Groundwater in the Adour-Garonne basin

ADOUR-GARONNE WATER AGENCY

SUSTAINABLE DEVELOPMENT MINISTRY

BRGM

Geoscience for a sustainable Earth

present

Groundwater-river interactions

We are on the bank of the Garonne, which flows down below. Bear in mind that waterways are in direct relationwith groundwater tables, which we call alluvial sheets. These sheets circulate in geological formations Made of gravel,sand and pebbles, deposited during the last glaciations, when it was very cold and very damp. Waterways tore off matter in the Pyrenees and deposited it here, in this plain. There were also alternations with hotter and drier periods, where waterways eroded and cut into their own alluvium. It's the alternation between these hot and cold phases which modelled the morphology of the alluvial sheet that we see here, in this plain. In autumn and winter, the water that hits the ground infiltrates the soil and replenishes the groundwater tables, which will slowly flow into the river to feed its flow. We know that in the summer, 80 to 100% of the water flow into rivers comes from groundwater. So, it's important to manage the collection of groundwater in advance to guarantee a minimal flow into rivers for recreational uses, such as canoeing and swimming, as well as to produce potable water and allow for aquatic life.

ADOUR-GARONNE WATER AGENCY

BRGM

Geoscience for a sustainable Earth

present

Groundwater hydrobiology

Under the top stones, you find the crustaceans, insect larva and molluscs that feed the river fauna, particularly the fish. But it's deeper down that a permanent fauna, will colonise the groundwater. But to observe and collect them, we need equipment that will allow us to reach this deep zone. The well screen is 40 cm deep. Using a pump, I will suck up water, sediment and fauna, that can be found deeper down. At the bottom of the net, we look at everything over 2/10th of a millimetre. The gravel and grains of sand that the pump sucked up, and the insect larva and small crustaceans which were in the interstices of these grains. It is in this sand and gravel, and particularly in the interstices, in the gaps, that we find the animals we've just collected. This is where they eat by filtering the groundwater. Their current inventory is extremely interesting and will allow us to discover a whole new field of relatively unknown fauna.

ADOUR-GARONNE WATER AGENCY

SUSTAINABLE DEVELOPMENT MINISTRY

Geosciences for a sustainable Earth

BRGM

present

The Sedimentary aquifers of the Adour-Garonne district

Here we are in Blaye, in a remarkable spot. This is a place where we see a rock type that can be found 300 metres Under the city of Bordeaux and which holds the largest water reserve that supplies the town in potable water. This rock seems very solid When you see it here, but actually it's like a sponge. All of these small cavities situated in the rock under the city contain millions of cubic metres of water to supply the city. The age of the sediment, which occurred in a marine environment, corresponds to about forty million years ago. But the age of the water corresponds to the rainwater that fell around the time when the Lascaux caves were painted 20 to 22,000 years ago. This is a great example of an aquifer reservoir that's completely loose. It's sand from the Landes region. This geological formation is found throughout the Aquitaine between the Garonne and the Adour rivers. This is a very important formation. It contains huge quantities of water because it is highly porous. It provides hundreds of millions of cubic metres that will mainly serve to supply agriculture. This spring here illustrates perfectly the fact that limestone, which appears to be something very compact, is actually relatively porous. Large amounts of water come out of the rock's interstices, but also from the cracks. This water will come out in specific places because there are impermeable layers underneath like marls. This aquifer also supplies Bordeaux with huge quantities of water.

ADOUR-GARONNE WATER AGENCY

SUSTAINABLE DEVELOPMENT MINISTRY

Geosciences for a sustainable Earth

BRGM

present

Karstic aquifers

It all starts in the clouds and rain. When water hits the ground, if the soil is impermeable, the water will flow. It might create small streams, like this one. If the rock is permeable, the water will infiltrate. Here we have a special case: limestone. Generally, limestone is impermeable. If you put water on a piece of limestone, it will take a long time to seep through. But limestone has another property: the rock is very hard. As proof, during erosion, it keeps its relief. We can see the cliff very well. Since this rock is very hard, if there are deformations, the rock will fracture. These cracks will allow water to seep through. Here, an additional process comes into play. When water penetrates the limestone, it dissolves the rock, digs into it and creates galleries. As is the case here. This in turn allows the streaming water to infiltrate very easily. We saw the moment when the water entered the ground. This is where it comes out, the resurgence. For years, we thought that in these limestone massifs, water circulation was limited to this entrance and this exit, to this gallery. Not at all. It is far more complex. From the moment when the water hits the massif surface, until the moment it comes here, it will flow through vast spaces. There will be huge spaces of emptiness, and it's these spaces which we define with the term "karst". That's what a karstic aquifer is. It is the study of these exits here, which will allow us to understand the full history of this water, from the time it hit the massif surface until the point when it arrives here. This water tells us a story. We must decipher it. We have many elements to do so. And so, we will be able to observe that these limestone terrains contain a huge amount of water and are extremely important resources. They will teach us how water circulates and is renewed.