Contaminant Mass Transfer from NAPLs to Water Studied in a Continuously Stirred Flow-Through Reactor

The release of nonaqueous-phase liquids (NAPLs) from porous media to groundwater is a widespread environmental problem. The mass transfer of individual NAPL components controls both the extent of groundwater contamination and the persistence of the residual NAPL phase. In order to quantify this key process, small-scale experimental studies on NAPL-water mass transfer were performed in a dynamic system mimicking environmental conditions with “clean” water continuously flowing through the NAPL pool. To describe this process, a modified simulation method was developed and validated by the experimental data. The experimental system consisted of a custom-designed flow cell (with NAPL and water) connected to the peripheral equipment (e.g., pump, water source). This continuously stirred flow-through reactor was used to perform mass transfer experiments with simple and complex model NAPL–water systems. To simulate the experimental data (concentration versus time profiles of individual NAPL compounds), an analytical solution of a standard mass transfer model was adapted in simple model NAPL systems, and a numerical method was employed for complex multicomponent model NAPL–water systems containing phenols, heteroaromatic compounds, and polycyclic aromatic hydrocarbons (PAHs). The numerical model was developed based on a mass balance equation and a general form of Raoult’s law. The simulated concentration profiles of the various solutes matched well the experimental data only if the nonideal behavior of the more polar solutes was accounted for. Using the developed numerical mode simulated mass transfer coefficients for individual NAPL components compared well with previously published values if available.