Measurement of pore connectivity to describe diffusion through a nonaqueous phase in unsaturated soils

Contaminant diffusion through a nonaqueous phase liquid (NAPL) in unsaturated soils may often be an important transport mechanism. A diffusion tube technique was used to measure the effective diffusivity through NAPL in unsaturated soils as a function of soil type (or pore size distribution), water saturation (milliliter water/milliliter void), and NAPL saturation (milliliter NAPL/milliliter void). Results indicated that the effective diffusivities depended on the ‘connectivityʹ of the NAPL. This ‘connectivityʹ increased with NAPL saturation, but was shown to have a more complex relation to the water saturation and pore size distribution of the soil. A series of mercury intrusion and extrusion experiments was also carried out to characterize the pore size distribution and pore ‘connectivityʹ of the model soils. Using this characterization of the pore size distribution, along with a previously developed diffusional resistance model, the effective diffusivity was reasonably predicted for the soils and saturations examined in this study.