Measurement of Diffusion Coefficients in Porous Media

The role of diffusion in aquifers is gainning recognition as an important process controlling contaminant plume longevity.  Diffusion of contaminants into, and later back-diffusion out of, low permeability geologic features is thought to be an important rate limiting process for plume control or removal by pumping.  Currently, much of what is known about the role of diffusion stems from conceptual models supported by numerical modeling.  The diffusion coefficients used in these investigations are commonly obtained from empirical estimates of free solution diffusion coefficients modified for tortuosity.  This work seeks to make laboratory measurements of the diffusion of conservative tracers in media found in aquifers, ranging from silt to sand.  The effective diffusion coefficients are compared for the various media, and for layered systems, to evalute the assumptions that are currently widely used.