Wettability hysteresis and its implications for DNAPL source zone distribution

Subsurface heterogeneity at sites contaminated with nonaqueous phase liquids (NAPLs) reduces the effectiveness of traditional remediation measures. One cause may be the increased proportion of NAPL that is hydraulically isolated due to capillary trapping in heterogeneously-wetted materials. This study examines the wettability of ten materials, ranging from minerals, such as calcite and dolomite, to carbonaceous materials, such shale and coal, in air and water, NAPL and air, and NAPL and water systems. The wettability differed depending on which phase the solid material was initially immersed in: the less crystalline solids, if initially contacted by water were water-wet, but if initially contacted by NAPL were NAPL-wet. This difference, termed here wettability hysteresis, was observed for a suite of halogenated NAPLs and was independent of equilibration time. The degree of wettability hysteresis was greatest in the NAPL and water systems, with the magnitude of the difference increasing with the carbonaceous materials. Since the degree of capillary trapping in subsurface materials is related to wettability, the phenomenon of wettability hysteresis suggests that system history is a factor that may increase the heterogeneity of NAPL source zones.