| Abstract
| - Transport modeling, risk assessment, and the evaluationof remediation strategies at contaminated sites require theknowledge of gas diffusivities in soil. A field method ispresented, which determines the tortuosity factor and themass fraction in the air phase of a volatile compound insitu. The compound is injected into the unsaturated zonetogether with a conservative gaseous tracer to form a pointsource. Concentrations are monitored at the injectionpoint during 8 h and evaluated with an analytical equationfor reactive transport. The air-filled porosity is determinedindependently. From these data, both the effective andthe sorption-affected diffusion coefficients are obtained.Results are reported for volatile organic pollutants in botha lysimeter and a sandy soil. The measurements showgood reproducibility. Batch experiments suggest that tracerswere not truly conservative at subsurface temperatures.This may lead to a systematic underestimation of the effectivediffusion coefficient by less than 10%, but the sorption-affected diffusion coefficients were probably overestimatedby 15−20%. Nevertheless, the in situ method can avoidconsiderable uncertainties associated with choosingappropriate empirical relationships for the tortuosity factoror deviations from natural soil conditions in laboratoryexperiments.
|
