Infiltration and Distribution of Elemental Mercury DNAPL in Water-Saturated Porous Media: Experimental and Numerical Investigation

Abstract

Liquid elemental mercury occurrence in the subsurface as dense non-aqueous phase liquid (DNAPL) is reported worldwide in proximity of several industrial facilities, such as chlor-alkali plants. Insight into Hg0 DNAPL infiltration behavior is lacking and, to date, there are no experimental observations of its infiltration and distribution in water-saturated porous media, except for capillary pressure-saturation column experiments. To better understand the processes governing elemental mercury DNAPL flow behavior, a series of flow container experiments were performed using mercury DNAPL (in sands and glass beads) and tetrachloroethylene (PCE) (in sands). While liquid Hg0 was not able to infiltrate in the sand-filled container due to an overall lower permeability of the sample and a defect of the setup, in the glass beads experiment mercury DNAPL infiltration occurred. Dual gamma ray measurements showed that, in glass beads, liquid Hg0 preferentially migrated towards higher porosity zones. As for PCE, infiltration and distribution of Hg0 DNAPL are strongly affected by the heterogeneities within the porous formation. However, compared to other DNAPLs, liquid Hg0 shows a strong attenuation potential of gamma rays. Finally, numerical simulations of the glass beads experiment showed an overall good agreement with the experimental results, highlighting that, among the factors influencing the prediction of liquid Hg0 migration in water-saturated porous media, the most critical are (i) the knowledge of the inflow rate, (ii) the reliable estimation of the porous formation permeability, and (iii) the accurate representation of the correlation between retention properties and intrinsic permeability.