Has the torrential rains in recent weeks really boosted the water tables? How does groundwater recharge? Along the water, hydrogeologist Jean-Christophe Maréchal, project manager at BRGM (the geological and mining research office).
A water table is a reservoir of underground water that is located under our feet. It should be understood that the basement is made up of large, completely empty parts. In hard rocks, for example, there are cracks, fractures, and this void regularly fills with rainwater. They infiltrate and fill this void, which constitutes the water table. They are not underground lakes as we often imagine.
Following a rain, we will have water infiltration. This water will gradually fill the water table which can start to overflow. This is called a flood by rising water table. So we have infiltration phases, then underground flows. The water can then come out in springs or in rivers for example.
In fact, this water is in perpetual motion! It is filtered by the process of infiltration through the ground. You should know that the older the water, the longer it will circulate in the basement, the more it will undergo this self-purification phenomenon, and the better its quality will be.
90% of the water tables are supplied by rainfall. Sometimes there may be river infiltration, but this is a marginal phenomenon. A lot of water has fallen in recent weeks, so that’s good news for the water tables in anticipation of summer and the risk of drought.
Currently, most aquifers are in a satisfactory state of filling. There are a few exceptions in the Rhone and Saône corridor, but most of the water tables are above annual averages at this time of year. Groundwater also reacts depending on the nature of the soil.
In France, we find almost all types of rocks present in the world. There are water tables which will react very quickly, in which the water circulation is done very quickly. Others will have much slower circulation rhythms with much more inertia. We are talking about hundreds, even thousands of years to renew these water tables.