EXPERIMENTS ON SOLUTE TRANSPORT IN AGGREGATED POROUS-MEDIA - ARE DIFFUSIONS WITHIN AGGREGATES AND HYDRODYNAMIC DISPERSION INDEPENDENT

Two region models for solute transport in porous media assume that hyd rodynamic dispersion in mobile water and solute diffusion within immob ile water regions are independent. Experimental and theoretical result s for transport through a macropore indicate that hydrodynamic dispers ion and solute exchange are interdependent. Experiments were carried o ut to investigate this problem for a column packed with spherical poro us aggregates. The effective diffusion coefficient of a tracer within the agreggates was determined from specific experiments. The dispersiv ity of the bed was determined from experiments carried out with a colu mn filled with nonporous beads. We took advantage of the dependence of hydrodynamic dispersion on density ratios between the invading and di splaced solutions to obtain a set of breakthrough curves corresponding to situations where the diffusion coefficient remains constant, where as the dispersivity varies. Simulations reproduce correctly the experi ments. Small discrepancies are noted that can be corrected either by i ncreasing the dispersion coefficient or by fitting the external mass t ransfer coefficient. Increased dispersion coefficients probably reveal a modification of Taylor dispersion due to solute exchange. The fitte d external mass transfer coefficients are close to the values obtained with classical correlations of the chemical engineering literature.