Multiphase and Multi-component Interactions through the Unsaturated Saturated Zone Field and Model Study

Abstract

An integrated approach of field and model investigations was implemented to an aquifer underlying urban infrastructure. The study focuses on the transport of Trichloroethylene (TCE) through the unsaturated-saturated zone. Simulations, subject to isothermal conditions, addressed a threedimensional continuum involving three interacting mobile phases: aqueous, NAPL (Non-Aqueous Phase Liquids) and gaseous. The mathematical model considers water, air and TCE as components in equilibrium partitioning between the three mobile fluids, while for the latter we account also for sorption on the solid matrix. Predictions of migration patterns were due to a continuous spill from a single NAPL source situated at the soil surface.

The specific flux of the gaseous phase proved to be directed to the surface at the vicinity of impervious segments, implying that TCE vapors may surround building foundations. We note a vertical, gravity driven, NAPL vertical displacement of the gaseous phase the velocity of which is also influenced by the gaseous pressure gradient. Fluids densities depend on TCE concentration, we thus note a density driven flow pattern. As TCE concentration decreases away from the NAPL stem, the gradient of the gas density yields its horizontal displacement, and vertical outward shift resulting from the vanishing capillary pressure. The gaseous velocity far away from the stem is associated only with the gas pressure gradient and is diminished at the unsaturated-saturated boundary, as the relative permeability of gaseous phase decreases near the capillary fringe.