Simulation of solute transport through heterogeneous networks: analysis using the method of moments and the statistics of local transport characteristics

Abstract

Abstract We used a time domain random walk approach to simulate passive solute transport in networks. In individual pores, solute transport was modeled as a combination of Poiseuille flow and Taylor dispersion. The solute plume data were interpreted via the method of moments. Analysis of the first and second moments showed that the longitudinal dispersivity increased with increasing coefficient of variation of the pore radii CV and decreasing pore coordination number Z. The third moment was negative and its magnitude grew linearly with time, meaning that the simulated dispersion was intrinsically non-Fickian. The statistics of the Eulerian mean fluid velocities $${\hat{{\boldsymbol{u}}}}_{{\boldsymbol{i}}}$$ uˆi , the Taylor dispersion coefficients $${\hat{{\boldsymbol{D}}}}_{{\boldsymbol{i}}}$$ Dˆi and the transit times $${\hat{{\boldsymbol{\tau }}}}_{{\boldsymbol{i}}}$$ τˆi were very complex and strongly affected by CV and Z. In particular, the probability of occurrence of negative velocities grew with increasing CV and decreasing Z. Hence, backward and forward transit times had to be distinguished. The high-τ branch of the transit-times probability curves had a power law form associated to non-Fickian behavior. However, the exponent was insensitive to pore connectivity, although variations of Z affected the third moment growth. Thus, we conclude that both the low- and high-τ branches played a role in generating the observed non-Fickian behavior.