Source du Lez

The source du Lez is a karstic spring. It is the main outlet of the karstic aquifer of the garrigues of Bas-Languedoc. It gives rise to the Lez river, which then flows south and crosses the city of Montpellier. It is the site of the second largest groundwater pumping operation in the world, supplying drinking water to the Montpellier Metropolitan Area. Its mean annual discharge is 2.6 cubic metres per second; but it depends on the Mediterranean climate, with major floods linked to Cévenol episodes, which can exceed 10 cubic metres per second, and long low-water periods at the end of which the natural discharge can fall to 0.4 cubic metre per second.

The catchment area of the source du Lez does not correspond to a simple geological structure. It is established within a vast set of terrains located between the coastal rivers Hérault and Vidourle, of diverse natures: karstic terrains mainly constituted by the limestone series of the Upper Jurassic; non-karstic terrains where runoff contributes to feeding the karstic aquifer.

The reservoir feeding the source du Lez is complex. It is mainly constituted of Jurassic and Berriasian limestones in stratigraphic continuity, but can also extend, through abnormal contacts, to the limestones of the Upper Valanginian (“calcaires miroitants”), of the Upper Hauterivian, or even to the lacustrine limestones of the Lutetian. The floor of the aquifer is constituted by the black marls of the Lias outcropping at the heart of the Pic Saint-Loup anticline. The roof is mainly constituted of marls and marly limestones of the Lower Valanginian, classically considered impermeable.

Sources: Christelle Batiot-Guilhe, Jean-Luc Seidel (UMR HydroSciences Montpellier), Jean-Christophe Maréchal, Bernard Ladouche (Bureau de Recherches Géologiques et Minières), Vincent Prié (Biotope), Pierre-Olivier Malaterre and Maxime Jay-Allemand (G-eau), Flavie Cernesson, Pierre-Yves Vion (TETIS); Arnaud Vestier (Montpellier Agglomération), “Système karstique du Lez : projet gestion multi-usages”: https://eau34.ouvaton.org/files/FichesLez.pdf; article “Lez (fleuve)” of the collaborative encyclopedia Wikipédia, in French: https://fr.wikipedia.org/wiki/Lez_(fleuve); IGN Rando > Source du Lez: https://ignrando.fr/fr/pointsinteret/fiche/details/id/2713424; Marjolet G., Salado J. “Le système karstique de la source du Lez (Hérault)”. In: Méditerranée, deuxième série, tome 32, 1-2-1978, pp. 71-83. DOI: https://doi.org/10.3406/medit.1978.1771 www.persee.fr/doc/medit_0025-8296_1978_num_32_1_1771; Maurice Brunel (Professeur honoraire à l’Université de Montpellier), “L’eau des garrigues de Montpellier et sa gestion.”, in L’eau des garrigues de Montpellier et sa gestion, séance du 10 mai 2021: https://www.ac-sciences-lettres-montpellier.fr/academie_edition/fichiers_conf/BRUNEL-2021.pdf