Experimental Investigation of Plume Dilution and Dilution Enhancement in Three-Dimensional Heterogeneous Porous Media

We investigate dilution and dilution enhancement of continuously emitted solute plumes in saturated porous media. In particular, we study the effects of flow-focusing in high permeability inclusions and of helical flow in three-dimensional spatially non-stationary heterogeneous porous media. We perform tracer laboratory experiments in fully three-dimensional homogeneous and heterogeneous systems at different flow velocities (1, 3 and 5 m/day). Fluorescein and dissolved oxygen were used as conservative tracers. In the case of flow-focusing, the outcomes of fully 3-D experiments were compared with the results of analogous experiments performed in a quasi 2-D setup. We experimentally quantify dilution by determining the flux-related dilution index using flow rates and concentrations measured at the inlet and the outlet ports. Forward numerical simulations were used to interpret the experimental observations. The results show that dilution is enhanced due to flow-focusing in high-permeability inclusions and by helical flow. However, the degree of dilution enhancement by flow-focusing is dependent on the spatial arrangement of the high-permeability inclusions. Furthermore, although dilution is stronger in three-dimensional than in two-dimensional systems, dilution enhancement due to flow-focusing is more effective in two-dimensional heterogeneous porous media. Finally, helical flow produced by the macroscopically anisotropic porous media entails twisting of streamlines, which increases the lateral mass exchange and thus considerably enhances plume dilution.