well-homogenized. In the
case of a poor mix (channeling of fl ow), the calculation will be off.

Problem #6—Aquifer discharge

Figure 6 (after Lacroix, 1991) shows the piezometry of the Lower Durance
aquifer. This representation reveals limits to the incoming fl ux.