Advances in micro-scale imaging techniques, such as X-ray microtomography, have provided new insights into a broad range of porous media processes. However, direct imaging of flow and transport processes remains challenging due to spatial and temporal resolution limitations. Here, we investigate the use of dynamic three-dimensional neutron imaging to image flow and transport in Bentheim sandstone core samples before and after in-situ calcium carbonate precipitation. First, we demonstrate the applicability of neutron imaging to quantify the solute dispersion along the interface between heavy water and a cadmium aqueous solution. Then, we monitor the flow of heavy water within two Bentheim sandstone core samples before and after a step of in-situ mineral precipitation. The precipitation of calcium carbonate is induced by reactive mixing of two solutions containing CaCl2 and Na2CO3, either by injecting these two fluids one after each other (sequential experiment) or by injecting them in parallel (co-flow experiment). We use the contrast in neutron attenuation from time-lapse tomograms to derive three-dimensional fluid velocity field by using an inversion technique based on the advection-dispersion equation. Results show mineral precipitation induces a wider distribution of local flow velocities and leads to alterations in the main flow pathways. The flow distribution appears to be independent of the initial distribution in the sequential experiment, while in the co-flow experiment, we observed that higher initial local fluid velocities tended to increase slightly following precipitation. These findings suggest that neutron imaging is a promising technique to investigate dynamics processes in porous media.

The spatial distribution of reactants in heterogeneous media plays a central role in chemical reactions, which, as contact processes, depend on mixing. For fast reactions, diffusion is unable to smooth out structure in reactant distributions on scales relevant for reaction, leading to incomplete mixing. Thus, well-mixed reaction models tend to overestimate reaction rates, as they assume that all solute is available for reaction and do not take into account mass-transfer limitations. Although different models have been proposed to capture this phenomenon, linking pore-scale structure, flow heterogeneity, and local reaction kinetics to upscaled effective kinetics remains a challenging problem. We develop a novel theoretical framework to quantify these dynamics for fluid--solid reactions, with the fluid phase undergoing advective--diffusive transport. Our approach is based on the concept of inter-reaction times, which result from the waiting times between contacts of transported reactants with the solid phase. We use this formulation to quantify the time evolution of total mass for a catalytic degradation reaction, and test its predictions against numerical simulations of advective--diffusive transport in stratified channel flow and Stokes flow through a beadpack.

Dissolved Oxygen (DO) plays a key role in reactive processes and microbial dynamics in the critical zone. While the general view is that oxygen is rapidly depleted in soils and that deeper compartments are anoxic, recent observations showed that fractures can provide rapid pathways for deep oxygen penetration, triggering unexpected biogeochemical processes. As it is transported in the subsurface, DO reacts with electron donors, such as $Fe^{2+}$ coming from mineral dissolution, hence influencing rock-weathering. Yet, little is known about the factors controlling the spatial heterogeneity and distribution of oxygen with depth. Here we present analytical expressions describing the coupled evolution of DO and $Fe^{2+}$ as a function of fluid travel time in crystalline rocks. Our model, validated with reactive transport simulations, predicts a linear decay of DO with time, followed by a rapid non-linear increase of $Fe^{2+}$ concentrations up to an equilibrium state. Relative effects of the reducing capacity of the bedrock and of transport velocity are quantified through a Damkohler number, capturing key hydrological and geological controls of $Fe^{2+}$ and DO distributions in the subsurface. This framework is used to investigate contrasted DO and $Fe^{2+}$ concentrations observed in two crystalline catchments. These differences are explained by the Damkohler number: one system is reaction-limited while the second is transport-limited. We show that hydrological and geological drivers can be discriminated by analyzing both O$_2$ and $Fe^{2+}$. These findings provide a new conceptual framework to understand and predict the evolution of DO in the subsurface, a key element in the critical zone.

Mixing along the salt-freshwater interface is critical for geochemical reactions, transport and transformation of nutrients and contaminants in coastal ecosystems. However, the mechanisms and controls of mixing are not well understood. We develop an analytical model, based on the coupling between flow deformation and dispersion, that predicts the mixing dynamics along the interface for steady state flow in coastal aquifers. The analytical predictions are compared with the results of detailed numerical simulations, which show that non-uniform flow fields, inherent to seawater intrusion in coastal aquifer, result in a non-monotonic evolution of mixing width and mixing rates along the interface. The analytical model accurately captures these dynamics over a range of freshwater flow rates and dispersivities. It predicts the evolution of the mixing width and mixing rates along the interface, offering a new framework for understanding and modeling mixing and reaction processes in coastal aquifers.