An invisible ocean of limestone
Beneath the flat, fertile plains southwest of Paris lies one of the largest groundwater reservoirs in France: the Beauce Aquifer. This subterranean body of water is not a vast cavernous lake, but rather a thick layer of porous limestone that is saturated with fresh water. The aquifer stretches over an area of approximately 14,000 square kilometers, spanning two regions, Centre-Val de Loire and Île-de-France. Its total storage capacity is estimated to be 20 billion cubic meters, a volume about eighteen times that of Lake Annecy.
The water is held within Eocene-Oligocene lacustrine limestone, a type of sedimentary rock formed in ancient lakes. This geological formation acts as a sponge. The Beauce Aquifer is classified as an unconfined aquifer, meaning its upper surface, the water table, is at atmospheric pressure and can rise and fall. This makes it susceptible to infiltration from the surface—both for replenishment from rainfall and for the introduction of pollutants. The natural discharge from this immense reservoir feeds into the Loire River, with an average inter-annual flow of about 10 cubic meters per second.