Mixing processes of autogenic and allogenic waters in a large karst aquifer on the edge of a sedimentary basin (Causses du Quercy, France)

International audienceOn the edge of sedimentary basins, karst aquifers can be fed by several water origins from both autogenic and allogenic recharge. In some cases, water origin assessment is difficult and issues in water management may arise. The main goal of this study is to understand what controls hydrodynamical and geochemical variations at the outflow of a quite complex and large karst system. More precisely, this study illustrates how a consistent observational setup can be developed, based on a multi-proxy approach that can be used for tracing water origins, evaluating mixing phenomena, and contributions in karst aquifers considering both autogenic and allogenic recharge. The Ouysse karst system (650 km2), located in western France, provides the opportunity of studying water-mixing processes in binary karst systems fed by allogenic and autogenic recharges. Global water chemistry, hydrograph and chemograph analysis during a flood event, and source-mixing calculation were used to evaluate groundwater-flow origins and the contribution of each water type during the studied flood event: (i) karstic water; (ii) evaporite water; (iii) water from igneous-metamorphic rock aquifers. In terms of resource management, the information obtained can be used as a basis of forecasting and management actions

Mobile Sources Mixing Model Implementation for a Better Quantification of Hydrochemical Origins in Allogenic Karst Outlets: Application on the Ouysse Karst System

International audienceOn the edge of sedimentary basins, karst aquifers can be fed by several water sources from both autogenic and allogenic recharge. In some cases, assessing water origins can be hard and cause some difficulties for water resource management. The main goal of this study is to show the implementation of the mobile sources mixing model approach. More precisely, this research develops how a monitoring method using a multi-proxy approach can be used to quantify waters sources contributions from several origins at the outlets of a karst system. The study site is the Ouysse karst system, located in western France. The site offers the opportunity to understand the mixing processes between allogenic and autogenic water recharges. The karst system covers a 650 km(2) watershed, and is fed by three different chemical facies: (i) Autogenic water from the direct infiltration on the karstified limestones with high HCO3- values (median: 436 mg.L-1); (ii) Water coming from sinking rivers fed by spring coming from igneous rocks with low mineralization but relatively higher K+ values (median: 4.2 mg.L-1); (iii) Highly mineralized water coming from deep evaporitic layers and feeding another sinking river with very high sulfate concentrations (median: 400 mg.L-1). Sliding window cross-correlation analyses and hydrochemical analyses during a flood event are performed to implement a mobile source mixing model approach. This approach shows significant differences with a simple fixed source mixing model and appears more reliable but requires more time and money to carry out. The results and conclusion of this study will be used for forecasting and managing operational actions for resource management