Transport in Porous Media Containing Residual Hydrocarbon. II: Experiments

When liquid hydrocarbons or nonaqueous-phase liquids (NAPLs) become entrapped below the water table, flowing ground waters carry soluble NAPL components away from the spill zone. Transport of these dissolved NAPL components is controlled by several processes including advection, dispersion, sorption to aquifer materials, and liquid-liquid partitioning. To better understand these processes, miscible displacement experiments were conducted to generate breakthrough curves (BTCs) of pentafluorobenzoic acid (PFBA), benzene, and toluene on sand columns with and without a fixed decane residual. A departure from equilibrium transport is observed in BTCs from the sand-decane system. These BTCs show characteristics of early breakthrough, asymmetry, and tailing. The cause of nonequilibrium is hypothesized to be rate-limited solute exchange between decane and water. A new transport model, capable of handling time-dependent exchange processes, is successfully applied to reproduce experimental BTCs. Results indicate that time-dependent partitioning becomes increasingly important as the solute decane-water partition coefficient and the aqueous-phase fluid velocity increase.