In situ method to measure effective and sorption-affected gas-phase diffusion coefficients in soils

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.