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Mixing interfaces as reactive hotspots of porous media flows: theoretical upscaling, experimental imaging and field scale validation

Periodic Reporting for period 4 - ReactiveFronts (Mixing interfaces as reactive hotspots of porous media flows: theoretical upscaling, experimental imaging and field scale validation)

Reporting period: 2020-04-01 to 2021-09-30

In porous media, mixing interfaces such as contaminant plume fringes or boundaries between water bodies create highly reactive localized hotspots of chemical and microbiological activity, whether in engineered or natural systems. These reactive fronts are characterized by large concentration gradients, complex flow dynamics, variable water saturation, fluctuating redox conditions and multifunctional biological communities. The spatial and temporal variability of velocity gradients is expected to elongate mixing interfaces and steepen concentration gradients, thus influencing biogeochemical reactivity. However, a major issue with porous media flows is that these essential micro-scale interactions are inaccessible to direct observation. Furthermore, the lack of a validated upscaling framework from fluid- to system-scale represents a major barrier to the application of reactive transport models to natural or industrial problems.

The objective of the ReactiveFronts project is to address this knowledge gap by setting up a high level interdisciplinary team that will provide a new theoretical understanding and novel experimental imaging capacities for micro-scale interactions between flow, mixing and reactions and their impact on reactive front kinetics at the system scale. ReactiveFronts develops an original approach to this long-standing problem; combining theoretical, laboratory and field experimental methods.The focus on reactive interface dynamics, which represents a paradigm shift for reactive transport modelling in porous media, requires the development of original theoretical approaches (WP1) and novel milli-microfluidic experiments (WP2). This forms a basis for the study of complex features at increasing spatial scales, including the coupling between fluid dynamics and biological activity (WP4), the impact of 3D flow topologies and chaotic mixing on effective reaction kinetics (WP3), and the field scale assessment of these interactions (WP5).
During the first period of the project, the ReactiveFronts team has obtained significant modelling and experimental breakthroughs at the interface of fluid mechanics, geophysics, biogeochemistry and microbiology. Concerning the modelling of coupled mixing and reaction processes in porous, we have demonstrated the existence and origin of chaotic stretching in 3D granular media (Turuban et al. PRL 2018) and quantified its consequences on pore scale mixing dynamics through the lamella mixing theory (Lester et al. JFM 2016). Using this theory, we have shown how fluid stretching processes control the statistics of concentration gradients in porous media (Le Borgne et al. JFM 2017) and may be detected through geoelectrical measurements (Gosh et al. GRL 2018). These results open a range of perspectives, which will be explored by the team during the next part of the project, to investigate the interactions between mixing and biogeochemical processes. In particular, a novel experimental setup, based on optical index matching and laser-induced fluorescence, has been developed to provide an experimental demonstration of the chaotic nature of mixing in 3D porous media (Heyman et al. in preparation, Souzy et al. in preparation). A full experimental validation of the lamella theory has been obtained through a shear flow experiment developed at Aix-Marseille university (Souzy et al. JFM 2018). Finally, the exploration of reactive mixing processes in the field, using coupled hydrogeological and genomics technics, has led us to the discovery of deep microbial hot spots triggered by mixing in fractures (Bochet et al., under review in Nature Geoscience). Based on this observation, a new microfluidic experiment has been set up to investigate the link between mixing and microbiological processes.
The demonstration of the chaotic nature of pore scale mixing in granular media (Turuban et al. PRL 2018, Heyman et al. in preparation, Souzy et al. in preparation), is an important breakthrough, which will significantly change the current vision of mixing processes in porous media. Consequences of this discovery on biogeochemical processes will be explored quantitatively using a new porous media prototype based on the functionalization of glass beads rendering interfaces fluorescent upon reaction (PhD thesis Hojjat Borhany, collaboration Khalil Hanna) and a novel microfluidic device enabling the study of bacteria growth under controlled solute and dissolved gas gradients (PhD thesis Antoine Hubert).

Upscaling of the impact of these microscale processes on field scale phenomena will be achieved through the reactive lamella theory (Le Borgne et al., GRL 2014 JFM 2015, 2017). While simulations of reactive and microbiological processes in porous media and turbulent flow generally require weeks of computer time and allow only limited analysis of the governing equations, the new mechanistic model that we have introduced breaks down the complex three-dimensional coupling between mixing and biogeochemical processes into a reduced-dimension equation, allowing fast simulations and facilitating fundamental understanding. This will enable to link these microscale interactions to field scale processes, including redox and rock dissolution processes in the critical zone (PhD thesis Charlotte Le Traon), deep microbial hot spot development (Bochet et al., under review in nature geoscience) and bioclogging processes in geothermal operations (collaboration with Antea group).
Chaotic mixing in porous media (Heyman et al. PNAS 2020)
cover of JFM showing mixing patterns in porous media (Souzy et al. 2020)