Visualization of DNAPL fingering processes and mechanisms in water-saturated porous media

Abstract

Groundwater contamination by dense nonaqueous phase liquids (DNAPLs) has received increasing attention in the last decade. The fingering process of DNAPL migration in porous me- dia remains an incompletely understood subject. The main reason is that natural porous media are opaque and hence very few visualizations are available. This paper presents the visual results of two-dimensional experiments in a glass tank in which DNAPL penetrated into water-saturated homogeneous porous media. The results provide a clear reference for conceptual models of DNAPL finger development due to immiscible flow instabilities. The fingering process can be divided into two stages, that is, the finger initiation stage and the finger elongation stage. At the finger initiation stage, many DNAPL protuberances appear at the interface between tetrachloroethylene (PCE) and water along the surface of the porous media. During the finger elongation stage, some relatively larger protuberances develop into primary fingers. Secondary fingers may develop on the existing primary fingers. The fingers grew downwardly in a winding manner with the mechanisms of shielding, tip- growing, splitting, and coalescing. The fingers grew linearly with time and had similar growth rate at the finger elongation stage. The average wavelength (space between fingers) of the primary fingers was 0.051 m and the average PCE content in the region with fingers was 2.5% (7.0% saturation).