Quantification of a River Contribution to a Karst Aquifer: A Coupled Geochemical and Discharge Modelling Study

This study deals with the quantification of a river contribution to a karst aquifer. The contribution of surface water to the discharge of karstic spring provides an additional unknown of the system functioning. To this aim, a rainfall-discharge model was developed according to geochemical measurements. This model intends to: i) simulate spring hydrograph and ii) describe seasonal variations of the river contribution.

The studied karstic spring is The Fontaine des Chartreux located in southern France. Its catchment represents 250 km² on a wide karstic plateau. Water quality of this spring is a key issue since this spring is the only drinking water supply for more than 30,000 people living in Cahors city. Geochemical analyses showed that the Lot River contributes to the spring discharge, but the extent and the temporal variations of this contribution are difficult to assess from geochemical data only.

Our approach consists in using a classical conceptual rainfall-discharge model (already applied successfully to other karstic systems (Plagnes et al, this issue)) to simulate spring discharge at the daily time step. This model is based on a production and a transfer function, and an additional function representing the Lot River contribution. With respect to geochemical analyses, this last function was set inversely proportional to spring discharge.

This work shows that a simple conceptual model can represent pretty well the global functioning of such a complex system. Although the simulation results are far from being perfect, the model allows quantifying the river contribution to the total discharge. It also provides a new tool for water resources management, and gives new information to help to determine groundwater protection zones.

This approach could be easily applied to other karstic systems with similar issues even when measurements of surface water lost in sinkholes are not possible or not available.