Review of Field Methods for the Determination of the Tortuosity and Effective Gas‐Phase Diffusivity in the Vadose Zone

Modeling the gas exchange flux between soil and the atmosphere, risk assessment, and the evaluation of remediation strategies at contaminated sites require the knowledge of gas‐phase diffusivities in the subsurface. We review methods to measure the tortuosity factor or the effective gas‐phase diffusion coefficient in situ. The strong dependency of these parameters on the structure and volume of the air‐filled pore space in the subsurface calls for an accurate and robust in situ measurement. A variety of approaches have been proposed during the last decades, each based on the observation and interpretation of gaseous tracer diffusion in near‐surface soils or the deeper vadose zone under various initial and boundary conditions. We briefly describe the conceptual basis and experimental setup of each method and give insight into error propagation. We then discuss 115 effective diffusion coefficients D e compiled from the original method papers and applications. In situ methods and laboratory measurements on undisturbed soil cores yield comparable results. The Penman relationship, D e / D m = 0.66θ a , sets an upper limit for the field‐determined effective diffusion coefficient in the case of uniform porosity. The Moldrup relationship,D e / D m = θ a 2.5 / θ t, originally proposed for sieved and repacked soils, gave the best predictions of several porosity‐based relationships, but the relative deviation between observed and predicted D e can be substantial. Therefore, the application of such a relationship for the site‐specific modeling of gas‐phase diffusion should be justified with in situ measurements, especially in heterogeneous environments.