Exploring asymptotically-Fickian chemical transport in porous media.

Transport of chemicals through geological porous media can exhibit a wide range of behavior, from Fickian to highly anomalous, including a range of asymptotically-Fickian behavior wherein transport conforms to Fickian behavior only at long times and/or large distances. Asymptotically-Fickian transport behavior remains under-explored in the literature, and in many cases transport is considered simply either Fickian or anomalous. Here, a spectrum-based perspective for transport of chemicals is presented, which can better describe the subtleties of asymptotically-Fickian transport. A continuous time random walk particle tracking (CTRW-PT) model is used to describe transport through porous media, considering a wide parameter space representative of a broad range of transport behavior, all within the asymptotically-Fickian regime. Experimental results of column experiments are then analyzed using the same CTRW-PT model. Combined, these two sets of results - pure simulations and modeling of experiments - show the validity and utility of allowing a wide parameter space for CTRW-PT modeling. Additionally, several easy-to-calculate parameters are assessed as possible indicators for trends in the degree of anomalous transport, particularly in the context of evolving systems.