Critical Path Analysis of the Hydraulic Conductivity of Fractal Soils at the US DOE Hanford Site

Critical path analysis from percolation theory is used to calculate the hydraulic conductivity, K, of soils with pore space compatible with a (sometimes complex) fractal description. The fractal descriptions were chosen according to the particle-size distributions of two different soils at the US Department of Energy Hanford Site. One soil exhibited a bimodal particle-size distribution, and treated as a "dual" fractal. Results were compared with measured hydraulic properties of the two soils. Analysis yielded excellent agreement with experiment in most investigated properties without use of fitting parameters. The agreement was at least sufficient to aid in interpreting phenomena that related to the bimodal particle-size distribution, such as a rapid increase in the spread of hydraulic conductivity values with reduction of matric potential. The least certain parameter for calculation of K is the "critical volume fraction," which describes the minimum water content for which an interconnected network of capillary flow exists. The values of the critical volume deduced, however, allowed consistent interpretation in both soils investigated, in contrast to fitted values of a "residual moisture content" obtained by application of the van Genuchten function. Further, these values were in accord with recent published values of the threshold moisture content for solute diffusion.