EVALUATION OF THE CTRW APPROACH FOR MODELING THE POROUS MEDIA CONTAINING DECIMETER-SCALE PREFERENTIAL FLOW PATHS

The effectiveness of the classical advection-dispersion model (ADM) in describing solute transport in heterogeneous aquifers has been challenged by many studies. In particular, it has been shown that the ADM is not appropriate when heterogeneity gives rise to preferential flowpaths. In such situations the transport patterns, generally referred as non-Fickian, are characterized by highly asymmetric plumes with anomalous early high-concentration peaks and late time low-concentration tails. A recent development in study of non-Fickian transport is the continuous time random walk (CTRW) formulation (Berkowitz and Scher; 2001, Berkowitz et al., 2001). While this approach has been successfully applied for fitting breakthrough curves from laboratory and field experiments, no study has been conducted to test its applicability to handle solute transport in porous media containing small-scale preferential flowpaths. In this study we developed a numerical experiment to evaluate the effectiveness of the CTRW approach relative to the classical ADM in reproducing the transport pattern when small-scale preferential flowpaths are present. Our reference is a 2-dimensional synthetic aquifer characterized by a network of 10 cm wide high hydraulic conductivity (K) channels, embedded in otherwise homogeneous matrix. The main features of the synthetic aquifer are the same as in Zheng and Gorelick (2003) and the contrast in hydraulic conductivity between the channel and the remaining portion of the aquifer is 100:1. Numerical simulations were used to characterize groundwater flow and contaminant transport patterns. Only advection and molecular diffusion are considered. Observation points were used to collect synthetic breakthrough curves for comparison with fitted solutions from the ADM and CTRW models. The K and dispersivity values assigned to the ADM are the effective K of the synthetic aquifer and the dispersivity obtained from spatial moments analysis of the simulated plumes respectively. The CTRW models parameters are obtained from fitting the breakthrough curves of the observation points. Quantitative analysis and goodness-of-fit statistics were used to compare the concentration distributions of the ADM and CTRW models with the reference distribution in the synthetic aquifer with preferential flowpaths.