Hydrochemical and Hydrodynamic Behavior of the Epikarst at the Lascaux Cave (Montignac, France)

Abstract

This study focuses on determining the hydrodynamic and hydrogeochemical behavior of the vadose zone of Lascaux karst (Dordogne, France). This work is of particular interest for the conservation of the famous prehistoric paintings of the Lascaux Cave. Indeed, unsustainable infiltration into the cave influences the microclimate parameters (temperature, humidity) and may play a role in aerologic phenomena (condensation, drying) that can affect the decorated walls. A continuous monitoring device for physical and chemical parameters (conductivity, pH, temperature, dissolved O₂) and fluorescence of organic matter are installed in the epikarstic spring located inside the cavity. In addition, weekly samples are performed at the epikarstic and Haute Fageotte springs. Weekly rainfall (major ions, water isotopes) and soil water (major ions, organic carbon) samples are performed to determine the input chemical signal in the karst system. The results show that water with shorter or longer residence time can be identified during the floods, considering electrical conductivity and the dissolved organic carbon as natural tracers. Moreover, when the system is highly water saturated, water with long residence time is solicited highlighting an increase of the area participating to the flow at the epikarstic emergence. The study of the calco-carbonic equilibrium of the epikarstic spring also confirms these observations. The calco-carbonic equilibrium shows interannual and seasonal variations related to climatic conditions, carbon dioxide concentration in soils and epikarst, and the flow conditions in the vadose zone. The relations between SI_c and PCO_2eq allow us to recognize the climate influence on the recharge geochemical signal of the Lascaux karst. It would be useful to estimate the global warming impact on the geochemistry of karst water.