PROBLEMS - Hydrogeology: Objectives, Methods, Applications - page 329

Answer 2: Between Pontarlier and Houtaud, the aquifer diverges; it is
supplied from the south. Between Houtaud and Vuillecin, the Drugeon
drains the aquifer. The Pont Rouge gravel quarry drains the downstream
portion of the aquifer.

Answer 3:
• headward (south of Houtaud), the gradient is: 2/1250 = 0.0016, or 1.6
m·km -1 or 1.6‰;
• in the center (south of Dammartin): 2/750 = 0.0027 or 2.7 m·km -1 or
2.7‰;

• to the north (east of Vuillecin): 2/1350 = 0.0015 or 1.5 m·km -1 or 1.5‰.

Problem #5—Mixing in an alluvial aquifer (Durance, from
Lacroix, 1991)

As in any alluvial aquifer, the shallow Lower Durance aquifer is fed both
by the river and by the hillside aquifers. From an isotopic point of view,
the river is characterized by its low oxygen-18 concentrations, tied to the
middle alpine zone elevation of the drainage basin. This negatively marked
water arrives in the aquifer, either at the level of the Durance minor bed,
or below plots irrigated with river water. The local rainfall (on the plain
and the neighboring hills) is more enriched in oxygen-18 (Mediterranean
climate and low elevation).
Various sampling locations provided the following oxygen-18 values
over the course of a summertime survey (Table 5):

Table 5 Oxygen-18 tracing in surface water and groundwater.

Location Type delta Location Type delta

Sénas Durance -10.90 Pey FOR aquifer -10.25

Orgon Durance -10.80 Puyv P 12 aquifer -9.35

Jou P 111 hillside -8.00 Mal F 14 aquifer -8.20

Mey P 101 hillside -6.95 Sen P 20 aquifer -10.00

Repa SE 4 hillside -7.50 Sen F 14 aquifer -8.40

Mal FE hillside -7.15 Mer P 2 aquifer -8.75

Mer F0 hillside -6.95 Nov aquifer -10.40

Che ROQ hillside -6.35 Gre STP aquifer -8.75

Pla JF hillside -7.00 Cav TAP aquifer -8.90

Question 1: How can the proportions of water originating from the Durance
and from irrigation be determined?

Question 2: What are the results of the above calculation?