Variabilité spatio-temporelle du fonctionnement d'un aquifère karstique du Dogger (suivis hydrodynamiques et géochimiques multifréquences ; traitement du signal des réponses physiques et géochimiques)

Les aquifères karstiques sont parmi les milieux souterrains les plus difficiles à caractériser. Ils sont connus pour avoir des réponses complexes aux événements pluvieux en raison de l hétérogénéité de la porosité allant de la porosité matricielle (micro-porosité) à la porosité de conduits (macro-porosité). La porosité de conduits génère des vitesses de transfert élevées au sein de l aquifère, à l'origine de la vulnérabilité des aquifères karstiques. L'objectif de cette thèse est de définir les modalités du transfert de l'eau et des solutés au sein d'un aquifère karstique du Dogger situé en Bourgogne. Deux approches complémentaires ont été utilisées: (i) l'analyse de chroniques piézométriques et débitmétriques à partir d'outils de traitement du signal et (ii) une caractérisation géochimique et isotopique des eaux souterraines basée sur l'utilisation d'une large palette de traceurs, notamment des traceurs de datation. Ces deux approches ont été appliquées à différentes échelles temporelles: un suivi haute fréquence (jusqu'à 1h) lors de pics de crue pour étudier les transferts rapides et un suivi à long terme (pluri-annuel) pour caractériser les transferts lents et étudier la variabilité saisonnière des modalités d'écoulement. Ce travail a permis de mettre en évidence une très large distribution des temps de résidence de l eau souterraine, de quelques heures, lors d événements de crue souligné par des temps de réponse pluie-niveau piézométrique faible (inférieurs à 70 h) à quelques années pour l écoulement de base mis en évidence par l utilisation de traceurs de datation (39Kr, CFC-12 et SF6). En réponse aux précipitations, différentes masses d'eau ont pu être décrites: (i) une eau à vitesse d écoulement lente, qui correspond à l écoulement de base mis en évidence par l'utilisation des traceurs de datation; (ii) une eau de recharge rapide dont le temps de résidence au sein de l'aquifère varie, selon les porosités empruntées, de quelques heures à quelques jours soulignés grâce au suivi en continu de la conductivité électrique, des éléments majeurs et du tritium; et (iii) des eaux stockées au sein de la zone non saturée, remobilisées lors de pics de crue mises en évidence par la variation des compositions géochimiques en tritium et Ca2+ des eaux souterraines lors de pics de crue. Malgré une forte hétérogénéité des processus d'écoulement, une logique spatiale des écoulements rapides apparaît. Le temps de réponse du niveau piézométrique à un événement pluvieux et le temps de résidence augmentent vers l aval du dôme hydrogéologique. L'étude temporelle de la réponse impulsionnelle par la méthode du corrélogramme croisé coulissant montre que le temps de réponse varie de façon saisonnière, et est plus court en été. La variation du temps de réponse pour un forage entre été et hiver est significative et peut atteindre une centaine d heures. Cette variabilité temporelle s'explique en partie par une variabilité de l'intensité des pluies (plus importantes en été) qui induit une variation des mécanismes d'écoulement au sein de l'épikarst. Lors d'événements de pluie intense, la saturation de l'épikarst est plus importante provocant des transferts latéraux en son sein et permettant de transférer les eaux vers les conduits les plus ouverts.Karst aquifers are among the most difficult to characterize. They are known for their complex hydrological responses to rainfall events due to the high heterogeneity of their porosity, going from matrix conduit porosity. The porosity of conduit induces a high transfer velocity in the aquifer which is the cause of the karst aquifer vulnerability. The objective of this study is to define the modes of water transfer and solute in a karst aquifer of Dogger in Burgundy. Two complementary methods are used: (i) the analyse of piezometric and debitmetric time series, and (ii) the geochemical and isotopic characterization of groundwater based on the using of large panel of tracers, notably some dating tracers. These two methods are applied at different temporal scales: a high frequency monitoring (1 hour time step) during discharge event to study fast water transfer and a long time monitoring to characterize slow water transfer and study the seasonal variability of water transfer processes. This study highlights a large distribution of residence time of groundwater, from several hours during a recharge event, underlined by short response time rainfall-piezometric level (less to 70 hours) to several years for the baseflow showed by the using of dating tracers (39Kr, CFC-12 et SF6). In response to rainfall events, several water bodies are described. The first one, with slow transfer velocity, corresponds to the baseflow and is showed by dating tracers. The second one, with a direct recharge and a residence time in the karst aquifer that varies with the porosity from several hours to several days, is described by a continous monitoring of electrical conductivity, majors elements and tritium. Finally, the water stored in the unsaturated zone, remobilized during recharge events, is revealed by the variation of geochemical composition in tritium and Ca2+ in groundwater during recharge events. Despite of the high heterogeneity of water transfer processes, the fast water transfer is spatially organized. The response time of piezometric level to rainfall events and the residence time increase toward the hydrogeological downhill. The temporal study of the impulse response using the method of the sliding-window cross-correlation shows that the response time varies seasonally, being shorter during the summer. The variability of the response time is significant, up to a hundred hours. This temporal variability could be explained in part by a variability of the rainfall intensity which induces a variability of water flow processes in the epikarst. During intensive rainfall, the saturation of the epikarst is higher which induces lateral transfers and allows transferring water toward large conduits.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF

Inter-comparison exercises on dissolved gases for groundwater dating - (1) Goals of the exercise and site choice, validation of the sampling strategy

International audienceTwo international inter-comparison exercises devoted to dissolved gases and isotope analyses in groundwater, used as tools for groundwater dating were organized in 2012 in France (IDES- Université Paris Sud - CNRS and OSUR - Université Rennes 1- CNRS). The goal was to compare sampling and analytical protocols through results obtained by the community of groundwater dating laboratories. The two exercises were: GDAT1 on three supply boreholes in a homogeneous sand-aquifer of Fontainebleau (Paris Basin, France) and GDAT2 on two supply boreholes (shallow and deep) in a fractured rock aquifer in French Brittany. This twostep exercise is the first exercise which included a large number of gases and isotopes usually used in groundwater as dating tools and also permit to discuss the uncertainties related to sampling protocols issuing from each laboratory methods. The two tests allowed 31 Laboratories from 14 countries to compare their protocols for both sampling and analyses. This paper presents the participants and parameters measured, and focuses on the validation of the sampling strategy. Two laboratories analyzed CFC and SF6 samples collected at regular intervals during the sampling operations in order to verify water homogeneity. The results obtained by the two "reference" laboratories along with monitoring of field parameters showed no clear trend of gas concentration or physic-chemical properties. It can be concluded that the pumped groundwater composition remained constant during sampling. This study also shows the potential for relatively constant pumped groundwater composition from a specific well despite the complexity and/or mixing processes that may occur at a larger scale in the aquifer

Comparing carbonate and organic AMS- (super 14) C ages in Lake Abiyata sediments (Ethiopia); hydrochemistry and paleoenvironmental implications.

Le Laboratoire de Géologie de la Matière Organique est intégré dans l'Institut des Sciences de la Terre d'Orléans - ISTO - CNRS Université d'OrléansWe studied a 12.6-m-long sequence from Lake Abiyata (Central Ethiopia) to establish a reliable and accurate chronology for use in global paleoclimatic reconstructions. The 26 accelerator mass spectrometry radiocarbon (AMS 14C) ages, performed on carbonates and organic matter, define 2 parallel chronologies, representing the complete Holocene period. However, these chronologies show a significant discrepancy from 500 to 900 BP in depth; ages obtained on carbonates were always older than those on organic matter. The hydrogeological and geochemical behavior of the Lake Abiyata basin has shed light on this discrepancy. We found that the carbonate crystallization is due mainly to the mixing of lake waters with groundwaters from the multi-layered aquifer contained in the 600-m-thick basement of the lake. The 14C activity of total dissolved inorganic carbon (TDIC) measured by AMS from bottom and surface lake waters (111.4 and 111.8 pMC, respectively) confirms that the mixing occurs at the water-sediment interface. This evidence of groundwater participation in the carbonate crystallization calls into question the current paleoclimatic reconstructions based on inorganic carbonates in lakes. Specific attention should thus be given to the respective proportions of each end-member in the mixing for the quantitative estimation of the groundwater input. This will help to validate the paleoenvironmental reconstructions and to highlight an eventual diagenetical evolution of inorganic carbonates during burial, via the study of pore waters

A New Technique to Determine the Phosphate Oxygen Isotope Composition of Freshwater Samples at Low Ambient Phosphate Concentration

The oxygen isotope composition of dissolved inorganic phosphate (d18Op) offers new opportunities to understand the sources and the fate of phosphorus (P) in freshwater ecosystems. However, current analytical protocols for determining d18Op are unable to generate reliable data for samples in which ambient P concentrations are extremely low, precisely the systems in which d18Op may provide new and important insights into the biogeochemistry of P. In this paper, we report the development, testing and initial application of a new technique that enables d18Op analysis to be extended into such ecosystems. The Twist Spinning Mode (TSM) protocol described here enables >1000 L of sample with a P concentration <0.016 mg P L-1 to be initially processed within the field in approximately 24 hours. Combined with a new freeze-drying method to maximise the yield and minimise the contamination of silver phosphate generated for isotope ratio mass spectrometry, the TSM protocol is able to generate accurate and precise d18Op data. We evaluated the TSM protocol using synthetic test solutions and subsequently applied the protocol to samples from locations around the Saint-Lawrence River in Montreal, Canada. We believe that the novel technique reported here offers the methodological basis for researchers to extend the application of d18Op into a much wider range of freshwater ecosystems than has been possible to date

A portrait of wellbore leakage in northeastern British Columbia, Canada

Oil and gas well leakage is of public concern primarily due to the perceived risks of aquifer contamination and greenhouse gas (GHG) emissions. This study examined well leakage data from the British Columbia Oil and Gas Commission (BC OGC) to identify leakage pathways and initially quantify incident rates of leakage and GHG emissions from leaking wells. Three types of leakage are distinguished: “surface casing vent flow” (SCVF), “outside the surface casing leakage” (OSCL), and “cap leakage” (CL). In British Columbia (BC), the majority of reported incidents involve SCVF of gases, which does not pose a risk of aquifer contamination but does contribute to GHG emissions. Reported liquid leakage of brines and hydrocarbons is rarer. OSCL and CL of gas are more serious problems due to the risk of long-term leakage from abandoned wells; some were reported to be leaking gas several decades after they were permanently abandoned. According to the requirements of provincial regulation, 21,525 have been tested for leakage. In total, 2,329 wells in BC have had reported leakage during the lifetime of the well. This represents 10.8% of all wells in the assumed test population. However, it seems likely that wells drilled and/or abandoned before 2010 have unreported leakage. In BC, the total GHG emission from gas SCVF is estimated to reach about 75,000 t/y based on the existing inventory calculation; however, this number is likely higher due to underreporting

Identification des circulations souterraines dans le cas de deux aquifères sableux (Sables de Fontainebleau(78- Yvelines) et Sables de l'Astien (34- Hérault))

ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Pore water isotope fingerprints to understand the spatiotemporal groundwater recharge variability in ungauged watersheds

International audienc

Etude des modes d'acquisition de la signature isotopique en carbone des eaux souterraines et préservation du signal environnemental lors de la recharge des aquifères

La gestion des ressources en eau souterraine consiste en partie à l estimation du temps de séjour de l eau dans l aquifère. La décroissance radioactive du 14C au cours du temps est une méthode couramment utilisée. Cependant, elle repose notamment sur l estimation de l activité 14C (A14C) initiale des eaux souterraines. L eau se minéralise en carbone lors de son transfert à travers la zone non saturée (ZNS) par interaction avec le CO2 et la matrice carbonatée, interactions qui font l objet de cette étude. Un équipement spécifique au prélèvement du gaz, de l eau et de la matrice a été mis en place sur deux sites d étude en France, les sables de Fontainebleau non carbonatés (Yvelines) et les sables astiens carbonatés (Hérault). Les mesures in-situ sont couplées à une approche numérique afin de mieux contraindre les différents facteurs à l origine de la composition isotopique du CO2 à la base de la ZNS. En l absence de carbonates, le 13C du CO2 dépend du 13C moyen du CO2 produit, des quantités de CO2 produit et du coefficient de diffusion du CO2. Les variations saisonnières du 13C du CO2 en sub-surface tendent à disparaître en profondeur. Avant 1950, l A14C du CO2 peut être considérée égale à celle du CO2 atmosphérique. Après 1950, suite aux essais nucléaires atmosphériques, elle est intermédiaire entre celle du CO2 atmosphérique et celle de la matière organique à taux de renouvellement rapide. En présence de carbonates, le 13C du CO2 augmente (l A14C diminue) avec la profondeur. Cette évolution dépend du flux de précipitation/dissolution des carbonates, des quantités de CO2 produit et du coefficient de diffusion du CO2 dans la ZNS.The management of groundwater needs the estimation of the water residence time in aquifers, that can be done through the measurement of 14C radioactive decay of the Total Dissolved Inorganic Carbon (TDIC). This approach requires the knowledge of the TDIC 14C initial activity in recharge water. The groundwater mineralisation occurs in the Unsaturated Zone (UZ), where water interacts with CO2 and carbonates (if existing). We focus here on these interactions between both CO2, TDIC and carbonates. Two UZ have been investigated in France: the carbonate-free Fontainebleau sands and the carbonated Astian sands. Each site is equipped with a specific and experimental equipment to collect groundwater, UZ water, gas and solid matrix (organic matter and carbonate). The field-data are completed by a numeric approach in order to evaluate the different factors that define the CO2 isotopic composition at the bottom of the UZ. The 13C of CO2 depends on the mean 13C of produced CO2, the amount of produced CO2 and the diffusion coefficient of CO2. Subsurface seasonal variations of 13C disappear with depth. Before 1950, the 14C activity of CO2 can be assumed equal to atmospheric 14C level. After 1950, due to the atmospheric nuclear test, it depends on 14C activity of young organic matter and atmospheric CO2. The presence of carbonates leads to a 13C-enrichment of and a 14C-depletion of CO2 with depth, depending on the carbonate precipitation/dissolution flux, the amount of produced CO2 and the diffusion coefficient of CO2.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF