Abstract
Water resources will be one of the major challenges of the coming decades. The questioning of the sustainability of low flows in the Mediterranean area, and more particularly in Cevennes* rivers (*Mountain Range in South-Central France), has been identified as a strong research axis on an international and regional scale. This thesis aims to study the role of groundwater in maintaining low flows in Cevennes rivers using a geochemical approach coupled with a geological and geographical analysis. It is organised around the following three questions: - Can seasonal variations in the geochemical signature of groundwater be used as an indicator of the severity of low water levels, or even as a proxy for calculating the flow of Cevennes rivers? - Is it possible to identify contributions of the different geological reservoirs through a geochemical approach for maintenance purposes? - Can we explain differences in specific flows and more generally dynamics observed during low-water period in Cevennes watersheds through the prism of water contributions quantifying of the various geological reservoirs? The methodology developed is based on a weekly or bi-weekly monitoring of physicochemical parameters, major ions and stable isotopes of the water molecule during low-flow period in both groundwater and surface water. Groundwater is thus categorised by geological facies to identify all the end members present in the basin. These geochemical poles are then validated by two complementary and independent approaches: a leaching approach of rock samples representative of the reservoirs and a statistical classification. Once poles are identified and consolidated, a geochemical model (G-EMMA) is used to quantify the contribution of the various reservoirs present in the basins during low water period. The monitoring of geochemical parameters has made it possible to characterise the seasonal variability of the surface water signature, and highlight the link between river flow and electrical conductivity. This relationship allowed the development of a preliminary method to estimate flow by measuring electrical conductivity in flows. The calculation of groundwater contributions according to their geological reservoir of origin and its analysis coupled with the flow rate in individual basins highlight: (1) the preponderant role of the black mica-schist reservoir, despite its limited spatial extent, (2) significant differences in the slope of the recession curves of contributions of the different reservoirs to surface water flows between the beginning of Summer and low water level, leading to an inversion of reservoirs contributions that are predominant to the base flow when approaching low flow period. This approach was transposed to the study of 8 sub-basins of the Gardon d'Anduze and the Galeizon watershed, a tributary of the Gardon d'Alès, and explains the significant differences in specific low flow rates observed between these different watersheds, including the significant differences in run-off generation during low water period of geological reservoirs. The major role of two reservoirs (black mica schists and sedimentary series) in the support of low flows can be observed in the majority of the basins and allow the identification of trends in run-off generation and persistence of contributions of the observed reservoirs. The information obtained from this approach can be derived to explain the dynamics of reservoirs and their run-off generation during low flows. The analysis of geochemical monitoring results of surface and ground water demonstrates its utility in understanding and anticipating the criticality of low water levels.