PNNL

Abstract

The nonaqueous-phase liquid (NAPL)-water interfacial area is an important parameter which influences the rate of NAPL dissolution in porous media. The aim of this study was to generate a set of baseline data for specific interfacial area for a two-phase entrapped NAPL-water system in well-characterised porous media, and subsequently use these data to evaluate two current theoretical models. The first model tested distributes entrapped NAPL over the pore classes based on Land’s algorithm and assumes the resulting blobs to be spherical. The other model is thermodynamically based, assuming that reversible work done on the system results in an increase in interfacial area, such that the area between drainage and imbibition retention curves can be related to the interfacial area. Interfacial tracer tests (IFTT) were used to measure specific entrapped NAPL (hexadecane)-water interfacial areas in columns packed with four grades (12/20, 20/30, 30/40, 40/50) of silica sand. Using the anionic surfactant dihexylsulfosuccinate (Aerosol® MA80), IFTT gave specific interfacial areas between 58 cm-1 for the finest sand and 16 cm-1 for the coarsest, compared to values of between 33 cm-1 and 7 cm-1 for the first model and between 19 cm-1 and 5 cm-1 for the thermodynamic model. Results from the literature suggest that non-spherical blobs shapes occur relatively frequently; hence it is reasonable to suggest that the assumption of spherical NAPL blobs may explain the underprediction by the first model. The thermodynamic model underestimates the interfacial area because it assumes that entrapment occurs only within the largest pores. A modified version of the latter model, allowing entrapment across all pore classes, yielded values between 58 cm-1 and 13 cm-1. Of the models tested the modified thermodynamic model best predicts the interfacial area.