Hydrochemical characterization of transfers in karst aquifers by natural and anthropogenic tracers. Example of a Mediterranean karst system, the Lez karst aquifer (Southern France).

Abstract : The Lez karst spring, located in the Mediterranean basin (southern France), supplies with water the metropolitan area of Montpellier (France) since the 19th century. Since 1981, an intense pumping is being performed directly in the main conduit with a maximum exploitation flow rate of about 1,700 l/s. To improve the understanding of groundwater origins and circulation dynamics in this karst system, as well as the impact of three decades of intense water exploitation, groundwater samples have been collected during various hydrologic conditions since March 2006. The springs and wells of the Lez karst system as well as surrounding springs and wells have been monitored for: physicochemical parameters, major and trace elements, Total Organic Carbon (TOC), faecal and total coliforms, 18O, 2H, d13CTDIC and 87Sr/86Sr. During the first recharge events of autumn, high-mineralized waters have been observed at the Lez spring. This singular behaviour was monitored in a fine time-lag. A multivariate statistical analysis revealed the existence of different water-types discharging at the Lez spring. A coupled approach integrating geochemistry and isotopes were applied and provided insight into the different end-members, associated lithologies and the main reactions that control groundwater chemistry. Between the five distinguished water-types, the two more contrasting ones are emphasized: the first one correspond to more geochemically evolved, long residence-time waters, issued from deep layers where evaporite fingerprinting was identified. They are characterized by high mineralization and high concentrations in Cl, Na, Mg, Li, B and Br elements, high Sr/Ca, Mg/Ca and Cl/Br ratios and enriched d13CTDIC and 87Sr/86Sr. Between all the studied springs and wells, this chemical fingerprinting has been uniquely observed for the Lez spring groundwaters. The second water-type corresponds to low mineralized waters with high concentrations in NO3, bacteria and TOC, and represents the flux of rapid infiltration waters. They underline the vulnerability of the system to surface infiltration and anthropogenic contamination through the infiltration of waters by sinkholes and well-developed fracture networks. Hydrograph deconvolutions using multiple tracers were used to estimate the participation of two or three end-members in the various flood-events that occurred between 2008 and 2010. If we use chloride as tracer, the mean estimated participation of the different water types are, as follows: 12% for deep waters; 5% for recent waters and 83% for main aquifer waters. The comparison between present and former studies carried out before the installation of the pumping plant (1973-1974 dataset), indicates historical changes in water hydrogeochemistry, evidencing a decrease of the deep compartment participation to the outflow of the Lez spring. This change in water hydrogeochemistry may be attributed to the intense pumping of the karst system and, in the absence of noticeable climatic changes, traduces the direct consequences of anthropogenic forcing on the overall functioning of the aquifer. The multi-tracers approach combined to hydrodynamics appears as a very efficient tool for characterizing groundwater flows and their origins and seems to be potentially applicable to other similar complex Mediterranean karst systems that were subjected to deep karstification during the Messinian crisis. The features of this crisis seem to play a relevant role on the hydrogeological behaviour of the aquifer and chemical characteristics of waters by the participation of a deep compartment to the outflow of the karst system.
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Hydrology. AgroParisTech, 2010. English. 〈NNT : 2010AGPT0082〉
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Cristina Caetano Bicalho. Hydrochemical characterization of transfers in karst aquifers by natural and anthropogenic tracers. Example of a Mediterranean karst system, the Lez karst aquifer (Southern France).. Hydrology. AgroParisTech, 2010. English. 〈NNT : 2010AGPT0082〉. 〈pastel-00569544〉



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