HS8.1.3

Light Non-Aqueous Phase Liquids (LNAPLs) are one of the most important sources of soil and groundwater contamination worldwide. When they infiltrate through the unsaturated zone, part of the LNAPLs remains trapped by capillary forces. The others accumulate above the top of the water table, forming a floating ‘free’ phase able to generate a long-term dissolved LNAPL plume that durably alters the quality of the water resource. Seasonal variations in the groundwater level lead to significant vertical spreading of these light petroleum hydrocarbon contaminants at the capillary fringe, favoring their release into the air and groundwater. In the climate change context, the IPCC predicts an intensification of these groundwater level variations over the next century in response to variations in rainfall intensity and frequency, whose effects are increased by the use of water resources. This context may strongly impact the mobilization of these organic contaminants and their release to the environment. To study these phenomena, it is, therefore, essential to better understand the impact of the groundwater level fluctuation patterns on the LNAPLs mobilization processes. To this end, an original experimental system combining indirect geophysical (complex electrical conductivity, permittivity), in-situ physical-chemical (pH, Eh, temperature), and geochemical measurements was developed at the GISFI station (Homécourt, France). This device allows the assessment and the comparison of the amount and nature of LNAPLs release into the atmosphere and water from contaminated soil during two groundwater level fluctuations scenarios: one corresponding to the ‘actual’ rainfall pattern based on regional climate records; the other based on the predictions of the most extreme IPCC scenario. This study will be conducted at different scales (laboratory decametric columns and 2 m³ lysimeters) and on soils of different geological complexity. The remobilized hydrocarbons will be collected via suction cups and gas collection chambers as the groundwater table fluctuates and will be regularly analyzed (GC-MS, FTIR). The complementarity of the monitoring methods aims to provide a better understanding of the fate of these organic pollutants at contaminated sites and the evolution of the associated environmental risks in the coming years, under the expected effect of climate change. Preliminary results concerning the hydrocarbon pollution migration through the unsaturated zone and the distribution of the LNAPL will be presented to illustrate the capacity of this new instrumental system.

This work is partly funded by the DEEPSURF project "Lorraine Université d’Excellence", ANR-15-IDEX-04-LUE".

How to cite: Cavelan, A., Golfier, F., Colombano, S., Enjelvin, N., Davarzani, H., Deparis, J., Lorgeoux, C., Tinet, A.-J., Oltean, C., and Faure, P.: Impact of groundwater level variations induced by climate change on the mobilization of light refined petroleum hydrocarbon contaminants (LNAPLs), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4081, https://doi.org/10.5194/egusphere-egu21-4081, 2021.

Monitoring and characterization of flow and transport processes in the subsurface has been a key focus of hydrogeological research for several decades. Such processes can be relevant for numerous applications, such as hydrocarbon and geothermal reservoir characterization and monitoring, risk assessment of soil contaminants, or nuclear waste disposal strategies.

Monitoring of flow and transport processes in the subsurface is often challenging, as they are usually not directly observable. Here, we present an approach to monitor saline tracer migration through a weakly fractured crystalline rock mass by means of Ground Penetrating Radar (GPR), and we evaluate the data quantitatively in terms of a flow velocity field and localized difference GPR breakthrough curves (DRBTC).

Two comparable and repeated tracer injection experiments were performed within saturated rock on the decameter scale. Time-lapse single-hole reflection data were acquired from two different boreholes during these experiments using unshielded and omnidirectional borehole antennas. The individual surveys were analyzed by difference imaging techniques, which allowed ultimately for tracer breakthrough monitoring at different locations in the subsurface. By combining the two complimentary GPR data sets, the 3D tracer velocity field could be reconstructed.

Our DRBTCs agree well with measured BTCs of the saline tracer at different electrical conductivity monitoring positions. Additionally, we were able to calculate a DRBTC for a location not previously monitored with borehole sensors. The reconstructed velocity field is in good agreement with previous studies on dye tracer data at the same research locations. Furthermore, we were able to resolve separate flow paths towards different monitoring locations, which could not be inferred from the electrical conductivity sensor data alone. The GPR data thus helped to disentangle the complex flow field through the fractured rock.

Out technique can be adapted to other use cases such as 3D monitoring of fluid migration (and thus permeability enhancement) during hydraulic stimulation and tracing fluid contaminants – e.g. for nuclear waste repository monitoring.

How to cite: Giertzuch, P.-L., Shakas, A., Brixel, B., Doetsch, J., Jalali, M., and Maurer, H.: Inferring saline tracer transport characteristics from time-lapse GPR experiments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7541, https://doi.org/10.5194/egusphere-egu21-7541, 2021.

We develop a methodology to estimate soil hydrodynamic parameters from a water infiltration experiment monitored with a GPR (Ground Penetrating Radar). Such an experiment, carried out on both controlled and natural site, consists in applying a water charge in a tank on the soil surface. During the water infiltration, the water layer thickness above the soil surface in the tank and the GPR response on the infiltration water front are monitored. The infiltration experiment is then modelled numerically using hydrogeological parameters which describe the constitutive relationships between water content, pressure and hydraulic conductivity. In that goal, we use the WAMOS-1D code which combines the 1D Richards equation and the Mualem – van Genuchten model. From the hydrogeological models outputs and petrophysical relationships, corresponding GPR velocity models are created to generate the resulting GPR signals. Then, an inversion algorithm couples both the hydrogeological and the geophysical models to seek the optimal hydrodynamic parameters. The inverse problem objective function is calculated from the estimated arrival time of the GPR waves reflected by the water infiltration front and compared to the measured ones. Preliminary inversion tests explore the hydrodynamic parameters space using synthetic data. First results show that the saturated hydraulic conductivity parameter can be estimated. Further tests are performed to improve both our experimental set-up and methodology and allow an estimation of the other hydrodynamic parameters. An emerging idea is to complete the objective function by analyzing the arrival time corresponding to additional reflectors to the water infiltration front.

How to cite: Moua, R., Lesparre, N., Girard, J.-F., Belfort, B., and Lehmann, F.: Estimate of hydrodynamic parameters with a coupled hydrogeophysical inversion using GPR surveys, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8705, https://doi.org/10.5194/egusphere-egu21-8705, 2021.

An integrative interdisciplinary approach is currently developed to investigate groundwater systems in alpine and prealpine environments and how they respond to hydrological extremes such as droughts, heavy rain and floods in terms of water quantity, hydrochemical quality, and ecological status. The new approach is aimed at improving the understanding of the interaction between physical, chemical, and biological processes in groundwater responses to extreme events as well as developing indicators suitable for an integrative monitoring and management of the aquifers. For this purpose, observation wells of the existing state hydrographic monitoring net have been selected within the Austrian part of the Mur river basin, stretching from the alpine origin to the national border in the foreland. The investigation area thus comprises diverse hydrogeological settings and land-use types. The selected observation wells have long-term records of groundwater levels and are used for sampling campaigns under different hydrological conditions. Groundwater level fluctuations are evaluated using drought indices and statistical approaches, such as auto-correlation and cross-correlation with precipitation and stream stages. Our hydrochemical analyses of groundwater and surface waters also consider compounds indicative of agricultural sources (e.g., nitrate), wastewater-borne micro-pollutants, and stable isotopes of water. These indicators are used to identify different drivers controlling water origin and quality. The ecological status is characterized using microbiological measures, such as total number of bacteria and microbial activity, groundwater fauna, and the qualitative composition of dissolved organic matter (DOM). First results demonstrate a deterioration of water quality from groundwater to surface water and from the alpine region towards the foreland, corresponding to the more intense agricultural and urban land use in the foreland. Linkages between water quality and hydrological conditions are currently being evaluated and will be further examined using UV-Vis spectrometry for high-resolution in-situ monitoring of water quality changes (DOM and nitrate) at selected observation wells.

How to cite: Birk, S., Haas, J., Retter, A., Collenteur, R., Brielmann, H., Stumpp, C., and Griebler, C.: Integrative hydrogeo-ecological assessment of the quantitative and qualitative response of groundwater to hydrological extremes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13111, https://doi.org/10.5194/egusphere-egu21-13111, 2021.

Multiphase fluid flow is a common process in geological systems and has important applications such as aquifer remediation and Carbon Capture and Storage (CSS). Understanding how pore-scale fluid displacements link to the macroscopic descriptions of multiphase flow forms an important gap in our current understanding of this process. At the mesoscale, between the pore and the continuum scale, the distribution of the fluids in the pore network develops into different patterns depending on e.g. flow regime, pore geometry and surface chemistry. Over the years, significant effort has been put into identifying the underlying pore-scale displacement mechanisms[e.g. 1] and classifying these displacement patterns in model porous media based on e.g. the capillary number, viscosity ratio and wettability[2]–[4]. However, subsurface rocks tend to be far more complex in terms of pore structure and wettability than the model materials on which these classifications are based. We hypothesize that pore-scale complexities might induce local variations in the viscous-capillary force balance which could translate in qualitatively new multiphase flow behavior.

To test this hypothesis, we use fast laboratory based X-ray microtomography to image n-decane-brine drainage and imbibition experiments performed on two medium-grained calcareous sandstone samples of the Luxembourg Sandstone Formation (lower Jurassic) at slow flow rates (Ca 10^-9). One of these samples was treated using octadecyltrichlorosilane (OTS) to induce an mixed wettability distribution. The experiments were imaged continuously at 60s per 360° rotation using a laboratory based X-ray microtomography scanner optimized for fast image acquisition to generate a time series of images with a reconstructed voxel size of 8µm/vx. We quantify fluid displacements on a pore-by-pore basis to investigate the times scales associated with the fluid displacements. We identify a previously undescribed type of filling event that occurred during water-flooding under mixed-wet conditions, where certain large pores fill at a time scale that is four orders of magnitude slower than the Haines jumps that occur in neighboring pores. This displacement type is responsible for about 20% of the total displacement of the n-decane phase in our sample during water-flooding. The rate-limited behavior of these events can be explained by the fact that under mixed-wet conditions the persistent connectivity of the fluid phases allows the invasion of poorly connected, large pores through low-conductivity pore regions which locally control the flow rates.

[1]        R. Lenormand, C. Zarcone, en A. Sarr, ‘Mechanisms of the displacement of one fluid by another in a network of capillary ducts’, J. Fluid Mech., nr. 135, pp. 337–353, feb. 1983.
[2]        R. Lenormand, E. Touboul, en C. Zarcone, ‘Numerical models and experiments on immiscible displacements on immiscible displacements in porous media’, J. Fluid Mech., nr. 189, pp. 165–187, jun. 1988.
[3]        B. Zhao e.a., ‘Comprehensive comparison of pore-scale models for multiphase flow in porous media’, Proc. Natl. Acad. Sci., vol. 116, nr. 28, pp. 13799–13806, jul. 2019, doi: 10.1073/pnas.1901619116.
[4]        R. Holtzman, ‘Effects of Pore-Scale Disorder on Fluid Displacement in Partially-Wettable Porous Media’, Sci. Rep., vol. 6, nr. 1, p. 36221, dec. 2016, doi: 10.1038/srep36221.

How to cite: Mascini, A., Boone, M., Cnudde, V., and Bultreys, T.: Investigating wettability-controlled fluid displacements in heterogeneous rock using fast laboratory based X-ray microtomography, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15042, https://doi.org/10.5194/egusphere-egu21-15042, 2021.

Hydraulic tomography is a state of the art method for inferring hydraulic conductivity fields using head data. Here, a numerical model is used to simulate a steady-state hydraulic tomography experiment by assuming a Gaussian hydraulic conductivity field (also constant storativity) and generating the head and flux data in different observation points. We employed geostatistical inversion using head and flux data individually and jointly to better understand the relative merits of each data type. For the typical case of a small number of observation points, we find that flux data provide a better resolved hydraulic conductivity field compared to head data when considering data with similar signal-to-noise ratios. In the case of a high number of observation points, we find the estimated fields to be of similar quality regardless of the data type. A resolution analysis for a small number of observations reveals that head data averages over a broader region than flux data, and flux data can better resolve the hydraulic conductivity field than head data. The inversions' performance depends on borehole boundary conditions, with the best performing setting for flux data and head data are constant head and constant rate, respectively. However, the joint inversion results of both data types are insensitive to the borehole boundary type. Considering the same number of observations, the joint inversion of head and flux data does not offer advantages over individual inversions. By increasing the hydraulic conductivity field variance, we find that the resulting increased non-linearity makes it more challenging to recover high-quality estimates of the reference hydraulic conductivity field. Our findings would be useful for future planning and design of hydraulic tomography tests comprising the flux and head data.

How to cite: Pouladiborj, B., Bour, O., Linde, N., and Longuevergne, L.: Hydraulic tomography using joint inversion of head and flux data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15255, https://doi.org/10.5194/egusphere-egu21-15255, 2021.

A pore-scale description of the transport and mixing processes is particularly relevant when looking at biological and chemical reactions. For instance, a microbial population growth is controlled by local concentrations of nutrients and oxygen, and chemical reaction are driven by molecular-scale concentration gradients. The heterogeneous flow field typically found in porous media results from the contrast of velocities that deforms and elongates the mixing fronts between solutes that often evolves through a lamella-like topology. For continuous Darcy type flow field a novel framework that describes the statistical distribution of concentration being transported was recently developed (Le Borgne et al., JFM 2015). In this model, concentrations in each lamella are distributed as a Gaussian-like profile which experiences diffusion in the transverse direction while the lamella is elongated by advection along the local flow direction. The evolving concentration field is described as the superposition of each lamella. We hypothesize that this novel view, while perfectly predicting the distribution of concentration for Darcy scale mixing processes, will breakdown when the processes description is at the pore scale. Indeed the presence of solid and impermeable boundaries prevents lamella concentration to diffuse freely according to the a Gaussian shape, and therefore changes the mixing front profile, the lamella superposition and elongation rules. Previous work (Hamada et al, PRF, 2020) demonstrated that the presence of solid boundaries leads to an enhanced diffusion and thus fast homogenization of concentrations. In a purely diffusive process the local mixing time is reduced by a factor of ten with respect to the continuous case and concentration gradient are dissipated exponentially fast while a power law decrease is observed in continuous medium. To investigate the impact of these mechanisms on mixing we developed an experimental set-up to visualize and quantify the displacement of a conservative tracer in a synthetic porous medium. The designed apparatus allows to obtain high resolution concentration measurements at the pore scale. We show that the resulting mixing measures, computed in terms of concentration probability density function and dilution index values, diverge qualitatively and quantitatively from what happens in a continuous domain. These observations suggest that description of pore-scale diffusion-limited mixing requires model that takes into account the confined nature of porous medium, otherwise we will tend to overestimate concentration value and neglect the fast diffusion dynamic taking place at microscopic level.

How to cite: Hamada, M. and de Anna, P.: Diffusion limited mixing in heterogeneous porous media, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15721, https://doi.org/10.5194/egusphere-egu21-15721, 2021.

How to cite: de Anna, P., Pahlavan, A. A., Yawata, Y., Stocker, R., and Juanes, R.: Chemotaxis under flow disorder shapes microbial dispersion in porous media, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16083, https://doi.org/10.5194/egusphere-egu21-16083, 2021.

The transport of colloids in porous media is governed by deposition on solid surfaces and pore-scale flow variability. Classical approaches, like colloid filtration theory (CFT), do not capture behaviours observed experimentally, such as non-exponential steady state deposition profiles and heavy tailed BreakThrough Curves (BTC). In the framework of CFT, a key assumption is that the colloid attachment rate 𝑘 is constant and empirically estimated via a posteriori macroscopic data fitting. We design a novel experimental set-up based on time-lapse microscopy and continuous injection of fluorescent monodisperse colloids into a folded microfluidics device (1mt total length) designed with a controlled level of 2D spatial disorder. This set-up allows us to i) measure both BTC and deposition profile over several orders of magnitude and ii) to perform particle tracking and Lagrangian analysis of single colloid's trajectories. Based on this analysis, we propose a stochastic model that takes into account pore scale heterogeneities in terms of correlation length, velocity and attachment rate distribution, that captures the anomalous behaviour shown by the experimental data.

How to cite: miele, F., dentz, M., morales, V., and de Anna, P.: Filtration by porous media: a microfluidics approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16173, https://doi.org/10.5194/egusphere-egu21-16173, 2021.

Predicting dispersal patterns is important to understand microbial life in porous media as soils and sedimentary environments. We studied active and passive dispersal of bacterial cells in porous media characterized by two main pore features: fast channels and dead-end cavities. We combined experiments with microfluidic devices and time-lapse microscopy to track individual bacterial trajectories and measure the breakthrough curves and pore scale bacterial abundance. Escherichia coli cells dispersed more efficiently than the non-motile mutants showing a different retention in the dead-end pores. Our findings highlight the role of diffusion dominated dead-end pores on the dispersal of microorganisms in porous media.

How to cite: Scheidweiler, D., Bordoloi, A. D., and de Anna, P.: Microbial dispersal throgh dead-end cavities, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16206, https://doi.org/10.5194/egusphere-egu21-16206, 2021.

Heterogeneity in porous media may occur due to non-uniformity in the sizes or the shapes of grains that comprise the medium. We investigate the transport of colloids in a heterogeneous porous medium engineered in microuidic channels and featuring complex grain structures. Using experiment and numerical simulation, we investigate the velocity fields and the breakthrough curves of colloidal transport in a model porous medium by emphasising on the effects of dead-end pores. We characterize the porous structure via image processing and isolate dead-end sites from the remaining pore spaces. The study reveals complex flow structures inside dead-end sites that contribute to the small-scale velocity and long tails in the breakthrough curve. We provide a statistical model to capture the complex dynamics of the breakthrough curve.

How to cite: Bordoloi, A., Scheidweiler, D., and de Anna, P.: Effect of dead-end pores on anomalous transport in porous media, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16218, https://doi.org/10.5194/egusphere-egu21-16218, 2021.