MASS-TRANSFER IN STRUCTURED POROUS-MEDIA - EMBEDDING MESOSCALE STRUCTURE AND MICROSCALE HYDRODYNAMICS IN A 2-REGION MODEL

The objective of this research is to incorporate mesoscale structure a nd microscale hydrodynamics into a solute transport model and to evalu ate the worth of structural information in predicting solute movement within structured porous media. The structure of porous media is embed ded into a two-region solute transport model in which the inter-region mass flux is characterized by mesoscale mass-transfer coefficients. T he model was used to predict solute breakthrough in an undisturbed, fr actured saprolite soil column. Diffusive mass transfer at the aggregat e scale was explicitly accounted for by incorporating inter-region mas s flux across the interface of fractures and matrix blocks. The other model parameters, including pore-region porosity, hydrodynamic dispers ion coefficients, and flow velocities, were estimated independently us ing laboratory tracer injections, As a result, predictions were 50% mo re accurate than those obtained using a simple single-fracture concept ual model that represents a least-information scenario. Intra-region d ispersion coefficients were also calculated theoretically by using a T aylor dispersion equation and available data on fracture apertures. Us ing the theoretically calculated dispersion coefficients resulted in a n additional 50% gain of prediction accuracy, The theoretically calcul ated dispersion coefficients were orders of magnitude smaller than the experimentally estimated values that were obtained by fitting a discr ete-fracture model to the experimental data. This result suggests that mesoscale spreading of tracer in structured porous media may be large ly attributed to inter-region mass transfer. (C) 1998 Elsevier Science B.V. All rights reserved.