The GAIA-program: a scientific approach dedicated to the understanding of the deep groundwater circulations in the south Aquitaine Basin.

International audienc

Water sampling in low productive boreholes: how to ensure of the representativeness of sampling?

International audienceGood practices of wellbore purging advice to draw three to five times the volume of the water column prior to sample. An extensive literature in the past 20 years has established the biases that may be linked to this procedure with emphasis on contaminant sampling and the benefit low-flow sampling may have to avoid redistribution of contaminants in boreholes because of the flow-weighted average character of pumping (Einarson, 2006, Handbook Environmental Site Characterization and GroundWater Monitoring). Low-flow sampling may produce flow-biased samples if operated in long-screened boreholes where vertical gradients exist (McMillan et al., 2014, J. Contam. Hydrol. 1699, 50-61) and is better suited for high permeability boreholes (ISO Standards). Water sampling in low permeability aquifers remains challenging especially for boreholes drilled for water table level monitoring (long-screened boreholes; pumps may not be lowered down in the screened interval). Fluid Electrical Conductivity logging prior to pumping and sampling may help in locating the productive levels albeit the information obtained under ambient flow conditions may be of less relevance than the data collected using salt injection (e.g. Lasher and Nel, 2013, Groundwater Division Conference, Durban). Fiber optic Distributed Temperature Sensing may also resolve hydraulic (McMillan, 2015, PhD dissertation) but is not of common use. We refer to investigations performed in two boreholes (Labruguière and Valdurenque) of low permeabilities (10 −6 to 10 −7 m.s −1), located in detritic formations in SW France. These two boreholes, of 170 m and 123 m depth respectively, have long screened sections (128-170 m and 75-123 m respectively). With such geometries, the sub-mersible pump cannot be placed in the screened interval to perform volume purge. This raises the question of how long the pumping has to be done to get water representative of the downhole chemistry. The purge of three times the volume of the water column is unrealistic. For Labruguière borehole, it would take 12 hours cumulated at ≤1 m 3 .h −1 pumping rate, each session cannot last more than 2 hours (dewatering) and 12 hours are needed to recover the water table level. We thus refer to deep sampling to assess the usefulness of such a method. Several levels were determined on the basis of ambient logging (temperature, conductivity, pH, dissolved oxygen, redox potential). In parallel we use pumping to assess the purge process inside the borehole and determine the minimum amount of drawn water needed to get water from the screens. This also highlights that alternative method to judge of representativeness, such as stabilization of physico-chemical parameters, may lead to false positives, i.e. the parameters were stabilized but the water chemistry was not that of the screened section. Three cycles of logging – deep sampling – pumping were done in each borehole. Based on field data and laboratory analyses, it appears that a protocol for deep boreholes characterization may refer to 1) borehole logging (information on ambient structure of the water column), 2) slight solicitation of the borehole by pumping (renewal of water at productive levels), and 3) deep sampling at the depth(s) suggested by borehole logs

Cyclic variations of sulfate and boron concentrations and isotopes in deep groundwaters in the Aquitaine Basin, France

International audienceConcentrations and isotope contents of major and trace elements are principal factors in determining the origin of the chemical composition of groundwater. This paper focuses specifically on the use of sulfur and boron isotopes to characterize the origin of cyclic variations in the deep Eocene aquifer in the Aquitaine sedimentary basin (southwest of France). It is part of a multi-layer system mainly composed of sands and sandstone deposits. Groundwater contained in this deep reservoir is known to present stable chemical compositions, allowing its use for various purposes like drinking water, geothermal energy, thermal activity and agricultural irrigation. However, among the dozens of wells exploiting this aquifer and despite the reservoir’s substantial depth, variations in sulfate concentration have been identified in a limited area of the reservoir. These fluctuations are cyclic and they seem to be correlated with water level variations due to gas storage activities nearby in the same aquifer. Regular water samplings and analyses of major and trace elements and their isotopes have identified that bore concentration variations are correlated with sulfate variations. A geochemical modelling approach based on water mixes elucidates the causes of these variations in chemical composition, especially the boron and sulfate concentrations and their respective δ11B and δ34SSO4 values. From these numerical results, we identify that different sources explain the variations of boron concentration, one part coming from the silicates alteration (present in sub-layers of the exploited aquifer) and the other part coming from the evaporites alteration (present in the underlying molasse unit). These results also confirm the existence of mass and potentially water transfers between the different sub-layers of the reservoir and with the underlying molasse aquitard, implying new constraints for the future hydrogeologic modelling

Characterization and sampling of a deep and heterogeneous aquifer -An application to the Paleocene -Eocene aquifer in the Aquitaine basin, France

International audienceGroundwater sampling in boreholes has been carried out for decades according to well-established protocols and regulations. An important requirement in this context is the need to purge the borehole prior to any sampling action, the volume of the purge being adapted to the water flow and the regulatory context. Contaminated site investigations have introduced the concept of water column heterogeneity in the screened section of boreholes, which also occurs in some uncontaminated boreholes or in long-screened boreholes. Specific guidelines and practices were thus introduced, in particular the concept of discrete sampling. This type of sampling can be advantageously used in deep boreholes, provided that there is some renewal of water at the screens or the perforated intervals. The present study aims to take a step forward in the characterization of deep boreholes set up in the Aquitaine basin in France, by defining a protocol for relatively short boreholes (depth < 170 m) and applying it to much deeper boreholes, formerly drilled for oil and gas exploration (sampling up to 1035 m deep). Acquisitions were performed to better characterize water chemistry, including some isotope considerations. They were based on physico-chemical logging and endoscopic inspections where technically possible. After a review of the information provided by the investigations on the characterization of the Paleocene – Eocene aquifer in southwestern France, a methodology is proposed to make sampling in deep boreholes with low-yield more reliable, by combining the abstraction of a reduced water volume and the use of a downhole sampling system

Testing different approaches to evaluate groundwater vulnerability to climate change

International audienceToday, the impacts of climate change to the groundwater resources started to be quite well described for quantitative aspects and hydrogeological risk assessment. Some studies, such as EXPLORE2070, allowed highlighting the importance of climate change on, groundwater recharge reduction in some parts of the French territory, a change in recharge seasonality or the elevation of the sea water level. If the estimation of the climate change impact is possible based climatic scenarios proposed by the IPCC for aquifers for hydrogeological models exist, it remains difficult and delicate for sectors without models. The evaluation of climate change impact is more difficult knowing that a reduction of efficient rainfall will be variable impact from one aquifer to another. Thus, analysing the sensibility of aquifers to climate changes needs to take into account different hydrogeological parameters. Multiples methodological approaches can be envisaged. The first approach developed was based on the use of IDPR (infiltration capacity), possible overflowing and drying areas, density of intermittent rivers, thickness of the unsaturated zone and hydrogeological dynamic (captive, free aquifer, cumulative effect, inertia,…). Mapping the groundwater vulnerability to climate change was then constructed using GIS multicriteria data treatment. Two calculations were carried out; a typology of aquifer comportment following two scenarios and the establishment of the vulnerability map combining the typology and the mean aquifer recharge. Other methods, such as the use of drought resistance indicators can also be relevant for climate change impact evaluation. This indicator is based on three criteria i) half-decay time of water levels, ii) index of development and persistence of water network (IDPR) and iii) size of the aquifer system as, together with the thickness, determine the importance of its inertia. Mapping the drought resistance could be useful for example to look at the resilience of aquifer systems (capacity of the system to come back to a normal status) after severe drought episodes, one of the effects of climate change being more intense drought periods. Confronting the different methods is a good way to evaluate uncertainties and to find new directions for investigations

Revision of radiocarbon ages in groundwater from the Eocene aquifer in the Aquitaine basin (France)

International audienceIn deep aquifers the complex flow pattern originating from the geological structure often leads to difficult predictions of water origin, determination of the main flow paths, potential mixing of waters. All these uncertainties prevent an efficient management of the resource. In the context of the Aquitaine basin (France), new investigations were done on the Eocene aquifer, a sandstone reservoir located in the southern sector of the basin (south of the Garonne river). The waters of this aquifer are used for various purposes such as drinking water, geothermal energy, irrigation, thermalism. Because of these different uses, geological, hydrogeological and geochemical studies were lead in order to better understand the main flow paths and potential interactions of these waters with sus-and subjacent deep horizons. However, due to the aquifer extensions (a bit less than 15 000 km²) and the scarcity of the access points (less than 50 operated wells unequally distributed on the aquifer area), many questions are still not solved. For the geochemical characterizations of these waters, previous sampling campaigns performed between 1990 and 2000 allowed defining the chemistry of the water and some isotopic properties (André et al., 2005, Journal of Hydrology 305, 40–62). Specific measurements were done on the estimation of the age of the waters. These first investigations gave 14 C activities ranging from 0.5 pMC (in the deepest parts of the aquifer) to about 20 pMC (close to the outcrops) with average values close to 3 pMC. New investigations were recently done to complement the characterizations of the groundwater. The main goal was to propose a new sampling method adapted to ancient waters but also to use the latest measurement tools. The sampling for 14 C measurements consists to use sampling inox bottles with a volume of 500 mL. Two taps at the inlet and outlet of the bottle allow a perfect isolation of the water from the atmosphere during the sampling time. Then, in lab, the bottle is connected to an analytical line including an AMS (Accelerator Mass Spectroscopy). Water samples were also collected to measure the 36 Cl content in order to determine (i) the potential mixing of the groundwater with oldest ones or (ii) a different origin of the waters according to the location in the reservoir. Results of 14 C analyses show that waters are oldest than the ages determined in previous investigations. The new measured 14 C activities are not exceeding 1 pMC for all the waters in the deepest parts of the reservoir, corresponding to ages older than 30,000 years. The new measurements also confirm the potential mixing of waters with youngest ones close to the outcrops. This re-estimation of the ages slightly modifies the flow paths proposed up to now, particularly close to the geological structures. Furthermore, these new data provide hints on the reservoir properties (like permeability) and it will help to constrain the hydrogeological models

A platform to harmonize the regional hydrodynamic models in the southwest of France

International audienceTo support the authorities in water resource management, different hydrodynamic models have been developed in south-western France (the new Large Region: Aquitaine - Limousin - Poitou-Charentes) for nearly twenty years. These models also contribute to understand the functioning of multilayered aquifers on this territory. Four developed models coexist and cover a total area of 82,570 km² (Jurassic and Cretaceous models in Poitou-Charentes, the North Aquitaine Model and the South Aquitaine Model). They cover 87% of the sedimentary formations of the new Large Region. In addition of these regional models, others models with refined geometry or mesh have been developed on a part of the same territory in order to respond to local and specific problems. All these models, developed independently from each other, will be integrated in a common platform in order to harmonize their functioning with the experience gained during the development and the use of each of them.Thus, the use of a geological modeler upstream to create geometrical models allows some flexibility in order to regularly integrate the latest geological data (used for the North Aquitaine Model). Moreover, the use of climate spatialized data (SAFRAN data from Météo-France) coupled with the use of a specific recharge module allows to homogenize and calculate in the same way the groundwater recharge in the different models (used for the Jurassic model). Including the restitution of surface runoff and rivers flow in modeling allows as well to properly constrain the distribution of effective precipitation between infiltration and runoff.Finally, a database of water withdrawals is built to handle and treat in the same way data from different origins with variable time step and specific issues to be considered in each model (water withdrawals in groundwater and/or surface for various uses as water supply, industry, irrigation, dam releases, sewage discharges, etc.).All of these works aim to consolidate regional hydrodynamic models and to allow the update and faster adaptation of these models

Groundwater management of large aquifers in southwestern France by regional hydrodynamic models

International audienceIn southwestern France, groundwater resources are withdrawn for various uses (drinkable water, irrigation, industry, geothermal energy, forestry, conservation of wetlands, shellfish breeding among others) which can sometimes cause conflicts between users. To help the management of these resources, regional hydrodynamic models have been developed. These models have been a support to the public policies for over 20 years.In the northern region, groundwater is in close relation with rivers, especially during periods of low-water levels. Indeed, groundwater used for irrigation impact not only the stream flows, but also the water supply for the second large wetland of France: the Marais Poitevin. The developed models have permitted to test the impact of several scenarios of water withdrawals. These tests help to determinate the amount of water that can be pumped to respect defined objectives as water levels in piezometer and river flows.These models were also used to test the impact of the eventual implementation of water tanks (400,000 to 800,000 m3). Further south, the major problem is the important reduction of the level of the Eocene aquifer in the department of Gironde. In this area, the regional hydrodynamic model has been developed since 1990. Its development allows the management of deep groundwater resources and contributes to validate strategies of exploitation based on different simulations. This model also allows to answer problems of overexploitation and to analyse areas where water savings could be done to avoid this overexploitation, and to estimate the impact of new resources.The extreme southern region has a significant particularity: on two different sites, groundwater is used for the storage of gas. The cyclic injection and withdrawal of gas impacts significantly the aquifer level. The proposed model is adapted for the groundwater resources knowledge and management of this case. Finally, these models were used to evaluate impacts of climate change on groundwater resources in order to allow authorities to evaluate strategies to adapt to this change (Project Explore 2070 for example)

Sedimentary record of the Paleogene deformation in the North Pyrenean retroforeland (Aquitaine Basin) and link to the Pyrenean orogeny.

International audienceThe North Pyrenean retroforeland (Aquitaine basin) display a quite different tectonic style compared to its well known southern counterpart, and is not well known due to a lack of accurate stratigraphy and geometry at the scale of the basin. Strong efforts have been made in the recent years to improve the knowledge of the Pyrenean orogeny and Aquitaine basin. Based on the integration of more than 1000 wells and 10000 km of seismic profiles we present a synthesis of the Paleogene sedimentary record of the Aquitaine Basin. Large-scale sections, thickness and facies map at the scale of 1 My, precise the timing, intensity and distribution of the deformation. Within Cenozoic, five major tecto- sedimentary cycles are identified, 2 Paleocene cycles :A Danian-Selandian cycle (P1), and a Thanetian one(P2)., And 3 Eocene cycles:- An Ypresian- Lower Lutetian cycle (E1) composed of prograding deltas.- A Lutetian- Bartonian cycle (E2) composed of mixed carbonate siliciclastic deposits.- A Priabonian cycle (E3) marked by a large scale infilling of the basin and deposition of fined- grained continental sedimentation.The timing of deformation is refined and confirms the occurrence of a relatively quiet period during Paleocene (Danian- Lower Thanetian) although with progressively increasing sedimentation rate, followed by a syn orogenic period during Eocene. The latter is separated in two steps:- Firstly the development of a large-scale flexural basin during Early Eocene. Onset of (lim- ited) thrusting and fault reactivation is registered around Paleocene- Eocene boundary in the eastern part of the basin.- Secondly a propagation of the brittle deformation in the foreland with strong inversion and thrusting in the foreland (Audignon etc.) and complete spatial reorganization of the subsidence pattern.- A Priabonian phase of homogenous moderate subsidence during Priabonian, particularly in the eastern part of the basin.This evolution is congruent with the southern foreland one and traduces common phases of evolution. The Ypresian- Lutetian period flexure is interpreted as the consequence of the loading by the growth of the orogenic prism that took place at that time and possibly started earlier during Paleogene. The Bartonian compressive deformation of the foreland correspond to the major phase of collision. These results were acquired during the Gaia project founded by TIGF, BRGM and Agence de l’Eau Adour/Garonne

Upper Cretaceous Paleogene evolution of the Pyrenean retroforeland (Aquitain basin). Insights for Iberia Eurasia convergence.

International audienceThe southern foreland (proforeland) of the pyrenean orogeny is one of the most well- known foreland in the world, on the opposite its northern retroforeland (i.e the Aquitaine basin) which display a quite different tectonic style is less documented apart from local petroleum studies. Strong efforts have been made in the recent years to improve the knowledge of the Pyrenean orogeny and Aquitaine basin. We present here a synthesis of the Aquitain basin evolution during Upper Cretaceous and Paleogene based on the integration of more than 800 petroleum wells and 5000 km of seismic profiles. An harmonized and detailed litho- and sequence stratigraphy was build up for the entire period allowing to recompose large scale sections, thickness and facies map at the scale of 1 My, and sedimentary response to compressive deformation. The distribution, style and timing of deformation of the retroforeland are precised thanks to refined stratigraphic constraints. Within Cenozoic, five major tecto- sedimentary cycles are identified: 2 Paleocene cycles : -A Danian-Selandian cycle (P1), and a Thanetian cycle (P2), both characterized by carbonate platform growth and limited compressive deformations. 3 Eocene cycles: -An Ypresian- Lower Lutetian cycle (E1) composed of prograding deltas from east to west that redistribute material coming from the emerging mountain belt in eastern Pyrenees. This progradation trend leads to relatively quick infilling of the available space and emersion of a large part of the basin. -A Lutetian- Bartonian cycle (E2) composed of mixed carbonate siliciclastic deposits. -A Priabonian cycle (E3) marked by a large scale emersion of the basin and deposition of fined-grained continental sedimentation (Molasses d’Aquitaine). The timing of deformation is refined and confirms (1) the onset of compressive deformations during upper Cretaceous, (2) the occurrence of a relatively quiet period during Paleocene, and (3) a major period of basin deformation during Eocene. Within the basin, compressive deformations take place at both basin and local scale the latter being mainly recorded as reactivation of inherited extensive faulting dating from Albian and /or late Variscan extension, and salt tectonics reactivation mainly in the western part of the basin. Within Eocene, two periods of deformation corresponding with the two successive tecto-sedimentary cycles (E1 and E2) are identified. Onset of compressive deformations is diachronous from east to west. Early Eocene deformations mainly take place in the eastern and central part of the basin, with renewed deformations around Paleocene- Eocene boundary, coeval with the onset of terrigenous input from the growing mountain belt, and major pulse by Upper Ypresian. Late Lutetian- Early Bartonian deformations during cycle E2 are recorded throughout the basin and trigger major depocenter shifts.This refined timing of these deformation correlates with the one of the southern Pyrenean foreland deformations and suggest common control. These results were acquired during the Gaia project founded by TIGF, BRGM and Agence de l’Eau Adour/Garonne which aims to constrain the nature and dynamics of deep Upper cretaceous and Tertiary aquifers of the Aquitaine basin