3D simulation of the permeability tensor in a soil aggregate on basis of nanotomographic imaging and LBE solver

The purpose of this hydropedological work is to investigate the feasibility of a method to calculate permeability of soil peds on the matrix pore scale resolution. This paper focuses on imaging of the intra-aggregate microstructure and, based on the three-dimensional (3D) images, quantification of the pore network connectivity and permeability tensor. Finally, lattice Boltzmann equation (LBE) simulations of Navier-Stokes flow in the thus derived pore network allow to compute the heterogeneous 3D flow velocity field. Nanotomographic X-ray absorption mode imaging of a single soil ped has been performed at the TOMCAT beamline of the Swiss Light Source synchrotron facility with 0.74 mu m spatial resolution. Segmentation of the 3D Nano-CT images into solid phase and pore space allowed to study the statistical properties of the connected pore network. The thus derived pore network data were used as direct input for the software package GeoDict integrated with a LBE algorithm to perform saturated water flow modeling. The soil ped features quantified from the tomographic images were pore and grain size distributions (PSD, GSD), porosity, percolation tensor in x, y, z-direction and percolation pathways (macropores). The PSD frequency has a peak in the range 3-8 mu m (mesopores) contributing 50% of the total. In general, the matrix pore structure and, hence, saturated flow field velocity of our sample is highly anisotropic. LBE-simulated pore scale fluid flow is used to ultimately determine gross parameters (coefficients) of Darcy's law such as the saturated hydraulic conductivity, K (s). The thus simulated K (s) = 105 +/- 24 cm day(-1) of a single soil ped (mean of three ROIs and directions) is one order of magnitude higher than the value of K (s) = 12 +/- 2 cm day(-1) predicted by a classical pedotranfer function approach. This K (s) underestimation by PTF is typical for macropore flow. Integration of 3D image evaluation with the LBE approach as an essential step toward understanding the highly heterogeneous intra-aggregate microstructure is now possible on the submicron scale. Modeling the resulting anisotropic fluid advection field is, however, just a first step for 3D models of biogeochemical reactions at soil interfaces on this pore scale.