Characterizing gas-water interfacial and bulk-water partitioning for gas-phase transport of organic contaminants in unsaturated porous media

This study was performed to investigate the impact of interfacial and bulk-water partitioning on the retention and retardation of gas-phase contaminants during transport in unsaturated porous media. Gas-flow experiments were conducted using columns packed with three types of sandy porous media. Moisture contents were 11.9%, 16.0%, and 9.4% for the columns packed with glass beads, silica sand, and aquifer material, respectively. Contaminant retardation was the sum of retention by the gas, aqueous, and solid phases and accumulation at the gas-water interface. The results indicated that 29-73% of total trichloroethene retardation was due to accumulation at the gas-water interface, depending on porous media type. Partitioning into the bulk water accounted for 12-30% of total trichloroethene retardation for the various systems. Given the potential significance of interfacial and bulk-water retention for contaminant transport and remediation, it is important to consider methods for characterizing these processes at the field scale. Gas-phase tracer experiments using a suite of phase-selective partitioning tracers in combination with nonreactive tracers, as presented herein, may be one viable approach. For example, CO₂ was used successfully to measure the bulk-water content of the system, and heptane was used to determine the effective gas- water interfacial area.