10 Juillet – Thesis defense - Guillaume Lorette

14 h Amphi Géosciences- Building B18 (Pessac)

Functioning and Vulnerability of a multilayered karst aquifer using multitracers approach and high resolution monitoring. Application to Toulon Sprrings (Dordogne, France).

This work is included in an approach for a better knowledge of karst aquifers. For this, the Toulon Springs pilot site was chosen, and provides the opportunity to study relationships between multilayered karst aquifers of the northern edge of the Aquitaine sedimentary basin. Toulon Springs are major regional springs and are located in Périgueux (Dordogne County, France). They have been supplying water to the metropolitan area of Périgueux since 1832.
This work aim to test an innovative multitracer approach coupled with a physicochemical high-resolution auto-monitoring to characterize functioning and vulnerability of a multilayered karst aquifer.
The use of a coupled hydrodynamic and hydrochemical approach has highlighted Toulon Springs supply by a deep and captive aquifer, in addition to a more subsuperficial aquifer. This information has been incorporated into the global hydrogeological situation of the study area to propose a new delineation of Toulon Springs hydrogeological cathment.
The use of a high-resolution monitoring enables to specify the hydrogeological functioning of the studied karst system. The analysis performed on several flood events, identify that mass transfer can range from a few days to a few weeks.
The use of nitrate isotope (δ15N-NO3- ; δ18O-NO3-) enables to identify two main nitrate sources in both surface water and groundwater: (i) inorganic fertilizer; (ii) sewage from individual house.
The last part of this work was to analyse Toulon Springs temporal vulnerability during floods to marker of contamination such as particles, dissolved organic carbon, nitrate, and bacteria. Hence, several water type from surface runoff, unsaturated zone and saturated zone were identify as responsible of contaminant transfer: (i) water from saturated zone is responsible of vulnerability to mineral particles ; (ii) water from unsaturated zone is responsible of vulnerability to nitrare  ; (iii) water from surface runoff is responsible of vulnerability to bacteria contamination.

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