资源环境科技发展态势分析平台

Characterizing the effects of subsurface fluid mixing on biogeochemical reactions is a key step toward monitoring and understanding a range of processes and applications in which fluids of different chemical compositions mix, such as aquifer remediation, CO2 sequestration, or groundwater-surface water interactions. Yet the development of noninvasive methods to monitor mixing processes remains an outstanding challenge. Mixing processes are controlled by concentration gradients that develop at scales below the spatial resolution of hydrogeophysical imaging techniques. To overcome this difficulty, we propose to focus on the geoelectrical response of mixing-driven chemical reactions, for which the products are electrically more conductive than the background solution. For such reactions, we investigate the impact of reactive mixing on the resulting effective electrical conductivity. The transport equations are solved using the Lamellar Mixing Theory for a range of velocity gradients representative of the stretching rates experienced by mixing fronts in heterogeneous porous media. Focusing on simple shear flows, we demonstrate that such reactions may result in substantial changes (several orders of magnitude) in the effective conductivity over time, thus providing geoelectrical signatures of reactive mixing dynamics. The temporal evolution of the effective conductivity depends on the relative orientations of the applied electrical potential gradient, mean flow, and velocity gradient, yielding different sensitivities to dispersion processes, stretching rates, and reaction kinetics. These results suggest that the use of chemical reactions with electrically conductive products could help to overcome present limitations of time-lapse geophysical imaging in the monitoring of the spatiotemporal dynamics of subsurface reactive mixing processes.

Plain Language Summary It is a well-known fact that mixing of fluids with different chemical compositions in the subsurface plays a key role in various biological and geochemical reactions. The simplest way to gain insight into any reaction is to have an accurate estimate of the mass of the products(s) and its distribution. When such reactions are accompanied by fluid flow, additional complexities arise because the flow sweeps away the reactants in different directions, leading to mixing and spreading. Since the flow in the subsurface is intricate, this creates complex mixing and reaction patterns. In addition to this, the opacity of the subsurface prevents us from having easy access to these reactions, which poses serious difficulties in properly monitoring the subsurface reactive mixing processes. In the present work, we show that it might be possible to indirectly monitor the state of the some special kinds of reactions by measuring the electrical conductivity of the subsurface. In more technical terms, here we show that for reactions generating products with significantly different electrical conductivities as compared to the reactants, the effective conductivity is intricately linked to the reaction rate, the mixing rate, and the geometry of the reaction hot spots.