Résumé
The Lez karst aquifer, located in the Mediterranean basin (southern France), supplies the Montpellier metropolitan region with potable water and has a maximum authorized pumping discharge of about 1700 L/s. The complexity of this system results from the heterogeneity of natural factors such as geological characteristics, spatial distribution of recharge processes, particularities of the Mediterranean climate, and the growing impact of human occupation. The particularity of this karst aquifer lies in the withdrawing of water directly from the main drain, which is located lower than the spring overflow level. The spring has been known to have a permanent water flow in the past; however, the recent increase in the water removal from the aquifer has caused the spring water flow to become only a seasonal phenomenon. Marjolet & Salado (1976) reported the results of studies of the Lez karst aquifer system and proposed a functioning model based on major elements data. In 1981, a pumping plant was installed for exploitation of the aquifer. Comparison between the previous and the present data indicates historical changes in the water chemistry. We suggest that the changes in hydrogeochemical characteristics are a consequence of the active exploitation of the karst aquifer by modifying the water circulation dynamics. This study aims to analyse the differences, to understand the water chemistry changes, and to better foresee the aquifer evolution for the future.