Linking Self-Organized Heterogeneities to Solute Transport in Mixing-Induced Precipitated Porous Media

Recent laboratory experiments in an intermediate-scale Hele-Shaw cell, designed to emulate a coarse sand aquifer, demonstrate that calcite precipitation induced by mixing leads to the formation of a self-organized, heterogeneous porous medium. This medium is characterized by elongated carbonate structures and internal preferential flow channels aligned with the main flow direction. The resulting transport behavior exhibits strong anomalous features, as evidenced by breakthrough curves showing earlier solute arrival, a distinct double peak, and pronounced tailing. In this article, we investigate the relationship between the self-organized heterogeneous structure of the porous medium formed through mixing-induced precipitation and its impact on solute transport. To achieve this, we analyze the spatial variability of hydraulic conductivity by implementing different permeability scenarios in a random walk particle tracking model. These scenarios, derived from image analysis of the precipitated structures, range from simple representations to increasingly complex configurations. Our results highlight the importance of capturing two key features to effectively describe solute transport. First, delineating the total precipitated area is crucial for accurately representing flow diversion caused by permeability reduction, which explains the emergence of the double peak in solute concentrations. However, fully capturing both the double peak transition and tailing requires representing the internal structure of the high-precipitation zones within the precipitated area, as these characterize internal preferential flow channels.