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Lagrangian modeling of mixing-limited reactive transport in porous media: multi-rate interaction by exchange with the mean
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  • Guillem Sole-Mari,
  • Daniel Fernàndez-Garcia,
  • Xavier Sanchez-Vila,
  • Diogo Bolster
Guillem Sole-Mari
Universitat Politècnica de Catalunya, Universitat Politècnica de Catalunya

Corresponding Author:[email protected]

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Daniel Fernàndez-Garcia
Universitat Politècnica de Catalunya, Universitat Politècnica de Catalunya
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Xavier Sanchez-Vila
Universitat Politècnica de Catalunya, Universitat Politècnica de Catalunya
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Diogo Bolster
University of Notre Dame, University of Notre Dame
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Abstract

The presence of solute concentration fluctuations at spatial scales much below the scale of resolution is a major challenge for modeling reactive transport in porous media. Overlooking small-scale fluctuations, which is the usual procedure, often results in strong disagreements between field observations and model predictions, including, but not limited to, the overestimation of e˙ective reaction rates. Existing innovative approaches that account for local reactant segregation do not provide a general mathematical formulation for the generation, transport and decay of these fluctuations and their impact on chemical reactions. We propose a Lagrangian formulation based on the random motion of fluid particles carrying solute concentrations whose departure from the local mean is relaxed through multi-rate interaction by exchange with the mean (MRIEM). We derive and analyze the macroscopic description of the local concentration covariance that emerges from the model, showing its potential to simulate the dynamics of mixing-limited processes. The action of hydrodynamic dispersion on coarse-scale concentration gradients is responsible for the production of local concentration covariance, whereas covariance destruction stems from the local mixing process represented by the MRIEM formulation. The temporal evolution of integrated mixing metrics in two simple scenarios shows the trends that characterize fully-resolved physical systems, such as a late-time power-law decay of the relative importance of incomplete mixing with respect to the total mixing. Experimental observations of mixing-limited reactive transport are successfully reproduced by the model.
Aug 2020Published in Water Resources Research volume 56 issue 8. 10.1029/2019WR026993