An experimental investigation of reaction zone controls on fluid flow and mass transport

This study examines homogeneous and heterogeneous mass transport in porous media featuring a solid-phase reaction zone that evolves and influences mass transport. It uses both flow-tank experiments and 2D reactive transport modeling to elucidate plume and reaction-zone behavior in situations with inherent complexity due to flow and transport and geochemical variability. The system is created by pumping a dilute Fe(ClO4)3 solution through a medium of glass beads and crushed calcite. As pH increases through the dissolution of calcite, amorphous ferric oxyhydroxide precipitates. The distribution of Fe, dissolved carbonate species, and pH are strongly coupled to the reaction zone because of the dissolution of CaCO3 and the precipitation of Fe(OH)3. Plugging influences the transport of dissolved constituents because it promotes fingering, hydraulic fracturing and flow bypassing. With the homogeneous systems, differences in the fingering produced by changing CaCO3 contents produced a heterogeneous distribution of Fe(OH)3(s) and dissolved Fe. As a result, a reaction zone with a complex shape developed due to the heterogeneity in hydraulic conductivity and CaCO3 content.