This work focuses on the experimental measurement and mathematical modeling of processes affecting the dissolution of nonaqueous phase liquids (NAPLs) entrapped in sandy porous media. Results of a series of laboratory‐scale one‐dimensional column dissolution experiments indicate that the length of time required to dissolve NAPLs and substantially reduce aqueous phase effluent concentrations is many times greater than predicted by equilibrium calculations. Experimental measurements clearly show an influence of both grain size and grain size distribution on the evolution of effluent concentrations. The longer cleaning times associated with coarse or graded media are attributed to the larger and more amorphous NAPL blobs associated with these media. A general correlation for transient dissolution rates is proposed which incorporates porous medium properties, Reynolds number, and volumetric fraction of NAPL. The model is calibrated with results from styrene dissolution experiments and is shown to adequately predict trichloroethylene dissolution rates in the same sandy media over the period of time required to dissolve the NAPL.

Results of an experimental investigation into steady state dissolution of nonaqueous phase liquids (NAPLs) entrapped within water saturated porous media are presented. The influence of porous media characteristics, NAPL type, and aqueous phase velocity on NAPL dissolution rates is explored through evaluation of a series of laboratory column studies. For many of the conditions tested, measured organic solute concentrations in the column effluents were below solubility limits, indicating nonequilibrium conditions. Entrapped NAPL distributions are examined and shown to depend upon porous media grading and mean grain size. Experimental results reveal a dependence of dissolution rate on the distribution pattern of entrapped NAPL, as well as upon aqueous phase velocity. A phenomenological model for the mass transfer process is developed which incorporates grain size parameters as surrogate measures of NAPL distribution. An additional set of column experiments, employing another porous medium and NAPL type, confirm the usefulness of the model as a predictor of steady state mass transfer rates in similar systems.

Federal legislation in the U.S. mandates increased production of biofuels. To meet the required demand, corn and “traditional” ethanol rendering crops will soak up irrigation water that could exacerbate water shortages in some regions. Further, the agricultural runoff will return much of this water steeped with eutrophication/hypoxia-causing nitrogen and phosphorus to sensitive inland and coastal waterways. Unless more sustainable crop methods are implemented and biotechnology provides more efficient bioenergy plants, the greening of fuel could come with the browning of water. Dominguez-Faus and coauthors present the data and wonder whether society will have to choose between drinking and driving.

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The objective of this work is to assess the potential significance of deviations from local equilibrium for the exchange of mass between residual nonaqueous phase liquids and the aqueous phase in the saturated groundwater zone. A one-dimensional convection-dispersion mass balance equation incorporating a first-order interphase mass transfer rate relationship and temporal changes in blob configuration is used to model this system. Analytical and numerical rnethods are employed to examine the steady state and transient behavior of the system under a variety of hypothetical aquifer conditions and pumping remediation schemes. Sensitivity of the model to several parameters including mass transfer coefficient, blob size and shape, and Darcy velocity is explored. Results of the theoretical assessment indicate that nonequilibrium effects could play a significant role in some contamination scenarios, primarily for large blob sizes and relatively high velocities. Design of soil flushing techniques will be impacted by these conclusions. Uncertainty in several parameter values used in this analysis indicate the need for further experimental investigation of this process. INTRODUCTION Spills or leaks of organic chemicals to the environment frequently result in the contamination of subsurface soils and groundwater. Many of these pollutants are only slightly soluble in water and thus may exist as virtually immiscible or nonaqueous phase liquids (NAPLs). Documentation of the existence of NAPLs in aquifer systems is growing [Atwater, 1984; Cohen et al., 1987; Feenstra and Coburn, 1986; Schwille, 1988], and the need for an enhancement of our uMerstanding of the fate and transport processes associated with these pollutants is becoming more evident [U.S. Environmental Protection Agency, (USEPA), 1987; Abriola, 9891.Migration of NAPLs in subsurface systems is a complex process. Following a spill or leak, NAPLs generally migrate downward through the vadose zone due to gravitational forces. If the spill is sufficiently large and the NAPL less dense than water, it will eventually reach the water table, where it will spread laterally in the capillary fringe zone. Alternatively, if the NAPL is heavier than water, it will continue to migrate vertically, displacing aquifer pore water [Schwille, 1988;Mackay et al., 1985]. Interfacial forces acting between the water phase or air phase and the NAPL will cause residual "blobs" of the organic phase to be retained within the unsaturated and saturated zones, as depicted schematically in Figure 1. Migration of the bulk organic phase as such will eventually cease when all of the fluid becomes trapped as discontinuous blobs, or when the NAPL encounters a low permeability stratum [Kueper et al., !989] and has insufficient pressure to force the nonwetting NAPL into the small pores of this layer. In this case a pool of NAPL collects on the low permeability stratum. The presence of NAPL in the subsurface represents a potential long-term source of pollution [Pinder, 1982; $chwille, 1988; Baehr, 1987]. In ...