Three-Dimensional Modeling of Multiphase Flow and Mass Transfer in Porous Media

Abstract

The number of documented aquifers contaminated by organic pollutants has rapidly increased within recent years, and it has become evident that they are one of the major threats to groundwater quality. Due to their hydrophobic nature, organic contaminants in aqueous systems primarily exist as a non-aqueous phase. This organic phase usually enters the subsurface in the unsaturated zone, where it forms a three-phase system with soil-water and soil-air. Although immiscible with water, most organic pollutants are soluble in water to an extent far beyond the threshold of drinking water standards. Since these dissolved fractions of the contaminant are much more mobile, their movement will often have the major impact on environmental and water resources concern. Once the pollutant is in the subsurface, one of three different pathways may dominate further migration: movement as a separate organic phase, dissolution and subsequent transport of dissolved fractions in water, or volatilization together with gaseous organics in soil-air. Depending on soil and fluid parameters, each of these migration paths might be important in natural aquifer systems. In most cases the different paths are strongly coupled and it is not obvious which path actually dominates. Thus, only a thorough investigation of the complex interactions between all of these mechanisms can give full understanding of the aquifer contamination that derives from these NAPLs (non-aqueous phase liquids).