Continuous Time Random Walks for Non-Local Radial Solute TransportReport as inadecuate

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1 Institute of Environmental Assessment and Water Research IDÆA, Barcelona 2 MIT - Massachusetts Institute of Technology 3 GR - Géosciences Rennes

Abstract : This paper derives and analyzes continuous time random walk CTRW models in radial flow geometries for the quantification of non-local solute transport induced by heterogeneous flow distributions and by mobile-immobile mass transfer processes. To this end we derive a general CTRW framework in radial coordinates starting from the random walk equations for radial particle positions and times. The particle density, or solute concentration is governed by a non-local radial advection-dispersion equation ADE. Unlike in CTRWs for uniform flow scenarios, particle transition times here depend on the radial particle position, which renders the CTRW non-stationary. As a consequence, the memory kernel characterizing the non-local ADE, is radially dependent. Based on this general formulation, we derive radial CTRW implementations that i emulate non-local radial transport due to heterogeneous advection, ii model multirate mass transfer MRMT between mobile and immobile continua, and iii quantify both heterogeneous advection in a mobile region and mass transfer between mobile and immobile regions. The expected solute breakthrough behavior is studied using numerical random walk particle tracking simulations. This behavior is analyzed by explicit analytical expressions for the asymptotic solute breakthrough curves. We observe clear power-law tails of the solute breakthrough for broad power-law distributions of particle transit times heterogeneous advection and particle trapping times MRMT model. The combined model displays two distinct time regimes. An intermediate regime, in which the solute breakthrough is dominated by the particle transit times in the mobile zones, and a late time regime that is governed by the distribution of particle trapping times in immobile zones.

Keywords : Random Walk Particle Tracking Stochastic Modeling Multirate Mass Transfer Continuous Time Random Walks Radial Transport Non-Local Transport

Author: Marco Dentz - Peter Kang - Tanguy Le Borgne -



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