Heterogeneity and macroscopic anisotropy of porous media play an important role for dilution and reaction enhancement of conservative and reactive plumes. In this study, we perform numerical simulations to investigate steady-state flow and transport in three-dimensional heterogeneous porous media. We consider two macroscopic anisotropic inclusions resulting in helical flows with twisting streamlines in a three-dimensional flow-through domain. The inclusions are obtained by alternating two layers of angled slices of coarse and fine porous media with different hydraulic conductivity. We investigate flow and transport scenarios considering different geometry and relative position of the two anisotropic inclusions yielding helical flow fields with different extent of interaction. We use metrics of stretching and folding to characterize the flow field and entropy-based metrics for the analysis of the conservative and reactive transport problems. The outcomes show that the two helices result in different patterns of twisting streamlines, which cause distinct deformation of the plumes. However, mixing and reaction enhancement could not be directly related to the extent of the flow field deformation: Configurations with strong deformation can result in only moderate mixing enhancement, whereas configurations with limited deformation of the flow field can lead to significant mixing of the solute plume. Finally, we explore the impact of different degradation rates on reactive transport and the role of reaction kinetics on the entropy balance for a reactant undergoing transport and mixing-controlled degradation in the twisting flow fields. The results show that strong mixing enhancement due to helical flow increases the importance of the reaction kinetics that becomes the rate-limiting process for solute reactive transport.

Transverse dispersive mixing plays an important role in controlling natural attenuation of contaminant plumes and the performance of engineered remediation strategies. The extent of transverse mixing can be significantly affected by porous media heterogeneity and anisotropy. For instance, flow focusing in the high-permeability inclusions leads to an enhancement of dilution and reactive mixing in steady-state solute transport. Numerous modeling studies have been performed to understand the mechanism of conservative and reactive transport in homogeneous and complex heterogeneous porous media. However, experimental investigations are necessary to show an intuitive phenomenon and to validate the modeling results. This paper briefly reviews recent laboratory experimental studies on dilution and reactive mixing of steady-state transport in saturated homogeneous and heterogeneous porous media. In this context, setups and measuring techniques are described in pore-scale and Darcy-scale experiments. Parameters quantifying dilution and reactive mixing in the experiments are also introduced. Finally, we discuss the further experimental works necessary to deepen our understanding of dilution and reactive mixing in natural aquifers.