TABLE OF CONTENTS
- PURPOSE OF THE STUDY AND GEOGRAPHICAL LOCATION
2. GEOLOGICAL AND HYDROGEOLOGICAL CONTEXT
- Geology
- Hydrogeology
3. FEASIBILITY OF WATER DRILLING
1.
PURPOSE OF THE STUDY AND GEOGRAPHICAL LOCATION
The Rock Project contacted SOURCE CONSULTING to find underground water beneath it's land.
The groundwater will be used for:
- domestic purposes
- drinking water
- garden, plantations
This parcel of land, with a surface area of around 4,200 m2, is located at an altitude of 450 metres above sea level.
The site is dominated by the ridge to the north (1,000 m above sea level) and overlooks the stream to the south (330 m above sea level).
The topography is marked by a very steep slope, particularly the cliff overlooking the site.
The hydrographic network is made up of steep, dry valleys that only drain during rainy periods. They join up with the stream which, during the summer months, no longer flows in its upstream section (according to our observations, a few pools are still filled with water).
The stream flows into the left bank of the river at an altitude of 200 m above sea level.
2.
GEOLOGICAL AND HYDROGEOLOGICAL CONTEXT
2.1 GEOLOGY
The geological structure of the study area is complex, due to the detachment of the sedimentary cover during the uplift of the Mountain massif.
The sedimentary layers slid south and south-west over the detached Triassic level. They also underwent tectonic deformations such as folds and numerous faults.
On the left bank of the river, there are 3 major geological units (visible on the surface), from the oldest to the most recent age:
1 The Jurassic, made up of limestone and dolomite, 300 to 350 m thick, which outcrops on the sides of the Lower river gorges and the mountain massif,
2 The Lower to Middle Cretaceous (Neocomian to Cenomanian), alternating limestone and marl, 300 to 600 m thick,
3 The Upper Cretaceous (Turonian-Senonian), alternating limestone and marl-limestone, 200 to 600 m thick.
According to the geological map, the Rock Project’s parcel is located at the junction between the Lower Cretaceous and the Upper Cretaceous.
The Upper Cretaceous is represented locally by limestone of Turonian age, clearly visible on the cliff face to the north of the plot.
At the base of the cliff, we find marl of Cenomanian age (Middle Cretaceous).
The southern boundary of The Rock Project’s plot is characterized by the presence of limestone blocks overhanging alternating marls and limestone banks up to a meter thick (Lower Cretaceous).
The general dip of these geological layers appears to be locally north-north-east.
2.2 HYDROGEOLOGY
Of the 3 major geological formations in the area, the Jurassic limestone constitutes the largest aquifer reserve.
This deep reservoir receives rainwater, which infiltrates more or less rapidly into the karstic fissures of the outcrops (in the open air), before emerging via springs located at its base, made up of the upper Triassic formations, which are reputed to be not very permeable.
We did not find any springs on the Jurassic outcrops on the left bank of the river, although data mentions "griffons on the left bank over a length of 300 m" and "the stream spring" at an altitude of 215 m.
This suggests that there are a few low-flow springs at or slightly above the river. It would appear, however, that the north-easterly dip of the layer favours underground flow towards the interior of the massif rather than along the banks of the river.
The limestones of the Turonian (Upper Cretaceous), when cracked, may be the site of a water table that flows over a more marly level. We did not find any springs at the base of the cliff.
Given the steepness of the cliff, it is unlikely to infiltrate rainwater, which tends to run off quickly into the valleys.
Finally, the limestone beds dating from the Lower Cretaceous period may constitute groundwater reservoirs, compartmentalized by marl levels.
We located three springs below, including 2 to the west and one slightly to the east of the Rock Project plot.
Their geolocation places them at altitudes of 350 m.
These altitudes are close together, which would indicate the existence of one or more juxtaposed reservoirs, limited at their base by a layer of impermeable marl. The photos below show the limestone rocks around the springs.
These springs, which trickle out in August, provide reserve of water, sufficient for the farm consumption.
The owner of the spring told us that he "pumps 10 m3/day" to fill his cistern, the overflow of which feeds a basin that always appears to be full.
This flow, which is lower than that of the spring, is more than sufficient for farming needs.
3.
FEASIBILITY OF DRILLING FOR WATER
In view of the elements presented above, the probability of successfully drilling a water well on the Rock Project’s plot of land would be more or less certain depending on the depths targeted:
The probability of finding water up to an altitude of 350 m, i.e. a borehole depth of 100 m, is medium because the water reserves in the Lower Cretaceous limestone are low.
The probability of finding water up to an altitude of 200 m, i.e. a borehole depth of 250 m, would be higher because the structure would cross the Jurassic limestones, which are known to be a large aquifer reservoir.
If the borehole were to successfully reach a water table in the Lower Cretaceous limestone it will therefore be necessary to install a water storage tank and optimize the pump's flow rate and operating times.
The exact location of the borehole on the plot will have to be decided in consultation with a drilling company, depending on access constraints and the installation of the drilling machine on variable terrain.
