Understanding everything about the groundwater in Aquitaine

Understanding Groundwater

Hydrogeology of the Aquitaine Basin and Monitoring Tools for Aquifers

France is divided into 7 large hydrographic basins. The one in the south-west covers several regions and numerous counties. It's called the Adour-Garonne drainage basin. The basin's natural boundaries directly result from the direction of rainwater flow. Rainwater infiltrates the ground and streams in the direction of the slope. This water joins small rivers, then forms larger rivers, and finally flows into the Atlantic Ocean. From a geological standpoint, the ground in the eastern basin is composed of ancient crystalline rock of magmatic origin. We also find limestone fields dating back to the Secondary Period, and detrital limestone, sand and molasse from the Tertiary. On the surface, the Landes Triangle is a vast sandy plain. The sand comes from matter torn from the Pyrenees and the south Massif Central during the Ice Age. All this is covered in places by alluvial deposits brought in over the course of time by rivers. The formation of these various soils is directly related to the basin's geological history, which started at the beginning of the Secondary, 250 million years ago. A series of marine transgressions and regressions, corresponding to the rise and fall of sea levels linked to climate change and plate tectonics, left alternating layers of sedimentary deposits which have turned into rock. Thus, the Aquitaine Basin is made up of superimposed sedimentary layers or strata. Tectonic movements began at the end of the Secondary and increased until the middle of the Tertiary. This caused the gradual rise of the Pyrenees, as well as ground deformations and faults. An east-west geological cross-section of the Aquitaine Basin shows a superimposition of sedimentary layers with a global westward slope.

The layers are affected by deformations such as folds and faults which make them either outcrop or sink deep down.

A north-south cross-section shows the deformation of the strata, with large folds like the anticlines at Villagrains and Jonzac. An anticline is a convex fold containing the oldest layers at its centre. Underground geological strata of limestone and sand are porous and permeable formations. They let water circulate easily and thus form fabulous reservoirs. We call them aquifers. Conversely, the layers of clay and marl are less permeable. They act as brakes to the waterflow and form aquitards. Thus, the geology of the basin is comprised of multiple layers, where permeable ones alternate with less permeable ones. Water circulates preferentially in aquifers by flowing from the highest points to the lowest ones. This water is constantly renewed, albeit very slowly. It must pass through cavities in the rock which are minuscule, and sometimes microscopic. The age of the geological layer helps regroup complex groups under one same name. We talk about aquifers that are Jurassic, Cretaceous, Eocene, Oligocene, Miocene, Pliocene and Quaternary. The deformations affecting the various layers can generate folds that bring the oldest formations to the surface. This is the case of the Cretaceous reservoir 800 m below Bordeaux, whereas it is near the surface of the Villagrains-Landiras anticline. Close to the surface, the aquifer has an advantage: it can be directly refilled by rainwater. But this also makes it more vulnerable to pollution.

What are the different types of aquifers?

Unconfined aquifers are the ones most commonly encountered on the surface. The aquifer is not completely filled with water. We distinguish a saturated and an unsaturated zone. When a reservoir gets deeper and is covered with an aquitard, water saturates the entire aquifer and forms a confined aquifer. This water is under pressure. If we drill in this aquifer, the water will rise higher than the ceiling of the confining layer. It can even spurt forth from the ground like an artesian well. This imaginary plane is called the piezometric surface of the aquifer. Piezometric surfaces are essential to understanding the behaviour of an aquifer. Here, we see the piezometric surface of the Eocene water table. We realize that there is a large drop in Bordeaux. It is one of the most exploited aquifers in Aquitaine for potable water and has been for over 60 years. Due to the overexploitation of certain aquifers, namely for potable water supplies, it has become essential to monitor aquifers and implement efficient management tools. For this, dynamic hydrological models have been developed. The north Aquitaine model, MONA, was conceived by the BRGM to find solutions for the severe drop in groundwater levels in the Eocene aquifer in Gironde.

MONA: What is it?

The MONA is a hydrodynamic model. It's a tool that helps us understand. It synthesises a large quantity of data from the entire Aquitaine Basin. And it's also a management tool used within the framework of public policies for groundwater management in the north Aquitaine Basin.

MONA: What's it for?

We use the north Aquitaine model to make groundwater diagnoses. We use it to carry out prospective simulations to help us find the true impact of the strategies implemented to manage this groundwater. Does saving water have an impact on the evolution of the resource's water? What impact will future production sites have? Will we find a new equilibrium? And what impact will climate change have on this groundwater?

How was MONA created?

The model is based on 3,248 boreholes, which enabled us to import the data necessary to reconstruct the geometry of the subsoil in depth. We also used seismic data and geological maps of the entire region.

But how do you go about making a model like this? Each aquifer is represented in 3 dimensions on a digital map with 500 meter interval lines. Each mesh contains the hydraulic properties of the aquifer. For example, its permeability and stockability, as well as infiltration values and the initial piezometric plane of the aquifer. This data enabled the creation of a hydrodynamic 3D model. The programme can model aquifers, follow their evolution, and carry out simulations.

The north Aquitaine model is at Version 4. It simulates water flow through 15 aquifers and 15 aquitards. It's a tool that is constantly being improved to synthesise all of our knowledge.

For one of the missions of hydrogeologists is to continually improve our knowledge of geology and understanding of ground and subsoil phenomena. The stakes are huge: respond to problems posed by global changes, climate change, the increasing scarcity of resources and pollution. Preserving underground water resources has become a major challenge for our society. For now and for future generations.

Understanding Groundwater

Hydrogeology of the Aquitaine Basin and Monitoring Tools for Aquifers

2) Hydrogeology of the Aquitaine Basin

Underground geological strata of limestone and sand are porous and permeable formations. They let water circulate easily and thus form fabulous reservoirs. We call them aquifers. Conversely, the layers of clay and marl are less permeable. They act as brakes to the waterflow and form aquitards. Thus, the geology of the basin is comprised of multiple layers, where permeable ones alternate with less permeable ones. Water circulates preferentially in aquifers by flowing from the highest points to the lowest ones. This water is constantly renewed, albeit very slowly. It must pass through cavities in the rock which are minuscule, and sometimes microscopic. The age of the geological layer helps regroup complex groups under one same name. We talk about aquifers that are Jurassic, Cretaceous, Eocene, Oligocene, Miocene, Pliocene and Quaternary. The deformations affecting the various layers can generate folds that bring the oldest formations to the surface. This is the case of the Cretaceous reservoir 800 m below Bordeaux, whereas it is near the surface of the Villagrains-Landiras anticline. Close to the surface, the aquifer has an advantage: it can be directly refilled by rainwater. But this also makes it more vulnerable to pollution. What are the different types of aquifers? Unconfined aquifers are the ones most commonly encountered on the surface. The aquifer is not completely filled with water. We distinguish a saturated and an unsaturated zone. When a reservoir gets deeper and is covered with an aquitard, water saturates the entire aquifer and forms a confined aquifer. This water is under pressure. If we drill in this aquifer, the water will rise higher than the ceiling of the confining layer. It can even spurt forth from the ground like an artesian well. This imaginary plane is called the piezometric surface of the aquifer. Piezometric surfaces are essential to understanding the behaviour of an aquifer. Here, we see the piezometric surface of the Eocene water table. We realize that there is a large drop in Bordeaux. It is one of the most exploited aquifers in Aquitaine for potable water and has been for over 60 years.