Recent Advances in Experimental Studies of Steady-State Dilution and Reactive Mixing in Saturated Porous Media

Abstract

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.At steady-state conditions, transverse dispersive mixing is the key process determining the length of the plume and the fate of the contaminant transport [6][7][8][9][10][11]. Even though the transverse dispersion coefficient is smaller than the longitudinal dispersion coefficient, contact areas between the mixing solutions are much larger in the transverse direction compared to the longitudinal direction. Only at the front of the plume does longitudinal dispersion play an important role in steady-state plume transport. This can be reflected by the steep concentration gradients found at the fringes of groundwater plumes in the field investigations of contaminant aquifers [12][13][14][15].Observation and measuring points in the field investigation of contaminant transport are generally coarse. Furthermore, the heterogeneity and anisotropy of porous media and the biogeochemical complexity of natural aquifer systems influence dilution and mixing. The interpretation separating different physical and chemical processes and the influencing factors is difficult in natural groundwater systems. A detailed analysis of the pure transverse dispersive mixing process and its influencing factors under well-defined and controlled conditions is only possible by performing laboratory experiments and numerical simulations. Numerous modeling studies have been conducted to investigate transverse dispersion of conservative and reactive solutes in homogeneous and heterogeneous porous media [16][17][18][19]. However, a major challenge of various numerical methods is their numerical dispersion and the resulting uncertainties. Therefore, laboratory experiments are always necessary to give an intuitive view of the phenomenon and to test and verify the numerical models.The objectives of this paper are to give a brief review o...

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