Résumé
The End-Member Mixing Analysis (EMMA) is used in hydrology to determine the origin of water from solute contents measurements. This method assumes that the water collected at a sampling point originates from a mixing between several end-members. Classically, in hydrology, the water sampled at the outlet of a small head watershed may result from a mixing between rainfall, soil water and groundwater. The objective of EMMA is to compute the relative contributions of the end-members and their evolution with time. This provides valuable information on the origin of water and hydrologic characteristics of the water cycle. Similarly, in hydrogeology, the origin of groundwater can vary according to hydrological conditions, during a pumping test for example or during a flood event. In this paper, this approach still poorly used in hydrogeology, is applied to two Mediterranean karst systems with contrasted objectives. The Lez karst system is a major resource for the water supply of Montpellier city in Southern France. During autumn, it is observed that the first rainy events create an increase of water mineralization at the main karstic spring. An EMMA analysis has been conducted on the spring water during three hydrological cycles. It determines the respective contributions of two carbonate reservoirs to the spring discharge that fluctuate according to hydrologic conditions. In addition, a contribution from a deep aquifer during the first rainy events is also highlighted. The Nimes city (Southern France) faced many flood events with devastating inundations. The main spring of the Nimes karst system is located in the centre of the city. Hydrochemical and water level data have highlighted the role of the karst groundwater in the flood genesis in surface streams. EMMA has confirmed the role of the epikarst during flood event once the karst system is saturated. The monitoring of water streams during high flow conditions shows the relative contributions of pre-event water stored in the karst and of epikarst water to the runoff. The application of EMMA to hydrogeology looks promising, especially in complex aquifers where the origin of groundwater is not straightforward.