Gas-phase diffusivity and tortuosity of structured soils.

Area of Science:

• Environmental Science
• Geotechnical Engineering
• Soil Physics

Background:

• Subsurface vapor migration modeling often uses soil-gas diffusivity models.
• Classical models, developed for structureless soils, fail to account for preferential diffusion through soil voids and fractures.
• This limitation can lead to underestimation of vapor intrusion risks.

Purpose of the Study:

• To investigate the accuracy of existing soil-gas diffusivity models for heterogeneous and fractured soils.
• To develop and validate a more appropriate model for predicting gas diffusion in such complex soil structures.
• To assess the impact of soil structure and porosity on gas diffusion coefficients.

Main Methods:

• Measurement of the ratio of the gas diffusion coefficient in soil to that in free air (D(p)/D(0)).
• Analysis of 42 intact, unsaturated soil cores from six Danish sites with varying structures.
• Application and comparison of classical and dual-porosity models for gas-phase diffusivity.

Main Results:

• Classical models adequately described diffusion in structureless fine sand.
• Glacial clay till and limestone soils exhibited dual-porosity behavior.
• A dual-porosity model successfully described gas diffusion in fractured soils, considering fractures and a less diffusive matrix.
• Fracture tortuosity in till and limestone correlated negatively with total porosity (<40%), indicating compaction effects.

Conclusions:

• Classical models may significantly underestimate subsurface gas diffusion in heterogeneous and fractured soils.
• A dual-porosity approach is crucial for accurately modeling vapor migration in complex soil environments.
• Soil structure, particularly fracture geometry influenced by compaction, plays a key role in gas diffusion.