AN EXPERIMENTAL INVESTIGATION OF RATE-LIMITED NONAQUEOUS PHASE LIQUIDVOLATILIZATION IN UNSATURATED POROUS-MEDIA - STEADY-STATE MASS-TRANSFER

Results of one-dimensional soil column experiments are presented to ev aluate the factors influencing volatilization of entrapped nonaqueous phase liquids (NAPLs) in unsaturated sandy porous media. Three-phase f luid saturations measured in Ottawa and Wagner sands were found to dep end upon porous media grain size and distribution, with residual water and NAPL saturations ranging from 8 to 16% and 4 to 10%, respectively . In general, residual NAPL saturations were 2-3 times less than NAPL entrapped in similar two-phase (organic-water) systems. During volatil ization of three single-component NAPLs (styrene, toluene, and tetrach loroethylene), contaminant vapor phase effluent concentrations deviate d from local equilibrium values by 10-40% for pore velocities ranging from 0.25 to 1.5 cm/s. In contrast to NAPL dissolution, mass transfer rates were foundto decrease with decreasing soil mean grain size. An e mpirical correlation based on the modified Sherwood number and Peclet number was developed which incorporates the soil mean grain size as a surrogate measure of NAPL distribution. The utility of this model is d emonstrated for the prediction of steady state volatilization rates in independent NAPL-porous media systems.