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GEOCHEMICAL EVOLUTION AND TIMESCALE OF SEAWATER INTRUSION INTO THE CONFINED COASTAL AQUIFER OF THE RHONE DELTA

Congress: 2008
Author(s): V. de Montety, O. Radakovitch, C. Vallet-Coulomb, B. Blavoux, V. Vallès, Y. Travi
V. de Montety(1), O. Radakovitch(2), C. Vallet-Coulomb(2), B. Blavoux(1), V.Vallès(1), Y. Travi(1) (1) LHA, Laboratoire d’Hydrogéologie d’ Avignon, Université d’Avignon, 33 rue Louis Pasteur, 84000 Avignon. Tel/fax : 04.90.14.44.92/89 veronique.de -montety@univ-avignon.fr, bernard.blavoux@univ-avignon.fr, vincent.valles@univ-avignon.fr (2) CEREGE, Centre Européen de Recherche et d’enseignement en Géosciences de l’environnement, Europôle de l’Arbois, BP 80, 13545 Aix-en-provence. Tel: 04.42.97.15.80 rada@cerege.fr, vallet@cerege.fr

Keyword(s): Seawater intrusion, Confined aquifer, Hydrogeochemistry, Isotopes, Water-rocks interaction
Article:
AbstractSince the last decades, coastal areas are subject to two main hazards: important anthropization and sea level rise; two points which have a strong impact on the groundwater resource and more particularly on its quality. Located between the two arms of the Rhône River, the Camargue delta gives a good example of such issues: the human pressure since the end of the nineteen century has led to a strong modification of the delta and its vicinity (huge industrial and harbour area on the other side of the Rhône River). Moreover, this low deltaic plain is particularly sensitive to the acceleration of the sea level rise estimated at 2mm/year since 1905 in this area. Within the framework of the ORE RESYST*,an affiliated LOICZ° programme, a hydrochemical and isotopic study of the saline confined aquifer of the Rhône delta has been carried out to better assess the consequence of these issues on the groundwater. Understanding the origin and mechanisms of the salinization process of the confined aquifer is an important point for preventing further deterioration of this groundwater resource. Groundwater presents strong values of electric conductivity (up to 58 mS/cm) rising from the north of the delta to the Mediterranean Sea. Chemical and isotopic (18O, 2H, and 13C) analyses performed on monthly samples from 8 boreholes revealed that seawater intrusion occurs in the aquifer. The seawater contribution varies in the groundwater from about 6% to 98%. However, the ideal two end-members mixing between freshwater and Mediterranean seawater is strongly modified by water/rock interactions. Two different zones have been identified within the aquifer: (i) in the less saline part of the aquifer (up to 20% of seawater), cation exchanges prevail whereas (ii) in the most saline part of the aquifer sulphates reduction processes and precipitation phenomena occur. In order to date the seawater intrusion, new samplings have been carried out for 3H and 14C analyses in the confined aquifer and its potential recharge sources (Mediterranean Sea, unconfined Crau aquifer, Rhône River). Except for one borehole, groundwater samples contain no tritium suggesting that penetration of Mediterranean seawater occur more than 40 years ago. Moreover, the absence of tritium in the groundwater indicates that the freshwater recharge from the Rhône River and the Crau aquifer is a slow process. The low 14C activities measured in groundwater seem to confirm the non-recent seawater source. However the high water/rock interactions have to be taken into account to correct the apparent groundwater aging. The resulting age of the groundwater would have important consequences in the hazard assessment of a sea level rise due to climatic changes. *Environment Research Observatory: Response of a delta system to external forçing °Liaison with Land-Ocean Interactions in the Coastal Zone
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