Mass transfer in structured porous media: embedding mesoscale structure and microscale hydrodynamics in a two-region model

Abstract

The objective of this research is to incorporate mesoscale structure and microscale hydrodynamics into a solute transport model and to evaluate the worth of structural information in predicting solute movement within structured porous media. The structure of porous media is embedded into a two-region solute transport model in which the inter-region mass flux is characterized by mesoscale mass-transfer coefficients. The model was used to predict solute breakthrough in an undisturbed, fractured saprolite soil column. Diffusive mass transfer at the aggregate scale was explicitly accounted for by incorporating inter-region mass flux across the interface of fractures and matrix blocks. The other model parameters, including pore-region porosity, hydrodynamic dispersion coefficients, and flow velocities, were estimated independently using laboratory tracer injections. As a result, predictions were 50% more accurate than those obtained using a simple single-fracture conceptual model that represents a least-information scenario. Intra-region dispersion coefficients were also calculated theoretically by using a Taylor dispersion equation and available data on fracture apertures. Using the theoretically calculated dispersion coefficients resulted in an additional 50% gain of prediction accuracy. The theoretically calculated dispersion coefficients were orders of magnitude smaller than the experimentally estimated values that were obtained by fitting a discrete-fracture model to the experimental data. This result suggests that mesoscale spreading of tracer in structured porous media may be largely attributed to inter-region mass transfer.