Elucidation of retention processes governing the transport of volatile organic compounds in unsaturated soil systems

Abstract

There is evidence to suggest that the air-water interface serves as an important retention domain for volatile organic compounds (VOCs) in vadose-zone soil systems. Gas-phase solute transport experiments were conducted to evaluate the influence of air-water interfacial adsorption on trichloroethene transport and retention. Mechanical mixing and diffusion were observed to contribute significantly to the transport of gases and vapors in unsaturated soils. The relative contribution of individual dispersion processes was governed largely by the differences in their diffusion coefficients, while changes in gas-phase tortuosity and linear velocity due to soil-water content changes represented secondary effects. Difluoromethane (DFM) was shown to hold promise as a reactive tracer for the in-situ measurement of soil-water content, as there was shown to be a linear relationship between DFM-estimated and measured soil-water contents. Heptane was shown here to exhibit nonideal tracer behavior that complicate its use in estimating air-water interfacial areas. Conversely, relatively ideal interfacial tracer properties were exhibited by decane. In excess of 90% of the decane retardation factor was contributed by adsorption at the air-water interface, rendering other forms of retention entirely secondary. Decane retardation factors were in an appropriate range for soil-water contents greater than ∼2.5%. Specific air-water interfacial areas estimated from decane retention data appear to be reasonable, based on comparison with the measured N₂√BET specific surface area of the porous media and comparison with literature data. The retention of TCE vapor in unsaturated soil systems to be influenced by several processes, including sorption to the solid surfaces, dissolution into bulk soil-water, and adsorption at the air-water interface. The retention of TCE in the system under various conditions was not predicted well by the traditional retardation equation. However, use of DFM (water-partitioning tracer) data and decane (interfacial tracer) data were observed to further improve agreement between predicted and experimental TCE retardation factors. Interfacial processes were shown to influence TCE retention most significantly at soil-water contents less than 5%, contributing 87% of the total measured retardation factor at 2% soil-water content.