Diffusion And Flow In Gravel, Soil, And Whole Rock

Abstract

Transport parameters (diffusion coefficients, D(θ), hydraulic conductivities, K(θ), and retardation factors, Rf were experimentally determined in unsaturated soil, gravel, bentonite, and whole rock over a wide range of water contents, fixed at desired levels using the Unsaturated Flow Apparatus (UFATM). Effective diffusion coefficients in all media were found primarily to be a function of volumeric water cintent (θ) and not material characteristics, except where the characteristics affect or determine water content. At high water contents, D(θ) gradually declines as water content decreases, from 10-5cm2/s at a θ of about 50% to 10-7cm2/s at a θ of about 5%, followed by a sharp decline as surface films become thin and discontinuous, and pendular water elements become very small, from 10-7cm2/s at a θ of about 5% to 10-10cm2/s at a θ of about 0,5%. The several whole rock cores studied behaved similary. In aggregate material such as gravel and soil where the particles themselves have significant porosity, only the surface water content, not the internal water of the particles, contributes to the diffusion coefficient and hydraulic conductivity under unsaturated conditions, although the internal water is very important in retardation and other chemical effects. Experimentally determined K(θ) compares favorably to van Genuchten/Mualem relationships calculated from laboratory-determined water retention versus matric potential data obtained on the same soils. Experimentally determined K(θ) for whole rock appears to validate capillary bundle theory.