Comparison of soil tortuosity calculated by different methods

Tortuosity is a parameter characterising the complexity of pore geometry in porous media for fluids and solutes to move through. It is loosely defined and has been calculated by different methods based on either the pore geometry or a special transport process. While it has been known that tortuosities calculated from different methods vary, it remains obscure if there is a one-to-one relationship between them, especially for soils which are not randomly structured but self-organised by a myriad of interactive biotic and abiotic processes. We studied this based on X-ray images of 30 soil aggregates taken from fields which have been under different land managements for more than 70 years and thus have contrasting structures. The tortuosity of every soil sample was calculated using three methods: viscous fluid flow, solute diffusion, and geometric structure of the pores, with the former two calculated from pore-scale simulations. The results showed that although the tortuosities calculated by all methods are correlated, their correlation is weak and there is no one-to-one relationship between them. On average, the tortuosity calculated from fluid flow is the highest and the geometrical tortuosity is the least, with that calculated from solute diffusion in between. The tortuosity calculated from all three methods decreases as porosity increases, but the coefficient of determination is low. We also found that the bulk diffusion coefficient cannot be predicted using geometrical tortuosity and porosity of the soils from the formulae suggested in the literature. These findings reveal that tortuosity is a process-dependent parameter rather than an intrinsic soil property, and that tortuosity calculated from different methods cannot be used interchangeably to estimate soil transport parameters.