[LUM#16] Aquifer! Aquifer! Do I look like an aquifer?

[LUM Magazine, Podcast] - Published on July 11, 2022 in Science-Society

What if the most important things were actually invisible to the eye? In France, around 75% of the water we consume comes from groundwater. With water crises looming, aren't these large reservoirs known as aquifers an underappreciated and underutilized resource? Between a lack of awareness here and ecological risks elsewhere, the sustainable exploitation of aquifers is still a work in progress.

The Amazon, Nile, Congo, and Yangtze Jiang are all rivers whose sheer size is as fascinating as it is frightening. And yet, "the water contained in rivers, streams, and lakes accounts for barely 1% of the Earth's liquid freshwater supply," explains Séverin Pistre, a researcher at HydroSciences Montpellier*. So where could the remaining 99% of water be? Right under our feet! More precisely, in the Earth's crust, which, with a composition of around 20 to 25% water, forms the planet's largest reservoir of fresh water. This is a high-quality resource, often much less polluted than surface water because it is sheltered in what specialists call aquifers.

Folded into karsts

Forget the seas and large underground lakes described by Jules Verne. The term aquifer, literally "water bearer," refers instead to groups of rocks in which rainwater infiltrates and circulates. There are three types, depending on the nature of the soil in which they form: "There are aquifers where water penetrates the pores of rocks, such as gravel or sand, " explains the hydrologist, "and others where it flows through harder rocks, traveling through cracks or lodging in interstices." While these two types of aquifers are the most common worldwide, there is a third type that is well known in the region: karsts.

Let's turn back the clock 250 million years. At that time, the region was covered by a deep ocean. As it receded, it left behind thick layers of sediment which, over time, formed limestone rocks. "In these easily soluble carbonate rocks, water creates its own spaces, forming drains, underground rivers, and then springs such as the Lez spring. The Demoiselles and Clamouse caves, for example, were filled with water before the water level dropped," explains Séverin Pistre. Aquifers are also found in China, Vietnam, Brazil, and elsewhere. Worldwide, one in four people drink karst water.

Aquifers, what to do?

So there is water everywhere, and in very large quantities! But you still have to go and get it, and to do that, it's best to have studied geology. France alone has more than 30,000 registered boreholes, while in India the figure rises to more than 2 million. Different drilling techniques are used depending on whether the aquifer in question is made up of sand or hard rock. "If we have hard rock known as 'bedrock', we know that the water is contained in fissures, and we can be within 5 meters of it," explains the researcher. In sand or gravel, the water table is widespread and the location of the borehole may be less precise. " If the nature of the soil is not already known, which is quite rare in France today, the geologist can use geophysics. One of the best-known methods involves sending an electric current into the ground to measure its apparent resistivity. "If the current travels through rocks that contain water, which is conductive, there will be low resistance. Another possibility is to look for a borehole or well pumping from the same aquifer in order to analyze the water. As it passes through rock, the water will dissolve it and become charged with ions, the analysis of which will reveal the rocks from which they originate. "We have thus observed that the chemistry of the Lez spring changes slightly between winter and summer, when the most superficial part of the aquifer is depleted and deeper aquifers take over."

And they pumped and pumped...

As the world prepares to face a water crisis, why not make greater use of this liquid treasure hidden beneath our feet? The first obstacle to this exploitation is the preservation of the resource, particularly when it comes to "mining" water from aquifers that can take thousands of years to recharge. This can be observed in arid or semi-arid countries that experienced wetter climates tens of thousands of years ago. "If we exploit them, we know that the water will not be renewed. When German troops from the Afrika Korps carried out drilling in North Africa, they found artesian wells from which water gushed without having to be pumped. Today, we have to go down several dozen meters to find it," continues Séverin Pistre.

Around the Mediterranean, karst aquifers ensure that the water table is replenished very quickly thanks to vast outcrops and the porosity of the rocks, which limits runoff. In a context where IPCC scenarios predict an increase in intense rainfall events in the region, karsts could thus become very interesting water reservoirs. However, certain precautions must be taken, because while the permeability of karsts allows for rapid groundwater recharge, it also offers little protection against pollution. "In France, politicians often have little knowledge of groundwater and take little interest in it. This is a mistake. Aquifers must be considered strategic reserves and included in a genuine resource protection policy, as recommended by the water agencies," concludes the hydrogeologist.

*HSM (University of Montpellier – CNRS – IRD)

Keywords: water, Environment, LUM#16, LumLu, Research, Company