A pluridisciplinary methodology for intregrated management of coastal aquifer - Geological, hydrogeological and economic studies of the Roussillon aquifer (Pyrénées-Orientales, France)

In order to study Mediterranean coastal water management, a pluridisciplinarity approach is developed. Reservoir geology and some of its tools, used in oil prospecting, are applied to build a detailed sedimentary model. The analysis of depositional environments and sedimentary process allows the correlation of pre-existing data (outcrop, borehole, and seismic profile) using Genetic stratigraphy (onshore domain) and seismic stratigraphy (offshore domain). The interpretation results in a better knowledge of the sedimentary geometries following correlations between onshore and offshore domains. It is thus possible to differentiate the coastal groundwater aquifers precisely and to establish their relative connections. At the same time, hydrogeological investigations such as hydrochemistry and geophysical prospecting allow us to elaborate the hydrogeological conceptual model of the case studies. Variable-density flow and solute transport simulations constitute the hydrogeological work. Experimental economy constitutes the third part of this integrated methodology. It assesses the effectiveness of institutional arrangements to cope with aquifer overexploitation. Feed-back from these three fields of research will also authenticate our methodology. This approach applied on Roussillon basin (South-west of French Mediterranean coastline) could be exported to many other coastal area

Hydro-socio-economic implications for water management strategies: the case of Roussillon coastal aquifer

In many Mediterranean coastal areas, agriculture, drinking water supply, tourism and industry strongly depend on the available groundwater resources. As a result of the significant economic development during the last three decades along the coast, abstractions from coastal aquifers have increased tremendously, frequently leading to overexploitation and saltwater intrusion. Geological, hydrogeological and socio-economical studies as a multidisciplinary approach on a coastal Mediterranean aquifer- the Roussillon - have been carried out in order to design effective water management strategies on areas sensitive to seawater intrusion risk. Geology provides geometry and architecture of the different aquifers, hydrogeology assess the seawater intrusion risk while socio-economic study includes consulting the stakeholders with the aim of suggesting water management and policy option acceptable to the majority of population. This paper then highlights the economic interests at stake, diversity of viewpoints expressed by stakeholders and political dimension of the issue, which are likely to be encountered for all similar situations on both sides of the Mediterranean Sea

Apport de la stratigraphie séquentielle à la gestion et à la modélisation des ressources en eau des aquifères côtiers

With the intense economic and demographic development, littoral zones have vital impact on the environment and, in particular, on the water resources. Although coastal groundwater management has many results from scientific research, one of the major problems which remains is the knowledge of the aquifers geometries. Geological interpretations of the Roussillon Plio-Quaternary basin, stemming from sequential stratigraphy, are compared, through a database, with the hydrogeology of this complex hydrosystem located in the coastal areas of Pyrénées-Orientales (France). Statistical study of the abstraction points (screen distribution, logs productivity...), functional analysis (piezometric time series analyses), hydrochemistry and electrical geophysics were used with the aim of building a hydrogeological conceptual model of the onshore / offshore basin scale flows. Sea, residual salinity areas and coastal rivers contribute to the increase in salinity of a superficial Quaternary unconfined aquifer overlying several Pliocene confined aquifers used for drinking water supply of the coastal area. Vulnerability of this good water both in terms of quality and quantity and its intrusion by saline water has been modeled. On the offshore domain, protective function of low and middle permeability layers is highlighted with respect to the safeguarding the quality of drinking water.Lieu de développement économique et démographique intense, les zones littorales font l'objet de pressions importantes sur l'environnement et, en particulier, sur les ressources en eau. Bien que la gestion des eaux souterraines côtières bénéficie de nombreux résultats issus de la recherche scientifique, une des problématiques majeures reste la connaissance de la géométrie des aquifères. Des interprétations géologiques de la genèse du bassin Plio-Quaternaire du Roussillon, issues de la stratigraphie séquentielle, sont confrontées, par l'intermédiaire d'une base données, à l'hydrogéologie de cet hydrosystème complexe localisé sur la partie littorale des Pyrénées-Orientales. L'étude statistique des points de prélèvement (distribution des crépines, productivité des forages...), l'analyse fonctionnelle (traitement du signal des chroniques piézométriques), l'hydrochimie et la géophysique électrique ont été utilisées afin d'élaborer un modèle conceptuel hydrogéologique des écoulements à l'échelle du bassin et de son prolongement vers le domaine offshore. La présence de la mer, de zones à salinité résiduelle et de cours d'eaux littoraux contribue à augmenter la salinité d'un aquifère libre supérieur (Quaternaire) sus-jacent aux différents aquifères captifs (Pliocène) exploités pour l'eau potable dans la zone littorale. La vulnérabilité face aux intrusions salines de cette ressource de bonne qualité, tant sur le point de vue quantitatif que qualitatif est appréhendée par modélisation. Dans le domaine offshore, le rôle protecteur des formations géologiques à faible et moyenne perméabilité est mis en évidence vis-à-vis de la préservation de la qualité de l'eau potable

Landslide motion observation on la Réunion Island (Indian ocean) seen by ALOS-2/PALSAR-2 based on image correlation techniques and SAR interferometry.

International audienceThe presented study consisted in processing ALOS-2 SM1 images by sub-pixel correlation and conventional differential SAR interferometry in order to assess the potential of these new data in the context of landslide deformation mapping. We selected the test site on la Réunion Island because of the presence of the well known Hell-bourg landslide, studied in the past by Remote Sensing methods, using both X, C and L-band interferometry.The study showed that even with a reduced data set, consisting of 7 images with time spans up to 120 days, ALOS-2 PALSAR data represent a unique tool for landslide displacement monitoring in the highly vegetated context of La Réunion Island. Both the L-band and the high spatial resolution (1.8m) allow better performances in the motion measurement than other SAR data such as C-band SAR, medium resolution L-band data or high resolution X-band with weeks time-span). We showed that the total deformation occurred between January 2016 and august 2014 reached up to 1.5m in azimuth and 70cm in slant range. The motion presents partitioning in different regimes. From the offset results, we can clearly delineate the trace of the scarps and the sectors with different velocity regimes. Time Series suggested us that significant motion occurred on a relatively short period between November 2014 and end January 2015. We suggest that this could be related to heavy rains occurring during the wet season.The use of L-band (or larger wavelength when available) with the higher possible resolution therefore appears as the more adapted Space-borne Earth Observation tools for monitoring of very slow to slow landslides in a tropical semi-vegetated context. In this perspective the current ALOS-2 has obvious interest, provided that we access to at least bimestrial acquisitions

Landslide displacement mapping based on ALOS-2/PALSAR-2 data using image correlation techniques and SAR interferometry: application to the Hell-Bourg landslide (Salazie Circle, La Réunion Island)

In the study described here, the capability of space-borne high-resolution L-band synthetic aperture radar (SAR) images (ALOS-2/PALSAR2 data in StripMap SM1 mode) for deriving and mapping two components of the deformation of slow landslides has been investigated. The deformation characterization was carried out on the basis of sub-pixel correlation offset tracking techniques and differential SAR interferometry. On the Hell-Bourg landslide (located on La Réunion Island, with displacements up to about 1 m/year), the deformation maps produced performed significantly better than the C-band or lower-resolution SAR data used in previous studies. A comparison was carried out with Global Navigation Satellite System data acquired on the test site. Even with a reduced image set (seven acquisitions), detailed deformation maps and information on deformation evolution during 2014–2016 could be generated. The combination of L-band data and high resolution was the source of the improved performance provided by this new kind of data

An innovative approach for very large landslide dynamic and hydrogeological triggering study by inverse modeling (Grand Ilet landslide, Reunion Island)

Landslide control mechanisms study and displacements modeling interest the scientific community since several decades, with a common objective: landslides prediction for humans and infrastructures protection. However many data acquisition, like pore water pressure or mechanical properties, are necessary for determinist model construction. It could be extremely complex for very large landslides in extreme climatic conditions. An innovative modeling method is proposed for very large landslide functioning characterization using the primary data rainfall and displacement. Here we study two very large landslides (450 Mm3) in a humid tropical climate (Salazie cirque, Reunion Island). We use an inverse modeling tool basing on a global approach, with Gaussian-exponential transfer functions. Transfer functions between the rainfall input signal and the velocity output signal (permanent GPS daily data)are determinated. Because of the gap displacement data, the hydrologic cycles 2010 and 2011 is selected for the calibration of transfer functions. Afterwards, we model the landslide velocity from rainfall signal since 2004 to 2011. In the case of Grand Ilet landslide, we study relations between transfer functions characteristics and the coupling between the displacements and the hydrogeological functioning. For cumulated displacements, final difference between simulations and observations for 7 years modeling is smaller than 5 %. Seasonal landslides velocity variations are accurately modeled during a period of 7 years. Bimodal transfer functions, with dissociation between rapid and slow impulse responses, are particularly effective for reproducing the recorded displacements. In particular, rapid response permits to model velocity increases after cyclonic events. In case of Grand Ilet landslide, transfer functions characteristics are strongly correlated with the landslide aquifer functioning. Indeed, influence times of rapid and slow responses are reliable with a double porosity effect. Rapid response is correlated with preferential flows in fractures network, whereas slow response is correlated with slow piezometric recharge of the porous medium. These results show that global approach is significant for very large landslide study, in Reunion Island context. Present method could then become a promising process for other very large landslide studies

Canne a sucre, herbicides et pollution des eaux a la reunion : bilan et perspectives au terme de la premiere decennie de suivi Sugarcane, herbicides and water pollution in reunion island: achievements and perspectives after ten year of monitoring

A La Réunion, les pesticides sont recherchés dans les eaux douces depuis 1999. Le BRGM a compilé des données de l'Office de l'eau, de l'Agence Régionale de Santé et de la Direction de l'Environnement couvrant 247 points d'eau entre mai 1999 et janvier 2010. Les recherches ont été positives sur 55% des points d'eau. Parmi les 398 substances recherchées, 72 ont été détectées et 65 quantifiées, parmi lesquelles 34 herbicides ou leurs métabolites, dont 17 liés au désherbage de la canne à sucre. Le nombre de dépassements du seuil de potabilité (0,1 μg/l) par substance s'élève à 251 pour 1811 détections, dont 1406 avérées (quantifiées). Les herbicides canne, glyphosate inclus, ont causé 82 % des dépassements. Deux anciens herbicides d'utilisation massive en canne, l'atrazine et le diuron, ont causé 80 % des pollutions avérées et 67 % des dépassements. Quelques dépassements sont dus à quatre herbicides encore en usage : ils sont testés en laboratoire sur des sols réunionnais en vue de déterminer leur potentiel de pollution diffuse dans le cadre du projet 'Transfert Phytos' porté par le BRGM Réunion

Analytical Solution for Modeling Discharge into a Tunnel Drilled in a Heterogeneous Unconfined Aquifer

International audiencePredicting transient inflow rates into a tunnel is an important issue faced by hydrogeologists. Most existing analytical solutions overestimate the initial discharge due to the assumption that drilling was instantaneous over the entire tunnel length. In addition, they assume a homogeneous system. An alternative model was recently developed for tunnels intersecting heterogeneous formations, but its application was reduced to the case of confined flow to deep tunnels in weakly diffusive aquifers. In this paper, we adapt existing analytical solutions for drainage systems to the specific case of a tunnel progressively drilled in a highly diffusive heterogeneous unconfined aquifer. The case of a tunnel overlying an impervious layer is analytically solved by applying the superposition principle, while the case of a tunnel constructed some distance above an impervious layer is solved by discretizing the tunnel length into subsectors. Both models can simulate transient discharge into a tunnel drilled at various speeds through a heterogeneous unconfined aquifer, and allow the prediction of discharge rates in shallow tunnels located in highly diffusive aquifers. We successfully applied this approach to a tunnel in heterogeneous volcanic rock

CANNE A SUCRE, HERBICIDES ET POLLUTION DES EAUX A LA REUNION : BILAN ET PERSPECTIVES AU TERME DE LA PREMIERE DECENNIE DE SUIVI

National audienceIn Reunion Island, water resources have been monitored for pesticides pollution since 1999. The BRGM, the French geological and hydrogeological survey, compiled data from three local agencies involved in water quality. The compilation included data from 247 sites monitored between May 1999 and January 2010. Positive results were found in 55% of the sites. Among the 398 substances checked, 73 were found and 65 were quantified: 35 of them were herbicides or herbicide degradates, and 17 were involved in sugarcane weeding. 1,811 results were positive, 1,407 results were quantified and 251 results had concentrations over the drinking water threshold (0.1 ppb). Sugarcane herbicides, including glyphosate, were responsible for 80% of the results over 0.1 ppb. Two sugarcane herbicides widely used in the past, atrazine and diuron, were responsible for 80 % of the quantified results and 67 % of the results over 0.1 ppb. Four herbicides currently in use have been found at concentrations over 0.1 ppb. They are being tested in laboratory by the BRGM in order to assess the pollution risks for groundwater when applied on some native soils.A La Réunion, les pesticides sont recherchés dans les eaux douces depuis 1999. Le BRGM a compilé des données de l'Office de l'eau, de l'Agence Régionale de Santé et de la Direction de l'Environnement couvrant 247 points d'eau entre mai 1999 et janvier 2010. Les recherches ont été positives sur 55% des points d'eau. Parmi les 398 substances recherchées, 73 ont été détectées et 65 quantifiées, parmi lesquelles 35 herbicides ou leurs métabolites, dont 17 liés au désherbage de la canne à sucre. Le nombre de dépassements du seuil de potabilité (0,1 µg/l) par substance s'élève à 251 pour 1811 détections, dont 1407 avérées (quantifiées). Les herbicides canne, glyphosate inclus, ont causé 80 % des dépassements. Deux anciens herbicides d'utilisation massive en canne, l'atrazine et le diuron, ont causé 80 % des pollutions avérées et 67 % des dépassements. Quelques dépassements sont dus à quatre herbicides encore en usage : ils sont testés en laboratoire sur des sols réunionnais en vue de déterminer leur potentiel de pollution diffuse dans le cadre du projet 'Transfert Phytos' porté par le BRGM Réunion. Mots-clés : ressources en eau -pollution -herbicides -canne à sucre -La Réunion