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Characterisation of underground flows in karstic aquifers by studying DOM fluorescence. Example of two Mediterranean systems (Lez and Causse d’Aumelas, Southeastern France)

Congress: 2008
Author(s): Christelle Batiot-Guilhe, Jean-Luc Seidel, Antoine Lafare, Hervé Jourde, Marie-Ange Cordier, Sandra Van-Exter, Claire Rodier
Christelle Batiot-Guilhe (1), Jean-Luc Seidel (1), Antoine Lafare (2), Hervé Jourde (1), Marie-Ange Cordier (1), Sandra Van-Exter (1), Claire Rodier (1) (1) HydroSciences Montpellier, UMR 5569, University of Montpellier 2, Place E. Bataillon, F-34095 Montpellier Cedex 5, France (2) BRGM Eau/RMD, 1039 Rue de Pinville, 34000 Montpellier

Keyword(s): karstic aquifers, Mediterranean systems
AbstractKarst aquifers are very particular hydrogeological systems, characterised by preferential flows and mixing of waters with different residence time. Recent works have shown that Total Organic Carbon (TOC) and Dissolved Organic Carbon (DOC) are relevant tracers of water infiltration within aquifers and specially in karstic areas. By using fluorescence spectroscopy, the type of organic compounds in solution and their natural or anthropogenic origin can be identify. Indeed, the fluorescence in natural waters originates mainly from organic acids (humic substances) and amino-acids. Recent works ([3], [2], [1], [4]) focused on the study of Dissolved Organic Matter (DOM) in natural waters by Excitation-Emission Matrix Spectroscopy (EEMS) and demonstrated that this technique is a relevant tool to perform a DOM qualitative and quantitative identification. We investigated this tracer on two karst systems, the “Lez” and the “Causse d’Aumelas” aquifers located in southeastern France, near Montpellier. These two karsts are constituted by secondary carbonate deposits, partly recovered by low permeable tertiary and/or secondary terrains. Their watersheds are respectively about 380 km2 and 200km2. These systems are more or less connected with intermittent streams, which may act as a contaminant source from surface waters to groundwaters. The study aims at understanding their behaviour during flood events and comparing infiltration flows within these two Mediterranean hydrosystems. In order to understand the interactions between surface waters and groundwaters, hydrodynamical and hydrochemical monitorings have been carried out at temporary and perennial springs, boreholes, swallow holes, and on the surface waters. Electrical conductivity, temperature and pH are also measured on each point and water samplings are completed twice a month or daily during flood events, in order to study hydrochemical tracers (major and trace elements, TOC, DOC, fluorescence of organic matter). The first results of the study upon the main perennial and temporary springs of these aquifers show very strong variations in TOC concentrations, about 0.5 ppm during dry season to more 10 ppm during flood events, which are clearly linked with fast infiltration. The main part of DOM at these outlets are characterised by couples of excitation-emission wavelengths (ExcMax/EmMax): 320-260 / 435-445 nm which corresponds to fulvic acids and identify a pedogenic origin. The study of DOM appears as a new relevant and easy tool to use in order to investigate karstic flows and highlight the existence of fast infiltration which is fundamental to estimate the vulnerability of the aquifer and to protect water resources. References [1] Baker A., Curry M. (2004). Fluorescence of leachates from three contrasting landfills. Water Research, 38, 2605-2613. [2] Coble P., (1996). Characterization of marine and terrestrial DOM in seawater using excitation-emission matrix spectroscopy. Marine Chemistry, 51, 325-346. [3] Coble G., Green S., Blough N. and Gagosian R. (1990). Characterization of dissolved organic matter in the Black Sea by fluorescence spectroscopy. Nature, 348, 6300, 432-435. [4] Spencer G.M., Baker B., Ahad J.M.E., Cowie G.L., Ganeshram R., Upstill-Goddard R.C., Uher G. (2007). Discriminatory classification of natural and anthropogenic waters in two U.K. estuaries. Science of the Total Environment, 373, 305-323.
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