title: Coastal groundwater salinization: Focus on the vertical variability in a multi-layered aquifer through a multi-isotope fingerprinting (Roussillon Basin, France).
abstract: The Roussillon sedimentary Basin (South France) is a complex multi-layered aquifer, close to the Mediterranean Sea facing seasonally increases of water abstraction and salinization issues. We report geochemical and isotopic vertical variability in this aquifer using groundwater sampled with a Westbay System® at two coastal monitoring sites: Barcarès and Canet. The Westbay sampling allows pointing out and explaining the variation of water quality along vertical profiles, both in productive layers and in the less permeable ones where most of the chemical processes are susceptible to take place. The aquifer layers are not equally impacted by salinization, with electrical conductivity ranging from 460 to 43,000μS·cm(-1). The δ(2)H-δ(18)O signatures show mixing between seawater and freshwater components with long water residence time as evidenced by the lack of contribution from modern water using (3)H, (14)C and CFCs/SF6. S(SO4) isotopes also evidence seawater contribution but some signatures can be related to oxidation of pyrite and/or organically bounded S. In the upper layers (87)Sr/(86)Sr ratios are close to that of seawater and then increase with depth, reflecting water-rock interaction with argillaceous formations while punctual low values reflect interaction with carbonate. Boron isotopes highlight secondary processes such as adsorption/desorption onto clays in addition to mixings. At the Barcarès site (120m deep), the high salinity in some layers appear to be related neither to present day seawater intrusion, nor to Salses-Leucate lagoonwater intrusion. Groundwater chemical composition thus highlights binary mixing between fresh groundwater and inherited salty water together with cation exchange processes, water-rock interactions and, locally, sedimentary organic matter mineralisation probably enhanced by pyrite oxidation. Finally, combining the results of this study and those of Caballero and Ladouche (2015), we discuss the possible future evolution of this aquifer system under global change, as well as the potential management strategies needed to preserve quantitatively and qualitatively this water resource.
authors.affiliation: BRGM, Avenue C. Guillemin, BP 36009, 45060 Orléans Cedex 02, France. Electronic address: e.petelet@brgm.fr.
authors.forename: Emmanuelle
authors.initials: E
authors.lastname: Petelet-Giraud
authors.affiliation: BRGM, Avenue C. Guillemin, BP 36009, 45060 Orléans Cedex 02, France.
authors.forename: Philippe
authors.initials: P
authors.lastname: Négrel
authors.affiliation: BRGM, Réunion Agency, 5, rue Sainte-Anne, CS 51016, 97404 Saint Denis Cedex, France.
authors.forename: Bertrand
authors.initials: B
authors.lastname: Aunay
authors.affiliation: BRGM Montpellier Agency, 1039, rue de Pinville, 34000 Montpellier, France.
authors.forename: Bernard
authors.initials: B
authors.lastname: Ladouche
authors.affiliation: BRGM Montpellier Agency, 1039, rue de Pinville, 34000 Montpellier, France.
authors.forename: Vincent
authors.initials: V
authors.lastname: Bailly-Comte
authors.affiliation: BRGM, Avenue C. Guillemin, BP 36009, 45060 Orléans Cedex 02, France.
authors.forename: Catherine
authors.initials: C
authors.lastname: Guerrot
authors.affiliation: BRGM, Avenue C. Guillemin, BP 36009, 45060 Orléans Cedex 02, France.
authors.forename: Christine
authors.initials: C
authors.lastname: Flehoc
authors.affiliation: Géosciences Montpellier, UMR 5243, Université de Montpellier, cc069, Place Eugène Bataillon, 34095 Montpellier Cedex 05, France.
authors.forename: Philippe
authors.initials: P
authors.lastname: Pezard
authors.affiliation: Géosciences Montpellier, UMR 5243, Université de Montpellier, cc069, Place Eugène Bataillon, 34095 Montpellier Cedex 05, France.
authors.forename: Johanna
authors.initials: J
authors.lastname: Lofi
authors.affiliation: BRGM, Avenue C. Guillemin, BP 36009, 45060 Orléans Cedex 02, France.
authors.forename: Nathalie
authors.initials: N
authors.lastname: Dörfliger
keywords: Barcarès and Canet sites; Coastal aquifer; Groundwater salinization; Isotopes; Roussillon Basin; Westbay System