PROBLEMS - Hydrogeology: Objectives, Methods, Applications - page 339

The hydrogeologist undertook a soil study, from which follow a few
excerpts:
“...The residence is located on a massive Eocene sandstone plateau.
This highly fractured formation weathers into decimeter to meter-scale
spheres and gives, at the surface, sandy clayey weathering material with a
permeability varying with the quantity of clay...
... downhill, at a distance of a hundred or so meters, the sandstone unit
is lined with a series of springs at an average elevation of 80.00 m, supported
by the Upper Cretaceous marl...”
The following work was undertaken:

• Auger reconnaissance:

Table 10 Result of the auger reconnaissance excavations (see location of points on Figure 13).

S1:
00-30 cm: organic-rich
soil
30-50 cm: sand
50 cm: no passage into
sandstone

S2:
00-20 cm: organic-rich soil
20-100 cm: clayey sand
100 cm: no passage into
sandstone

S3:
00-30 cm: organic-rich soil
30-50 cm: sand
50-150 cm: clayey sand and
sandstone
150 cm: limit of the tool

S4:
00-30 cm: organic-rich
soil
30-50 cm: clayey sand
150 cm: limit of the tool

S5:
00-20 cm: organic-rich soil
20-150 cm: clayey sand
150 cm: limit of the tool

• Constant level infi ltration tests (see location of points on the fl oor
plan):

Table 11 Results of permeability tests (see location of points on Figure 13).

E1: K = 5 mm·h -1 E2: K = 40 mm·h -1 E3: K = 50 mm·h -1

Question : Based on these observations, what advice can the hydrogeologist
give for this construction project?

Answer:

The fractured sandstone is permeable and the line of springs reveals the
presence of an aquifer located 40 or 50 m below the surface. The thickness
of the weathering mantle appears great enough at several locations to allow
the emplacement of the individual wastewater system, as long as the clay
content is not too high. Various areas of the plot can thus be eliminated:

• the zones outside the regulatory distances;
• the northern half, which is above the residence and included in the
previous zone;