Interpretation and nonuniqueness of CTRW transition distributions: Insights from an alternative solute transport formulation

Scott K. Hansen, Brian Berkowitz

Research output: Contribution to journalArticlepeer-review

16 Scopus citations

Abstract

The continuous time random walk (CTRW) has both an elegant mathematical theory and a successful record at modeling solute transport in the subsurface. However, there are some interpretation ambiguities relating to the relationship between the discrete CTRW transition distributions and the underlying continuous movement of solute that have not been addressed in existing literature. These include the exact definition of "transition", and the extent to which transition probability distributions are unique/quantifiable from data. Here, we present some theoretical results which address these uncertainties in systems with an advective bias. Simultaneously, we present an alternative, reduced parameter CTRW formulation for general advective transport in heterogeneous porous media, which models early- and late-time transport by use of random transition times between sparse, imaginary planes normal to flow. We show that even in the context of this reduced-parameter formulation there is nonuniqueness in the definitions of both transition lengths and waiting time distributions, and that neither may be uniquely determined from experimental data. For practical use of this formulation, we suggest Pareto transition time distributions, leading to a two-degree-of-freedom modeling approach. We then demonstrate the power of this approach in fitting two sets of existing experimental data. While the primary focus is the presentation of new results, the discussion is designed to be pedagogical and to provide a good entry point into practical modeling of solute transport with the CTRW.

Original languageEnglish
Pages (from-to)54-63
Number of pages10
JournalAdvances in Water Resources
Volume74
DOIs
StatePublished - 1 Dec 2014
Externally publishedYes

Keywords

  • Heterogeneous media
  • Pareto distribution
  • Random walk
  • Renewal process
  • Solute transport
  • Upscaling

ASJC Scopus subject areas

  • Water Science and Technology

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