Presentation: The potential of temperature and dissolved gas as smart tracers for process-based heterogeneity characterization

Event: AGU Fall Meeting 2019, San Francisco (USA) Presentation by Richard Hoffmann, Pascal Goderniaux, Alain Dassargues


Informative reference data for a realistic assessment of aquifer heterogeneity is a prerequisite for robust transport simulations. Structure-based imaging using salt or a dye as tracer with a known concentration and volume to observe transfer times, is a powerful hydrogeological tool in moderate heterogenous media. Solving then the advection-dispersion equation will explain most of the point to point transport behavior. But, once the aquifer heterogeneity is more complex, e.g. in a double porosity medium like chalk, matrix porosity linked to diffusion processes must be taken into consideration to avoid a biased interpretation of the tracer information. Thus, performing additional local process-based imaging using smart tracers as dissolved gas and hot or cold water, assists to explain the late-time tailing behaviors realistically.

Smart tracers were injected in a sub-horizontal fracture connecting two adjacent wells to provide data about the complementary behaviors of each tracer and to focus on matrix diffusion processes. One reference data set is a 70 hours injection of hot water (∆T = + 40 °C) complemented by two 10 minutes uranine pulse injections within an inflatable double packer system isolating the sub-horizontal chalk fracture of interest. The temperature signal arrives at a 7.55 m distance with a delay of 12 hours compared to the first uranine injection and shows a rebound after the injection stopped. Useful reference data for further numerical modelling consists now in (a) local fracture geometry information deduced from interpretation by analytical solutions and, (b) matrix diffusion information.

Numerical modelling of those smart tracer experiments may question deterministic models for predictions and motivates for data-driven prediction tools like Monte-Carlo simulation procedures within a direct predictive framework. Distance based global sensitivity analysis (e.g. simultaneous variation of multiple input variables like diffusion coefficient, aperture and matrix storage) will be considered accounting for temperature related changes of viscosity and density. Key information about the most influencing parameters are main model outcomes, as local process understanding is very useful for possible future upscaling in regional models made of structure-based imaging.

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Presentation: Groundwater – surface water interactions in a lowland stream valley: thermal characterization of groundwater upwelling in a wetland

Event: AGU Fall Meeting 2019, San Francisco (USA)
Presentation by Joel Tirado-Conde, Majken Looms, Peter Engesgaard


Wetlands are extremely dynamical systems and their behavior depends on the characteristics of the surroundings (topography, geology and vegetation, among others) as well as on meteorological and hydrological processes. Wetlands are wet partly because they receive groundwater (or drain water) through diffuse upwelling and through springs. Studying upwelling is of great importance to e.g. evaluate the overall ecology or capacity to remove nitrate of the wetland system. One problem is that diffuse upwelling is difficult locate and measure.

We analyze the temporal dynamics of a groundwater fed wetland in central Jutland (Denmark) by the use of various thermal methods across a lowland stream valley. A monitoring system consisting of Distributed Temperature Sensing (DTS), wells with temperature depth profiles and thermal infrared (TIR) imaging on a UAV, in conjunction with hydrological and atmospheric data, provide a quasi 3D time-lapse characterization of the thermal behavior of the system, both on the ground and in the subsurface, over a period of around two years.

By analyzing the temporal evolution of the temperature in both the wetland surface and the groundwater, we can infer potential locations of groundwater upwelling to the land surface and subsequent overland flow. This is relevant as previous studies have shown that it is a generally overlooked flow component that may have a big impact relative to base flow. Moreover, it serves as a test for the feasibility of using heat as a tracer to study groundwater – surface water exchanges in wetlands.

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Poster: Hydrogeophysical monitoring of a controlled infiltration experiment at the Ploemeur Hydrological Observatory (Brittany, France)

Event: Journées des Doctorants – Ecole Doctorale Géosciences, Ressources Naturelles et Environnement – Paris (France), 19/03/2019
Poster by Lara Blazevic, Ludovic Bodet, Laurent Longuevergne, Sylvain Pasquet, Damien Jougnot


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Publication: A cross-validation framework to extract data features for reducing structural uncertainty in subsurface heterogeneity

in Advances in Water Resources Vol 133 (November 2019)
by Jorge Lopez-Alvis, Thomas Hermans, Frédéric Nguyen


Spatial heterogeneity is a critical issue in the management of water resources. However, most studies do not consider uncertainty at different levels in the conceptualization of the subsurface patterns, for example using one single geological scenario to generate an ensemble of realizations.

In this paper, we represent the spatial uncertainty by the use of hierarchical models in which higher-level parameters control the structure. Reduction of uncertainty in such higher-level structural parameters with observation data may be done by updating the complete hierarchical model, but this is, in general, computationally challenging.

To address this, methods have been proposed that directly update these structural parameters by means of extracting lower dimensional representations of data called data features that are informative and applying a statistical estimation technique using these features.

The difficulty of such methods, however, lies in the choice and design of data features, i.e. their extraction function and their dimensionality, which have been shown to be case-dependent. Therefore, we propose a cross-validation framework to properly assess the robustness of each designed feature and make the choice of the best feature more objective. Such framework aids also in choosing the values for the parameters of the statistical estimation technique, such as the bandwidth for kernel density estimation.

We demonstrate the approach on a synthetic case with cross-hole ground penetrating radar traveltime data and two higher-level structural parameters: discrete geological scenarios and the continuous preferential orientation of channels.

With the best performing features selected according to the cross-validation score, we successfully reduce the uncertainty for these structural parameters in a computationally efficient way. While doing so, we also provide guidelines to design features accounting for the level of knowledge of the studied system.

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Publication: Application of Stable Isotopes of Water to Study Coupled Submarine Groundwater Discharge and Nutrient Delivery

in Water 11:9 (September 2019)
by Carlos Duque, Soren Jessen, Joel Tirado-Conde, Sachin Karan and Peter Engesgaard


Submarine groundwater discharge (SGD)—including terrestrial freshwater, density-driven flow at the saltwater–freshwater interface, and benthic exchange—can deliver nutrients to coastal areas, generating a negative effect in the quality of marine water bodies. It is recognized that water stable isotopes (18O and 2H) can be helpful tracers to identify different flow paths and origins of water. Here, we show that they can be also applied when assessing sources of nutrients to coastal areas.

A field site near a lagoon (Ringkøbing Fjord, Denmark) has been monitored at a metric scale to test if stable isotopes of water can be used to achieve a better understanding of the hydrochemical processes taking place in coastal aquifers, where there is a transition from freshwater to saltwater.

Results show that 18O and 2H differentiate the coastal aquifer into three zones: Freshwater, shallow, and deep saline zones, which corresponded well with zones having distinct concentrations of inorganic phosphorous. The explanation is associated with three mechanisms: (1) Differences in sediment composition, (2) chemical reactions triggered by mixing of different type of fluxes, and (3) biochemical and diffusive processes in the lagoon bed.

The different behaviors of nutrients in Ringkøbing Fjord need to be considered in water quality management. PO4 underneath the lagoon exceeds the groundwater concentration inland, thus demonstrating an intra-lagoon origin, while NO3, higher inland due to anthropogenic activity, is denitrified in the study area before reaching the lagoon.

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