Voici les éléments 1 - 10 sur 1021
  • Publication
    Accès libre
    A Framework for Untangling Transient Groundwater Mixing and Travel Times
    (2021-2)
    Popp, Andrea L.
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    Pardo-Alvarez, Alvaro
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    Scheidegger, Andreas
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    Musy, Stephanie
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    Purtschert, Roland
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    Kipfer, Rolf
    Understanding the mixing between surface water and groundwater as well as groundwater travel times in vulnerable aquifers is crucial to sustaining a safe water supply. Age dating tracers used to infer apparent travel times typically refer to the entire groundwater sample. A groundwater sample, however, consists of a mixture of waters with a distribution of travel times. Age dating tracers only reflect the proportion of the water that is under the dating range of the used tracer, thus their interpretation is typically biased. Additionally, end-member mixing models are subject to various sources of uncertainties, which are typically neglected. In this study, we introduce a new framework that untangles groundwater mixing ratios and travel times using a novel combination of in-situ noble gas analyses. We applied this approach during a groundwater pumping test carried out in a pre-alpine Swiss valley. First, we calculated transient mixing ratios between recently infiltrated river water and regional groundwater present in a wellfield, using helium-4 concentrations combined with a Bayesian end-member mixing model. Having identified the groundwater fraction of recently infiltrated river water (Frw) consequently allowed us to infer the travel times from the river to the wellfield, estimated based on radon-222 activities of Frw. Furthermore, we compared tracer-based estimates of Frw with results from a calibrated numerical model. We demonstrate (i) that partitioning of major water sources enables a meaningful interpretation of an age dating tracer of the water fraction of interest and (ii) that the streambed has a major control on the estimated travel times.
  • Publication
    Accès libre
    Late Holocene fine-grained sediments of the Balearic Abyssal Plain, Western Mediterranean Sea
    (2007-02-09)
    Zúñiga, D.
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    García-Orellana, J.
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    Calafat, A.
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    Price, N.B.
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    Sanchez-Vidal, A.
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    Canals, M.
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    Sanchez-Cabeza, J.A.
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    Masqué, P.
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    Fabres, J.
    The Late Holocene fine-grained sedimentation in the deep central Balearic Abyssal Plain, Western Mediterranean Sea, has been studied after the analysis of high quality sediment cores from three multicoring stations, named A, B and C. The coring stations are 25 km apart from each other and form a triangle that is at the greatest distance from the nearest landmasses. The sediments consist of foraminifer–pteropod oozes (layers U1 and U4 from bottom to top), two pteropod oozes (U2 and U5) and a turbidite layer (U3). The same sequence has been identified in the three cores.
    The turbidite layer U3 that lies on top of U2 (14C AMS dated at 1814 cal yrs BP) ranges from 5 to 11 cm thick and is clearly distinguished from the dominating hemipelagic sediments by colour variations, higher water content and the absence of planktonic foraminifer shells. Rather uniform values of organic carbon and nitrogen are also characteristic of the U3 turbidite unit, likely because of homogenisation during transport and emplacement. In addition, U3 unit shows higher contents of terrigenous elements such as K and Fe. Otherwise, the presence of zeolites in the turbidite layer supports the hypothesis of the Sardinian continental margin, where pyroclastic deposits are known, as the most likely source area for U3. Our results demonstrate the ability of turbidity currents originating on the margins surrounding the Balearic Abyssal Plain to reach the very centre of the basin during high sea level still stands as the present one.
  • Publication
    Métadonnées seulement
    A model of deep crustal fluid flow following the M-w=8.0 Antofagasta, Chile, earthquake
    (2004)
    Koerner, A.
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    Kissling, E.
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    We develop a model to test the hypothesis that a high fluid pressure pulse and subsequent fluid flow caused a spatially extensive increase in the V-P/V-S ratio following the M-w = 8.0 Antofagasta, Chile, subduction zone earthquake. The postseismic anomaly appeared within a 50-day period inside the forearc region of the Andes continental crust. We model this anomaly with a poroelastic medium that combines the fluid flow response to mean stress changes from slip along a dislocation plane, with a pore pressure pulse initiated by the coseismic rupturing of a seal separating hydrostatic-lithostatic fluid pressure conditions in the hanging walls and footwalls of the subduction zone, respectively. Variations in seismic velocity due to porosity and pore pressure changes are calculated using Gassmann's formula and the empirical law of critical porosity. We show that the slip-induced perturbation of the mean stress field is insufficient to explain the anomaly, but the release of lithostatic pressurized fluid trapped below the rupture plane ( within the oceanic crust) can explain the transient changes in seismic velocities. Our preferred model requires a very large intrinsic permeability of around 1 x 10(-1)3 m(2), suggesting a mechanism of a high-amplitude pressure pulse propagating through a highly permeable fracture system of the lower continental crust.
  • Publication
    Accès libre
    Your work is my boundary condition!: Challenges and approaches for a closer collaboration between hydrologists and hydrogeologists
    (2019-4)
    Maria, Staudinger
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    Michael, Stoelzle
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    Jan, Seibert
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    Markus, Weiler
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    Hydrologists and hydrogeologists both study the flux and storage of water with the numerous interactions and feedback mechanisms of surface water and groundwater. Traditionally however, focus, models and scales of the studies differ. In this commentary, situations are illustrated where boundary conditions that each discipline assumes, preserves and actively uses, can and have to be overcome. These situations occur when the domain of one discipline cannot be separated from the other one because of existing interaction and feedback mechanisms at the boundaries. Highlighted are especially these boundary conditions, where closer collaboration between catchment hydrologists and hydrogeologists would be most useful. Often such collaborations would be relatively straight-forward and rather requiring an increased awareness than novel methods.
  • Publication
    Accès libre
    A numerical analysis of dimensionality and heterogeneity effects on advective dispersive seawater intrusion processes
    Two-dimensional (2D) and 3D numerical simulations of the dispersive Henry problem show that heterogeneity affects seawater intrusion differently in 2D and 3D. When the variance of a multi-Gaussian isotropic hydraulic conductivity field increases, the penetration of the saltwater wedge decreases in 2D while it increases in 3D. This is due to the combined influence of advective and dispersive processes which are affected differently by heterogeneity and problem dimensionality. First, the equivalent hydraulic conductivity controls the mean head gradient and therefore the position of the wedge. For an isotropic medium, increasing the variance increases the equivalent conductivity in 3D but not in 2D. Second, the macrodispersion controls the rotation of the saltwater wedge by affecting the magnitude of the density contrasts along the saltwater wedge. An increased dispersion due to heterogeneity leads to a decreasing density contrast and therefore a smaller penetration of the wedge. The relative magnitude of these two opposite effects depends on the degree of heterogeneity, anisotropy of the medium, and dimension. Investigating these effects in 3D is very heavy numerically; as an alternative, one can simulate 2D heterogeneous media that approximate the behaviour of the 3D ones, provided that their statistical distribution is rescaled.
  • Publication
    Métadonnées seulement
    Decoupling of underground explosions by rubble-filled cavities
    (1993)
    Florence, Alexander L.
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    Keller, Carl E.
    A series of laboratory experiments has been performed to study the decoupling of explosions by placing them in rubble-filled cavities. The source conditions consisted of a tamped source (for reference), a spherical cavity, and two sizes of cylindrical cavity located at the center of a cylindrical core of granite. The rubble was well-graded granite aggregate compressed to provide a porosity of 20% (one test at 30%). Measurements consisted of particle velocities at several ranges from which displacements, reduced velocity and displacement potentials, spectra, and radiated kinetic energy were obtained. On the basis of the results of these small-scale experiments, conclusions are drawn concerning the extent of yield underestimation associated with the rubble-filled cavities and the dependence of the estimation on the decoupling criterion selected. Underestimation can amount to large factors, and a cavity test is identifiable by means of spectra in which the cavity natural frequency is evident.
  • Publication
    Accès libre
    Assessing the effect of chlorinated hydrocarbon degradation in aquitards on plume persistence due to back-diffusion
    (2018-4) ;
    Parker, Beth L.
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    This modeling study aims to investigate how reactive processes in aquitards impact plume persistence in adjacent aquifers. For that purpose themigration of a trichloroethene (TCE) plume in an aquifer originating from dense nonaqueous phase liquid (DNAPL) source dissolution and back-diffusion from an underlying reactive aquitardwas simulated in a 2D-numericalmodel. Two aquitard degradation scenarios were modeled considering one-step degradation from TCE to cis-dichloroethene (cDCE): a uniform (constant degradation with aquitard depth) and a nonuniform scenario (decreasing degradation with aquitard depth) and were compared with a no-degradation scenario. In the no-degradation scenario, a long-term TCE tailing above the Maximum Contaminant Level (MCL) caused by back-diffusion after source removal was observed. In contrast, in the aquitard degradation scenarios, TCE backdiffusion periods were shorter, whereby the extent of back-diffusion reduction depended on the aquitard degradation depth and the rate. For high degradation rates (half-life: 30–80 days), an aquitard degradation depth greater than 65 cm prevented TCE plume persistence after source removal but generated a long-term tailing above the MCL for the produced cDCE. For slow degradation rates (half-life: b200 days), TCE was only partially degraded after source removal, independent of the aquitard degradation depth, leading to a long-term dual contamination of the aquifer by cDCE and TCE. A sudden enrichment of 13C in TCE and cDCE was observed after source removal in the uniform and non-uniform degradation scenarios that was distinct from δ13C patterns observedwhen aquifer degradation occurs (continuous enrichment of 13C along the plume axis) and forwhen there is absence of degradation (no change of isotope ratios). This demonstrates that δ13C measurements in the aquifer can be used as a diagnostic tool to demonstrate aquitard degradation, which simplifies the identification of reactive processes in aquitards, as aquifers are usually easier to monitor than aquitards.
  • Publication
    Accès libre
    Implementation of the BDFGeotherm database (geothermal fluids in Switzerland) on Google Earth. Final report for the OFEN, Project No 101-842
    (OFEN, 2008)
    Sonney, Romain
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    Cattin, Stéphane
    La base de données BDFGeotherm, réalisée en 2007 sous ACCESS, a été modifiée dans le but d’améliorer sa diffusion et de faciliter son emploi. Le logiciel Google Earth et le site Internet du CREGE ont été utilisés pour convertir cette base regroupant des données sur les fluides thermaux souvent dispersées et difficiles d’accès. En téléchargeant le fichier “BDFGeotherm.kmz” depuis le site Internet du CREGE et en ouvrant ce fichier dans Google Earth, 84 sites contenant des informations sur les fluides thermaux apparaissent sur un fond de carte géographique. Chaque site est représenté par une épingle dont la couleur représente un intervalle de température mesurée. Le secteur nord du massif du Jura et la vallée supérieure du Rhône sont les deux zones où se concentrent la majorité des sites. Des données sur l’utilisation de l’eau, la géologie, le débit, la température et la minéralisation de l’eau apparaissent dans une nouvelle fenêtre en cliquant sur l’épingle des sites. Pour chacun d’entre eux, un lien vers le site Internet du CREGE permet d’obtenir des informations supplémentaires: description géographique, géologie du réservoir, propriétés hydrauliques, hydrochimie, isotopes et paramètres géothermaux. Pour un nombre limité de sites, des photos et des logs géologiques peuvent être visualisés et exportés.
  • Publication
    Accès libre
    Tracer and timescale methods for understanding complex geophysical and environmental fluid flows
    Deleersnijder, Eric
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    Haine, Thomas W.N.
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    Vanclooster, Marnik
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    Waugh, Darryn W
    Understanding advective-diffusive transport of trace constituents in natural fluid flows is an important challenge in Earth and environmental sciences with many diverse applications, including simulating the fate of contaminants, inferring the location of their source, and model assessment (e.g., [7,21]). Eulerian and Lagrangian methods are routinely used, including novel representations of mixing processes that resort to the so-called fractional-order diffusion. Moreover, geophysical and environmental fluid-flow models routinely produce huge amounts of output, and to make sense of these results sophisticated interpretation methods are required. Among these methods, an approach that is becoming progressively more popular consists in using real, or hypothetical, tracers to tag fluid masses and estimate associated timescales, such as age, residence time, and transit time. These timescales lead to very useful diagnoses that are increasingly applied in interdisciplinary environmental studies (e.g. [2,5]). This special issue presents a number of studies that are relevant to the above-mentioned field of research. Groundwater, soil water, riverine, estuarine, marine and ocean flows are considered, as well as the transport of sinking particles in water.
  • Publication
    Métadonnées seulement
    Carbon and chlorine isotope ratios of chlorinated ethenes migrating through a thick unsaturated zone of a sandy aquifer
    (2011) ;
    Aravena, R.
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    Shouakar-Stash, O.
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    Weisbrod, N.
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    Nasser, A.
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    Netzer, L.
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    Ronen, D.