Density-Driven Migration of Heavy NAPL Vapor in the Unsaturated Zone

Abstract:

Contamination of the subsoil is a major concern in industrially developed as well as developing countries. Liquids introduced into the unsaturated zone will migrate as a liquid phase, however, they will also vaporize and migrate in a gaseous state. In particular, vapor (gas) plumes migrate easily in the unsaturated zone. Heavy vapors migrate, preferentially downward, due to their greater density and thus pose a potential threat to aquifers.

Large scale column experiments and numerical simulations were conducted to investigate migration of carbon disulfide vapor. Carbon disulfide (CS₂), amongst others used for the manufacture of viscous rayon, is an industrial, non-polar solvent. It is highly volatile and characterized by a higher density than water (ρ = 1.263 g/cm³) and, above all, denser than air when in a gaseous state (1.6 compared to air).

The goals of these investigations were to quantitatively describe density-driven vapor migration in the subsurface at a large scale with clearly defined and controlled boundary conditions. The experiments were conducted in vertical, large columns (ID = 0.109 m) of 4 m length packed with dry porous medium in which the migration behavior of CS₂vapor was characterized. Different types of glass beads were used to investigate the influence of permeability. The porous medium was kept dry to avoid partitioning effects due to pore water. The upper and lower boundaries were open to the atmosphere and hence constant pressure boundaries which allowed for an unhindered migration of the heavy vapor injected in the middle section of the column. Gas samples were taken along the column throughout the experiment and analyzed using a GC (HP 6890 Series) to quantify time and space dependent migration.

The set-up of the experiment was numerically reproduced employing a 1-D, two-phase, two-component, isothermal model. Simulation results were compared with data from vapor migration experiments to verify the model. Variations were performed to investigate the parameter sensitivity and delineate influences from boundary conditions. The results of the numerical model confirmed the experimental results, thus the work greatly contributes to the general process understanding of density-driven vapor migration.