Marine and Petroleum Geology 53 (2014) 102e116
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Chlorine transport processes through a 2000 m aquifer/aquitard system R. Rebeix a, *, C. Le Gal La Salle a, P. Jean-Baptiste c, V. Lavastre a, b, E. Fourré c, F. Bensenouci d, J.M. Matray d, P. Landrein e, O. Shouakar-Stash f, S.K. Frape f, J.L. Michelot g, J. Lancelot a a
GIS-CEREGE-UMR 6635, Université de Nîmes, parc scientifique Georges Besse, 30035 Nîmes, France Université de Lyon, Université Jean Monnet e Sainte Etienne, CNRS UMR6524, LMV, F-42023 Saint Etienne, France LSCE, UMR 1572, CNRS-CEA-UVSQ/IPSL, 91191 Saclay, France d IRSN/PRP-DGE/SRTG/LETIS, France e ANDRA, rue Jean Monnet, 92290 Chatenay-Malabry, France f Department of Earth Sciences, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada g IDES, UMR CNRS e Université Paris-Sud, Bât. 504, 91405 Orsay, France b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 April 2013 Received in revised form 10 December 2013 Accepted 20 December 2013 Available online 28 December 2013
In the Paris Basin, in France, the Callovo-Oxfordian (COx) is currently studied over a 250 km2 surface area by the French national radioactive waste management agency in order to assess the feasibility of longterm underground nuclear waste repository. The COx is a 140 m thick clay-rich layer, which is part of the 2000 m aquitard/aquifer system constituting the sedimentary cover. In such sedimentary context, the transport processes of potential contaminants can be represented by both vertical diffusion and horizontal advection through the most permeable layers. Chloride is used as a natural conservative tracer, and is monitored in term of concentrations and isotopic composition (d37Cl) for both pore and groundwater. During this study, the samples were collected from three boreholes located in the center of the studied zone, one of them (EST433) going down to 2000 m depth. The main solute transport process is shown to be vertical diffusion from the massive Keuper halite level to the rest of the sedimentary pile. This global diffusive system can be occasionally disturbed by horizontal circulation of groundwater occurring in the Oxfordian and Dogger limestone formations. Therefore, these circulations cut the global diffusive system in a succession of independent diffusive systems. In this study the data set was implemented in a simplified 2D solute transport model and scenarii reproducing known history in term of paleo circulations inside the system, were applied and allowed to obtained a good fit of the data. Model results showed that paleo circulations, occurring between 145 Ma and 110 Ma, still have an impact on current distribution of chloride in the system, especially for d37Cl. The model highlights the need of the presence of a circulation spatially limited at the base of the Liassic formation to fit the data. The best fit obtained indicated current residence time of 500 ka in the Dogger and Oxfordian, with respective onset of the circulations at 20 Ma and 5 Ma. Ó 2013 Published by Elsevier Ltd.
Keywords: 37 Cl Diffusion Advection Groundwater Pore waters
1. Introduction Chlorine is a major constituent of brines derived from ocean waters and as only limited chlorine exchange is expected by watere rock interactions with carbonates, silicates or biologic media under most surface conditions, it is considered as a conservative tracer. * Corresponding author. CENBG CNRS IN2P3, University of Bordeaux, chemin du solarium, le Haut Vigneau, BP120, F-33175 Gradignan Cedex, France. E-mail addresses:
[email protected],
[email protected] (R. Rebeix). 0264-8172/$ e see front matter Ó 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.marpetgeo.2013.12.013
Therefore, Cl concentration in groundwaters and pore waters, through the studied 2000 m depth sequence, can be an appropriate tracer for the investigation of the sources of salt and hydrogeological connexion through the aquifer and aquitard levels. Furthermore, more specific informations on salt-sources and transport processes can be obtained with chlorine stable isotopes (37Cl and 35Cl). Consequently, the major contribution of 37Cl/35Cl ratio measured on pore waters and aquifer groundwaters can enable the understanding of the physical processes controlling the transfer of the solute through a sedimentary sequence.
R. Rebeix et al. / Marine and Petroleum Geology 53 (2014) 102e116
The major sources of chlorine in subsurface aqueous systems are trapped in seawaters and evaporate formations. Although, the chloride concentration and isotopic abundances (35Cl and 37Cl) of fluids were shown to be modified by transport processes (thermal/ molecular diffusion and osmosis/reverse osmosis) and/or mixing (advection/convection) (e.g. Desaulniers et al., 1986; Phillips and Bentley, 1987; Coleman et al., 1998; Eastoe et al., 2001). This effect generally occurs during successive steps of diagenetic and post-diagenetic evolutions of clayey and calcareous formations (e.g. Soler, 2001; De Marsily et al., 2002). The diffusion process occurs in response to gradients of concentration, and as 35Cl is more mobile than 37Cl, an isotopic fractionation is expected as a result of Cl-transfer. Therefore, the Cl-rich solution becomes enriched in 37Cl and the Cl-depleted solution becomes depleted in 37Cl. Desaulniers et al. (1986) reported a decrease in Cl concentration and d37Cl in pore waters of glacial deposits with increasing distance from the underlying bedrock. They were the first to attribute the depletion in 37Cl to the upward diffusion of Cl through low permeable sediments (Eggenkamp and Coleman, 2009 and reference therein). Some later works, further explained the observed chlorine isotope patterns with molecular diffusion (Coleman et al., 2000; Hendry et al., 2000; Waber et al., 2001; Lavastre et al., 2005). Although, ion filtration, which occurs when a saline solution is forced through a charged clay membrane, can also induce enrichment in 37Cl (Phillips and Bentley, 1987). Due to its higher mobility in comparison to 37Cl, the 35Cl-isotope is more readily repelled by the negative charge on clay minerals. Only few studies explained Cl-isotopes distribution in natural system by ion filtration. Even if, this process is fewly documented for its experimental and in situ aspects, the more representative study for this last point is the work of Godon et al. (2004), which have shown the 37 Cl enrichment in pore fluids of mud volcanoes related to ion filtration processes induced by compaction of the sediments. 1.1. The Paris basin and scope of the study The Paris Basin covers about 140,000 km2, or 25% of the French country and is filled with a thick accumulation of Mesozoic and Cenozoic sediments (up to 3000 m at its centre). This accumulation was accompanied with contrasting lithologies and permeabilities, creating thus a layered structure of alternating aquifers and aquitards. The Dogger and Triassic layers, which are known to be aquifers throughout the Paris Basin, have been extensively studied for petroleum, drinking water and geothermal energy potential (e.g. Matray et al., 1989; Matray et al., 1994). Since 1996, the French national radioactive waste management agency (Andra), investigates the eastern part of the Paris basin (France) in order to assess the feasibility of radioactive waste disposal in deep geological formations. First, Andra carried out a prospective program on aquifer/aquitard system through an area covering about 2000 km2 and 700 m in depth (from the upper Dogger to the Tithonian at the surface). In this work, in situ experiments, performed in an Underground Research Laboratory (URL) built in the COx for this purpose, were undertaken in the future host rock, which is the low permeability Callovo-Oxfordian clay formation (COx thereafter) located at about 490 m depth (Andra, 2005a). Geological (e.g. Lebon and Mouroux, 1999; Delay et al., 2007a; 2007b), geomechanical (e.g. Delay et al., 2007a; 2007b and references therein) and hydrogeological (e.g. De Marsily et al., 2002; Marty et al., 2003; Lavastre et al., 2005; 2010; Gaucher et al., 2004) studies allowed to confirm the potential high confinement of the COx and then assess the feasibility of HAVL-MAVL (High Activity Long LiveeIntermediate Activity Long Live) wastes repository in this geological formation. Following the prospective studies, a geological survey was carried out over a
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250 km2 surface area, called the “transposition zone” (TZ), located to the north/north-west of the URL and presenting similar geological properties than the previously investigated area (Fig. 1). Drilling campaign investigated the sedimentary cover from Dogger to Oxfordian formations at the TZ scale, and punctually the sedimentary cover from Buntsandstein to Oxfordian down to 2000 m in depth. The final objective of this second investigation is to identify a limited area (
