Department of Water Resources and Drinking Water, Swiss Federal Institute of Aquatic Science and Technology (Eawag), 8600 Dübendorf, Switzerland.
Department of Civil, Environmental and Geomatic Engineering, ETH Zürich, 8092 Zürich, Switzerland.
Environ Sci Technol. 2024 Jul 16;58(28):12643-12652. doi: 10.1021/acs.est.4c02799. Epub 2024 Jul 6.
Unsaturated porous media, characterized by the combined presence of several immiscible fluid phases in the pore space, are highly relevant systems in nature, because they control the fate of contaminants and the availability of nutrients in the subsoil. However, a full understanding of the mechanisms controlling solute mixing in such systems is still missing. In particular, the role of saturation in the development of chaotic solute mixing has remained unexplored. Using three-dimensional numerical simulations of flow and transport at the pore scale, built upon X-ray tomograms of a porous medium at different degrees of liquid (wetting)-phase saturation, we show the occurrence of chaotic dynamics in both the deformation of the solute plume, as characterized by computed chaos metrics (Lyapunov exponents), and the mixing of the injected solute. Our results show an enhancement of these chaotic dynamics at lower saturation and their occurrence even under diffusion-relevant conditions over the medium's length, also being strengthened by larger flow velocities. These findings highlight the dominant role of the pore-scale spatial heterogeneity of the system, enhanced by the presence of an immiscible phase (e.g., air), on the mixing efficiency. This represents a stepping stone for the assessment of mixing and reactions in unsaturated porous media.
非饱和多孔介质的特点是在孔隙空间中同时存在多种不混溶的流体相，在自然界中具有重要意义，因为它们控制着污染物的命运和底土中养分的可用性。然而，人们对控制这些系统中溶质混合机制的全面理解仍存在空白。特别是，饱和度在混沌溶质混合发展中的作用仍未得到探索。我们使用基于不同液体（润湿）相饱和度的多孔介质 X 射线断层扫描的孔隙尺度流动和输运的三维数值模拟，展示了溶质羽流变形的混沌动力学的发生，这一特征可以通过计算的混沌度量（李雅普诺夫指数）来表征，并且注入的溶质发生了混合。我们的结果表明，在较低饱和度下，这些混沌动力学会增强，即使在介质长度上的扩散相关条件下也会发生，并且较大的流速也会增强这种混沌动力学。这些发现强调了系统的孔隙尺度空间异质性的主导作用，这种异质性是由不混溶相（例如空气）的存在增强的，这对混合效率有影响。这为评估非饱和多孔介质中的混合和反应提供了一个起点。
