Experimental Investigation of Plume Dilution in Three-Dimensional Porous Media

Abstract

Transverse dispersion is of critical importance for plume dilution and transport in groundwater and represents a key mechanism controlling degradation of contaminant plumes. This work presents an experimental investigation supported by model-based interpretation of plume dilution in fully three-dimensional saturated porous media. High-resolution experiments were performed to study lateral mixing and dilution of steady-state plumes in homogeneous and heterogeneous experimental setups. The investigation aimed at the detailed study of key mechanisms controlling solute transport in saturated porous media and was structured according to a progressively increasing level of complexity. First, dimensionality and compound-specific effects on plume dilution were studied in homogeneous porous media. We performed multi-tracer (i.e., fluorescein and oxygen) experiments in a fully three-dimensional setup and compared the results with analogous experiments conducted in a quasi two-dimensional system. The concept of flux-related dilution index was used to quantify plume dilution and it was experimentally calculated considering the flow rates and the concentrations measured at the inlet and outlet ports. A semi-analytical solution was proposed to predict plume dilution in three-dimensional setup. Subsequently, the effects of flow focusing in high-permeability inclusions were investigated. The results show a dependence of transverse mixing on dimensionality and on the parameterization of transverse dispersion coefficients. Furthermore, the experimental results confirm that although dilution is stronger in three-dimensional porous media, dilution enhancement by flow-focusing in high-permeability inclusions is less effective in 3-D than in 2-D systems. The spatial arrangements of the high-permeability inclusions also significantly influence the dilution enhancement in three-dimensional heterogeneous porous media. Finally, transport and plume dilution in three-dimensional heterogeneous anisotropic porous media was investigated. Flow-through experiments performed in macroscopically anisotropic porous media provided the first experimental proof of the occurrence of helical flow. Such complex flow conditions, entailing streamlines twisting, were shown to significantly enhance plume dilution and mixing-controlled reactive transport.