Tracer diffusion coefficients in sedimentary rocks: correlation to porosity and hydraulic conductivity

Matrix diffusion is an important transport process in geologic materials of low hydraulic conductivity. For predicting the fate and transport of conta minants, a detailed understanding of the diffusion processes in natural por ous media is essential. In this study, diffusive tracer transport (iodide) was investigated in a variety of geologically different limestone and sands tone rocks. Porosity, structural and mineralogical composition, hydraulic c onductivity, and other rock properties were determined. The effective diffu sion coefficients were measured using the time-lag method. The results of t he diffusion experiments indicate that there is a close relationship betwee n total porosity and the effective diffusion coefficient of a rock (analogo us to Archie's Law). Consequently, the tortousity factor can be expressed a s a function of total porosity. The relationship fits best for thicker samp les ( > 1.0 cm) with high porosities ( > 20%). because of the reduced influ ence of heterogeneity in larger samples. In general, these correlations app ear to be a simple way to determine tortuosity and the effective diffusion coefficient from easy to determine rock porosity values. (C) 2001 Elsevier Science B.V. All rights reserved.