The Role of Diffusion in Solute Transport and Reaction in Porous Media: Results from Direct Visualization of Laboratory Experiments
Abstract
We present the results of a set of novel experiments that accurately image solute transport and reaction in porous media using conservative dyes and colorimetric reactions. These experiments address two basic questions: (1) How do chemicals mix so that they react in natural porous media? (2) When is solute spreading best modeled by Fickian dispersion and when is it best modeled as mobile-immobile domain mass transfer? These questions are intimately connected because small-scale mixing and reaction profoundly affects large-scale spreading and vice versa. Numerical simulations and experiments with conservative tracers indicate that diffusion may either increase or decrease large-scale spreading depending on the variance and connectedness of the hydraulic conductivity field. We find that rate-limited mass transfer between mobile and immobile regions best describes solute mixing and spreading in aquifers with geologically-realistic flow paths, even though the characteristics of these aquifers may have the same spatial statistics used in widely known stochastic theories that predict dispersive spreading. Experiments in homogenous media using reactions where two clear reactants combine to form a brightly colored product show that conventional models may underestimate the degree of chemical reaction because the reactants do not mix at the microscopic pore-scale. In other cases, our experiments show the reverse -- the degree of reaction is much greater than predicted because the reaction creates density fingering that causes mixing in a fashion similar to the phenomena of double-diffusive mixing but with the density instability caused by the creation of a high molecular weight product that is slow to diffuse. Finally, we discuss the effects of different spatial patterns of hydraulic conductivity on the degree of chemical reaction.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.H21E..06H
- Keywords:
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- 1829 Groundwater hydrology;
- 1831 Groundwater quality