Integrated Onshore-Offshore Investigation of a Mediterranean Layered Coastal Aquifer by Johanna Lofi1, Philippe Pezard2,Fred´ eric´ Bouchette2, Olivier Raynal2,3, Pierre Sabatier2,4, Nataliya Denchik2, Arnaud Levannier5, Laurent Dezileau2, and Raphael¨ Certain3 Abstract Most of the Mediterranean coastal porous aquifers are intensively exploited. Because of climatic and anthropogenic effects, understanding the physical and geological controls on groundwater distribution and flow dynamics in such aquifers is crucial. This study presents the results of a structural investigation of a system located along the coastline of the Gulf of Lions (NW Mediterranean). A key aspect of this study relies on an onshore-offshore integrated approach combining outcrops, seismic profiles, and borehole data analysis. This multidisciplinary approach provides constraints on pore-fluid salinity distribution and stratigraphic organization, which are crucial in assessing the modes of groundwater/seawater exchanges. Onshore, Lower Pliocene deposits dip gently seaward. They are unconformably overlain by Holocene clays in the lagoons. Offshore the Pliocene deposits either outcrop at the seabed or are buried below nonconsolidated sands infilling paleo-valleys. Beneath the lido, the groundwater salinity distribution consists of salty pore water, overlying fresher pore water. Active circulation of groundwater masses is inferred from the geophysical results. In particular, offshore outcrops and paleo-valleys may play an important role in salt water intrusion. Introduction and to predict seawater intrusion (Bear et al. 1999; Bar- Coastal aquifer structure and dynamics have been low 2003). In most porous coastal aquifers, fresh water studied for many decades for groundwater resources, pri- flows seaward. However, close to the coast, due to fluid marily with the aim to assess fresh groundwater reserves density differences generating density flows, seawater with higher density penetrates into the aquifer, below the fresh water. This forms an inclined transition zone (TZ) 1Corresponding author: Geosciences´ Montpellier—UMR where salt water/fresh water intermix. In natural condi- 5243—Bat. 22, Universite´ de Montpellier 2, Place E. Bataillon, tions, the geometry of this mixing zone (MZ) is related 34095 Montpellier Cedex 05, France; +33-4-67-14-93-09; fax: +33-4-67-14-32-44; johanna.lofi@gm.univ-montp2.fr to the hydrological properties of the aquifer, the physical 2Geosciences´ Montpellier—UMR 5243—Bat. 22, Universitede´ properties of both fluids (Henry 1964; Voss and Souza Montpellier 2, Place E. Bataillon, 34095 Montpellier Cedex 05, 1987; Croucher and O’Sullivan 1995), the geometry of France. the aquifer (Abarca et al. 2007), and/or the type of tides 3CEFREM, Universite´ de Perpignan via Domitia, 52 Avenue Paul Alduy, 66860 Perpignan Cedex, France. (Brovelli et al. 2007). The geometry and the extent of the 4Universite´ de Savoie, Laboratoire Environnement Dynamiques MZ will also depend of the degree of heterogeneity of et Territoire de Montagne, CNRS, UMR 5204, Le Bourget du Lac, the aquifer (Dagan and Zeitoun 1998; Held et al. 2005). France. As a result, saline and brackish groundwater can be found 5 Schlumberger Water Services, P.O. Box 553, 2600AN Delft, far onshore in coastal plains, and, conversely, fresh and The Netherlands. Received November 2011, accepted October 2012. moderately brackish groundwater of meteoric origin can © 2012, The Author(s) extend offshore (Hathaway et al. 1979; Groen et al. 2000; Ground Water © 2012, National Ground Water Association. Kooi and Groen 2003). The anthropogenic influence on doi: 10.1111/j.1745-6584.2012.01011.x salinity patterns, such as inland aquifer pumping resulting NGWA.org GROUND WATER 1 in seawater intrusion, has received by far the greatest Geological Context attention in hydro-geological studies (Bear et al. 1999). The study area is located along the coast of the Gulf However, salinity distribution relies strongly on the inter- of Lions’ passive margin, in the NW Mediterranean basin action between groundwater fluid dynamics and a geolog- (Figure 1). On this margin, the history of the coastal ical environment (commonly complex and anisotropic). porous aquifers is closely related to the Messinian Salinity The spatial distribution at several scales of porosity, Crisis event (MSC; Hsu¨ et al. 1973) which occurred hydraulic conductivity, and other hydraulic parameters approximately 5.5 Ma ago and exposed the Miocene in the subsurface are thus of major importance, and are margin to subaerial erosion (Lofi et al. 2005). This strongly related to geological context and depositional created deep valleys upstream (Clauzon 1973) and huge processes involved at geological time scales. Limited accommodation space for subsequent PQ sedimentation work has been done to understand the effect of geologic (Lofi et al. 2003; Duvail et al. 2005). heterogeneity on the exchange of groundwater and sea- The PQ sediments supplied from the Rhoneˆ and adja- water through coastal sediments, particularly offshore. In cent coastal rivers successively caused the progradation addition, beside modeling and geochemical studies per- of well-developed sedimentary wedges, building a new formed near submarine seeps or from borehole samples, continental shelf (Figure 1c). The current location of the there is little published about offshore aquifers. Geophys- onshore PQ depocenters is beneath the modern flat coastal ical investigations, generally carried out using seismic plains (see RCP, HCP, and RhCP in Figure 1a). They methods, allow the identification of subsurface geometries contain fresh groundwater resources enclosed in porous or active vent features (Kindinger et al. 1999; Swarzenski siliciclastic aquifers (Laurent 1993; Ambert et al. 1998; et al. 2001; Mulligan et al. 2007). Other techniques, such Dorfliger¨ 2003; Duvail et al. 2005; Aunay et al. 2006; as electrical tomography (Andersen et al. 2007) and air- Aunay 2007). These aquifers are associated with the borne or seafloor electromagnetics have proven valuable coarse-grained fraction of the PQ wedges. for evaluating offshore pore water salinities or distribu- In this context, our study focuses on the Maguelone tions (Hoefel and Evans 2001; Teatini et al. 2011). Studies area located on the northernmost part of the margin, using these techniques are rare and seldom combined with between the Herault´ and the Rhoneˆ coastal plains onshore studies (Evans and Lizarralde 2011; Teatini et al. (Figure 1b). The northern limit of the area is the Mesozoic 2011). Defining an accurate aquifer structure and het- limestone of the La Gardiole hill (∼200 m high; Figure 2a erogeneity both onshore and offshore therefore remains and 2b). As shown in Figure 2b, outcrops located at of primary importance for providing coherent conceptual the foot of the La Gardiole hill (gray) consist either of geological models which can lead to better predictions of Holocene (green) or Pliocene (Upl, yellow) formations. preferential flow pathways (Bowling et al. 2005; Cohen The latter also outcrop beneath the city of Villeneuve- et al. 2010). les-Maguelone` (VMc) and along the northern edge of Most of the Mediterranean coastal porous aquifers the Vic lagoon. Within the study area, the lagoons have are located in the Plio-Quaternary (PQ) deposits (Clau- been partially filled with Late-Holocene deposits with low zon 1973; Duvail et al. 2005; Boughriba et al. 2006; hydraulic conductivity (mainly clays and silts; Sabatier Aunay 2007; El Yaouti et al. 2009; Kouzana et al. 2010). et al. 2010) and have a water depth <2 m. They are These aquifers are “coastal detritic formations,” following limited southward by a barrier island (Maguelone lido) on the Custodio (2010) classification for European aquifers. which an experimental site has been located (Figure 2c, Understanding groundwater distribution and flows in see also “Data and Methods” Section). The Lez and such aquifers is of prime importance since in many Mosson rivers flow eastward of the study area (Figure 2a Mediterranean coastal areas, agriculture, drinking water and 2b). supply, tourism, and industry strongly depend on the available groundwater resources. Understanding physi- cal and geological controls on groundwater distribution Data and Methods and flow dynamics is thus critical and essential in order Studying the subsurface structure of the study area to anticipate salt water intrusion and to design ade- at several scales enables aquifer heterogeneity to be quate warning systems and water management tools. In assessed. Information on regional lithology and geometry such a context, this study focuses on a Mediterranean is provided by geological mapping (BRGM 1996). PQ porous system, set apart from the axis of the main Previous field works around VMc enabled interpretation coastal plains. We provide a careful integration, at several of sedimentary facies alongside dip measurements of scales, of various geological, hydrological, and geophys- outcropping strata (BRGM 1996; Raynal et al. 2010). In ical datasets to present a coherent image of subsurface this study, subsurface structure beneath the Maguelone aquifer structure, along an onshore-offshore cross section. lido has been characterized from nine boreholes (12 to Present-day hydrological processes are a significant con- 80 m deep) drilled in the framework of the ALIANCE, tribution to the observed distribution of groundwater MUSTANG, and ATIP PROGELAC projects (location in resources, and this emphasizes the importance of the off- Figure 2c).