Salinisation d'un aquifère captif côtier en contexte deltaïque - Cas de la Camargue (delta du Rhône, France) -

This study deals with the salinization of confined coastal aquifers in relation with sea level variations (past, future) and human pressures. The deep confined aquifer of Camargue has been studied in the framework of the ORE RESYST. This aquifer shows high salinities reaching that of the Mediterranean Sea near the shoreline. Two approaches have been jointly carried out: (i) an hydrodynamic study in steady and transient flow to understand the hydrogeological behaviour of the aquifer (flows, boundary conditions, hydrodynamic properties) and (ii) a monitoring of hydrochemical and isotopic natural tracers (18O, 2H, 13C TDIC, 3H and 14C TDIC) to determine the origin of the salinity of the aquifer. This work highlights the influence of the unconfined part of the aquifer (Crau aquifer) on the recharge of the confined aquifer. Thus, the high increase of salinity in the aquifer since 1969 is mainly due to natural or anthropogenic head depletion within the Crau aquifer. The confined aquifer shows rapid pressure transfers both from the Sea and the Rhone River whereas mass transfers are very low. Ionic and isotopic ratios highlight that strong salinities of the aquifer are due to a freshwater/seawater mixing, which is highly modified by geochemical interactions: cationic exchange, sedimentary organic matter degradation (sulphate reduction, methanogenesis), precipitation phenomena. The mean residence time of water is higher than fifty years and reflects the influence of a former marine intrusion related to the last marine transgression (upper Holocene).L'objectif de ce travail est de préciser les processus responsables de la salinisation des aquifères côtiers captifs en réponse aux variations du niveau marin (passées, futures) et aux pressions anthropiques. Cette étude a été réalisée sur l'aquifère profond captif de Camargue dans le cadre de l'ORE RESYST. Cet aquifère présente des salinités élevées atteignant celle de la Méditerranée à proximité de la côte. Deux approches ont été menées conjointement : (i) une étude hydrodynamique en régime permanent et transitoire pour préciser le fonctionnement hydrogéologique de l'aquifère (flux, conditions aux limites, propriétés hydrodynamiques) et (ii) un suivi des traceurs naturels hydrochimiques et isotopiques (18O, 2H, 13C CMTD, 3H et 14C CMTD) permettant de déterminer l'origine de la salinité de l'aquifère. Ce travail a mis en évidence le rôle de la partie libre de l'aquifère (aquifère de Crau) dans l'alimentation de l'aquifère captif. La forte augmentation de la salinité de l'aquifère captif depuis 1969 est ainsi principalement reliée à des baisses de charge naturelles ou anthropiques de l'aquifère de Crau. L'aquifère captif est soumis à des transferts de pression rapide depuis la mer mais également le Rhône qui s'accompagnent de transferts de masse très lents. En s'appuyant sur les rapports ioniques et isotopiques, ce travail a permis de démontrer que les fortes salinités de l'aquifère captif s'expliquent par un mélange eau douce/eau de mer fortement modifié par des interactions géochimiques : échanges de bases, processus de dégradation de la matière organique sédimentaire (sulfato-réduction, méthanogénèse), phénomènes de précipitations. Le temps moyen de séjour des eaux, supérieur à une cinquantaine d'année, témoigne de l'influence d'une intrusion marine ancienne en relation avec la dernière transgression marine (Holocène supérieur)

Salinisation d'un aquifère captif côtier en contexte deltaïque (cas de la Camargue (delta du Rhône, France))

L'objectif de ce travail est de préciser les processus responsables de la salinisation des aquifères côtiers captifs en réponse aux variations du niveau marin (passées, futures) et aux pressions anthropiques. Cette étude a été réalisée sur l'aquifère profond captif de Camargue dans le cadre de l'ORE RESYST. Cet aquifère présente des salinités élevées atteignant celle de la Méditerranée à proximité de la côte. Deux approches ont été menées conjointement : (i) une étude hydrodynamique en régime permanent et transitoire pour préciser le fonctionnement hydrogéologique de l'aquifère (flux, conditions aux limites, propriétés hydrodynamiques) et (ii) un suivi des traceurs naturels hydrochimiques et isotopiques (18O, 2H, 13CCMTD, 3H et 14CCMTD) permettant de déterminer l origine de la salinité de l'aquifère. Ce travail a mis en évidence le rôle de la partie libre de l aquifère (aquifère de Crau) dans l'alimentation de l'aquifère captif. La forte augmentation de la salinité de l'aquifère captif depuis 1969 est ainsi principalement reliée à des baisses de charge naturelles ou anthropiques de l'aquifère de Crau. L'aquifère captif est soumis à des transferts de pression rapide depuis la mer mais également le Rhône qui s'accompagnent de transferts de masse très lents. En s'appuyant sur les rapports ioniques et isotopiques, ce travail a permis de démontrer que les fortes salinités de l'aquifère captif s'expliquent par un mélange eau douce/eau de mer fortement modifié par des interactions géochimiques : échanges de bases, processus de dégradation de la matière organique sédimentaire (sulfato-réduction, méthanogénèse), phénomènes de précipitations. Le temps moyen de séjour des eaux, supérieur à une cinquantaine d année, témoigne de l'influence d'une intrusion marine ancienne en relation avec la dernière transgression marine (Holocène supérieur)This study deals with the salinization of confined coastal aquifers in relation with sea level variations (past, future) and human pressures. The deep confined aquifer of Camargue has been studied in the framework of the ORE RESYST. This aquifer shows high salinities reaching that of the Mediterranean Sea near the shoreline. Two approaches have been jointly carried out: (i) an hydrodynamic study in steady and transient flow to understand the hydrogeological behaviour of the aquifer (flows, boundary conditions, hydrodynamic properties) and (ii) a monitoring of hydrochemical and isotopic natural tracers (18O, 2H, 13CCMTD, 3H and 14CCMTD) to determine the origin of the salinity of the aquifer. This work highlights the influence of the unconfined part of the aquifer (Crau aquifer) on the recharge of the confined aquifer. Thus, the high increase of salinity in the aquifer since 1969 is mainly due to natural or anthropogenic head depletion within the Crau aquifer. The confined aquifer shows rapid pressure transfers both from the Sea and the Rhone River whereas mass transfers are very low. Ionic and isotopic ratios highlight that strong salinities of the aquifer are due to a freshwater/seawater mixing, which is highly modified by geochemical interactions: cationic exchange, sedimentary organic matter degradation (sulphate reduction, methanogenesis), precipitation phenomena. The mean residence time of water is higher than fifty years and reflects the influence of a former marine intrusion related to the last marine transgression (upper Holocene)AVIGNON-BU Centrale (840072102) / SudocSudocFranceF

Processes governing the supply of iron to phytoplankton in the Southern Ocean: a model study

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Recharge area of the Umbulan spring on the basis of the geology, hydrochemistry and isotopic approach, a high discharge spring of the Bromo-Tengger volcano, East Java

International audienceHydrogeochemical analysis and geological mapping, together with water Isotopes analysis, were performed to identify the recharge area of Umbulan spring, a high discharge spring located in the Bromo-Tengger volcano. The volcanic edifice, situated in a tropical climatic context, is the origin of exceptionally high discharge springs in such a volcanic context. This is the case of Umbulan spring whose discharge is about 3500 l/s that supply drinking water to the city of Surabaya, the second biggest city of Indonesia. Groundwater flows through fractured/weathered andesitic lava flow and pyroclastic deposits. The main groundwater outlet corresponds to gravity springs on the flanks of the volcano and to artesian springs in the plain. To improve the hydrogeological knowledge of the study area, the geological mapping of the North volcano flank has been performed to identify the aquiferous formations and refine the geological limits defined in the literature. Based on this geological survey, a new geological map was proposed. Water samples of gravity springs, artesian springs and deep wells were collected with elevations ranging from 40 to 2700 m above sea level, for water major ions elements and stable isotope (δ18O, δD). The meteoric local gradient of δ18O is assessed from the water isotope contents of springs, which are considered as “local pluviometer” representative of the precipitation isotope content at a given elevation corresponding to the mean elevation of their recharge catchment. Based on the analysis of the meteoric local gradient of δ18O, the mean elevation of Umbulan spring recharge catchment ranges between 700 to 1300 m a.s.l, which in agreement with geological observations. Many interrogations subsist but these first hydrogeological data collected in the field allowed to propose a first conceptual model of the Bromo-Tengger volcano, which will help improving the hydrogeological knowledge of the study area and thus preserve and manage the groundwater resource of Bromo-Tengger volcano

Water table variations in the hyperarid Atacama Desert: Role of the increasing groundwater extraction in the pampa del tamarugal (Northern Chile)

In the hyperarid Atacama Desert (Northern Chile), the economic and social development is supported using fossil groundwater. The groundwater extraction (GWE)has significantly increased over the last 30 years, reaching ∼4.2 m3.s−1 in 2018 (+1890%)at the Pampa del Tamarugal Aquifer (PTA). But opposite assumptions lead to uncertainties concerning the role of the increasing anthropogenic pressures and the ephemeral recharge events in the water table (WT)variations. This paper analyzes: (i)the long-term groundwater levels changes between the late 1950s (post Saltpeter Work)and the early 2010s, and (ii)the short-term response of groundwater levels, based on the analysis of the 1998–2018 WT time series at 10 observation boreholes. Results indicate that the WT variations in space and time are strongly related to the anthropogenic pressure changes. Since the late 1950s, the WT is declining in the major part of the PTA. Nevertheless, local reduction of GWE together with ephemeral recharge events in alluvial fans allowed local WT rises. But after a large GWE increase (+114%)between 2004 and 2006, all observation boreholes highlight a general WT decline (−9.8 ± 5.8 cm.yr−1). Over the years, anthropogenic pressures became the dominant factor of the WT variations and led to overuse the aquifer

Hydrochemical characterization of a complex Mediterranean karstic system (Thau lagoon area, Montpellier, France) for the regional management of the groundwater resources

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Characterization and Origin of karstic thermal waters of the Northeast of Algeria.

International audienceThe northeastern part of Algeria has numerous thermal karstic springs which present an economicalinterest for the national office of the Algerian tourism. This organization considers the setting-up ofinfrastructures dedicated to the thermal tourism. The springs are located in a region of a 535 000 kmCwhich extends from Bejaïa to El-Kala (Tunisian border). The geological context is marked by ageological structure inherited of the alpine tectonic episodes Priabonia n to Tortonian at the origin oflarge faults and thrust systems. An analysis of the hydrochemical char acteristics of these waters, hasbe realised, at the regional scale, mainly to determine the geotectonic factors being the cause of thechemical facies and rising, but also to identify one or several deep geothermal reservoirs. Twosampling campaigns were realized on 52 sites, one during low waters (September, 2014) and theother during high waters (May, 2015). The analysis concerned major and trace elements, and isotopes(O18, H2, H3, Sr87 Sr86). From binary diagrams and from ACP treatments, results allowed todiscriminate 4 geochemical groups of waters. (1) a HCO3-Ca type, slightly mineralized andcharacterizing hypothermal waters with fast circulations, (2) a HCO3-Na type, more mineralized andcharacterizing mesothermal waters with deeper circulations, marked by phenomena of base exchangebetween water and clay levels, (3) a Cl-Na type, characterizing hyperthermal waters with deepcirculations attested by chemical exchanges with sedimentary evaporite levels, finally (4) a SO4-Catype having acquired its mineralization in triassic formations. Furthermore, a high percentage (75%) ofhyper or mesothermal springs, with chloride or sulphate facies, are located along the large regionalfaults identified by geophysical investigation. These results will be clarified by the use of silicium andalkaline geothermometers and temperature estimation using saturation index

Spatial distribution of the iron supply to phytoplankton in the Southern Ocean: a model study

An upgraded version of the biogeochemical model SWAMCO is coupled to the ocean-sea-ice model NEMO-LIM to explore processes governing the spatial distribution of the iron supply to phytoplankton in the Southern Ocean. The 3-D NEMO-LIM-SWAMCO model is implemented in the ocean domain south of latitude 30° S and runs are performed over September 1989–December 2000. Model scenarios include potential iron sources (atmospheric deposition, iceberg calving and continental sediments) as well as iron storage within sea ice, all formulated based on a literature review. When all these processes are included, the simulated iron profiles and phytoplankton bloom distributions show satisfactory agreement with observations. Analysis of simulations points to the key role played by continental sediments as a primary source for iron. Iceberg calving and melting contribute by up to 25% of Chl a simulated in areas under influence of icebergs while atmospheric deposition has little effect at high latitudes. Activating sea ice-ocean iron exchanges redistribute iron geographically. Stored in the ice during winter formation, iron is then transported due to ice motion and is released and made available to phytoplankton during summer melt, in the vicinity of the marginal ice zones. Transient iron storage and transport associated with sea ice dynamics stimulate summer phytoplankton blooming (up to 3 mg Chl a m−3) in the Weddell Sea and off East Antarctica but not in the Ross, Bellingshausen and Amundsen Seas. This contrasted feature results from the simulated variable content of iron in sea ice and release of melting ice showing higher ice-ocean iron fluxes in the continental shelves of the Weddell and Ross Seas than in the Eastern Weddell Sea and the Bellingshausen-Amundsen Seas. This study confirms that iron sources and transport in the Southern Ocean likely provide important mechanisms in the geographical development of phytoplankton blooms and associated ecosystems.info:eu-repo/semantics/publishe

Hydraulic and geochemical impact of occasional saltwater intrusions through a submarine spring in a karstic and thermal aquifer (Thau lagoon area, Montpellier, France).

International audienceSubmarine springs are a common discharge feature of the karst aquifers along the Mediterranean coast. In some instances, occasional and localized saltwater intrusions can occur through the submarine spring and negatively impact the quality of the groundwater resource. The hydraulic and geochemical behaviour of a submarine spring discharging into the Thau Lagoon just offshore of the Balaruc peninsula near Montpellier, France, has been characterized to determine the impacts of such phenomena to better understand the dynamics of a regional karst aquifer and improve its groundwater management. This work is based on both historical and new hydrogeological and geochemical data, illustrating six occasional saltwater intrusion events (from 1967 to 2014) in the Thau Lagoon area (southern France).Hydraulic perturbation of the aquifer is propagated instantly within the Balaruc-les-Bains peninsula and reaches a distance of about 5 km upgradient within 9 d. Comparison of hydraulic heads during seawater intrusion events in 2010 and 2014 indicates an aggravation of the phenomenon with an increase in hydraulic head variations.In contrast, isotopic tracers (87Sr/86Sr, D/H, 18O/16O) and rare earth elements (REEs) demonstrate that the geochemical impact of these inversac events is only observed at the local scale but is still perceptible several years after the event. For example, some of the thermal wells had not recovered their initial geochemical state 20 and 40 months after the last two inversac events (2010 and 2014, respectively), suggesting a geochemical legacy of this phenomenon within the complex karst system. By contrast, an adjacent deep karst compartment located south of the study area is not affected by the saltwater intrusion and is characterized by distinctly different hydrodynamic behaviour.Overall, this work on occasional and localized saltwater intrusions constitutes a key step in understanding the dynamics of this complex karstic and thermal aquifer and will support the management of the groundwater resource

A lumped parameter model to evaluate the relevance of excess air as a tracer of exchanged flows between transmissive and capacitive compartments of karst systems

International audienceThe objective of this study was to evaluate the relevance of using excess air (EA) for the characterization of drain/matrix exchange in karst systems using a rainfall discharge model coupled with the simulation of EA measured at the outlet of the studied system. The conceptual model assumes a linear relationship between the formation of EA and the increase of hydrostatic pressure in the capacitive part of the aquifer. The simulated EA at the spring consists of the mixing of water circulating in the different compartments of the aquifer, with their own EA signature. The analysis is performed taking as an example the Durzon karst system (Larzac, France). The modeling is applied using daily rainfall discharge time series and 18 EA measurements at the main outlet of the karst system within 3 hydrological cycles. The main modeling results show that EA variations measured at the karst spring can be explained by recharge processes and exchange between conduit and matrix. EA measurements at the spring thus contain valuable information about the flow dynamics within the aquifer. Furthermore, results show that the use of EA measurements, despite their sparse temporal resolution, allows for reducing uncertainties in the estimation of some parameters of the reservoir model used for the simulation of karst spring discharge