title: In situ method to measure effective and sorption-affected gas-phase diffusion coefficients in soils.
abstract: Transport modeling, risk assessment, and the evaluation of remediation strategies at contaminated sites require the knowledge of gas diffusivities in soil. A field method is presented, which determines the tortuosity factor and the mass fraction in the air phase of a volatile compound in situ. The compound is injected into the unsaturated zone together with a conservative gaseous tracer to form a point source. Concentrations are monitored at the injection point during 8 h and evaluated with an analytical equation for reactive transport. The air-filled porosity is determined independently. From these data, both the effective and the sorption-affected diffusion coefficients are obtained. Results are reported for volatile organic pollutants in both a lysimeter and a sandy soil. The measurements show good reproducibility. Batch experiments suggest that tracers were not truly conservative at subsurface temperatures. This may lead to a systematic underestimation of the effective diffusion coefficient by less than 10%, but the sorption-affected diffusion coefficients were probably overestimated by 15-20%. Nevertheless, the in situ method can avoid considerable uncertainties associated with choosing appropriate empirical relationships for the tortuosity factor or deviations from natural soil conditions in laboratory experiments.
authors.affiliation: Swiss Federal Institute of Technology (EPFL), ENAC-ISTE-LPE, CH-1015 Lausanne, Switzerland.
authors.forename: David
authors.initials: D
authors.lastname: Werner
authors.forename: Patrick
authors.initials: P
authors.lastname: Höhener
mesh_terms: D000327:Adsorption; D004058:Diffusion; D004784:Environmental Monitoring / Q000379:methods*; D005544:Forecasting; D005740:Gases; D008962:Models, Theoretical; D018570:Risk Assessment; D012989:Soil Pollutants
chemical_list: D005740:Gases; D012989:Soil Pollutants