TRANSPORT IN POROUS-MEDIA CONTAINING RESIDUAL HYDROCARBON .2. EXPERIMENTS

When liquid hydrocarbons or nonaqueous-phase liquids (NAPLs) become en trapped below the water table, flowing ground waters carry soluble NAP L components away from the spill zone. Transport of these dissolved NA PL components is controlled by several processes including advection, dispersion, sorption to aquifer materials, and liquid-liquid partition ing. To better understand these processes, miscible displacement exper iments were conducted to generate breakthrough curves (BTCs) of pentaf luorobenzoic acid (PFBA), benzene, and toluene on sand columns with an d without a fixed decane residual. A departure from equilibrium transp ort is observed in BTCs from the sand-decane system. These BTCs show c haracteristics of early breakthrough, asymmetry, and tailing. The caus e 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 reproduc e experimental BTCs. Results indicate that time-dependent partitioning becomes increasingly important as the solute decane-water partition c oefficient and the aqueous-phase fluid velocity increase.