Modeling non-Fickian solute transport due to mass transfer and physical heterogeneity on arbitrary groundwater velocity fields

TL;DR

This paper introduces a hybrid particle tracking method called "CTRW-on-a-streamline" for modeling non-Fickian solute transport in heterogeneous groundwater flows, integrating complex heterogeneity and mass transfer effects.

Contribution

It develops a novel approach combining CTRW with explicit heterogeneous velocity fields, allowing separate parameterization of flow, heterogeneity, and mass transfer from data.

Findings

Method successfully models non-Fickian transport in heterogeneous fields.

Connections to classic CTRW and subordination are established.

Numerical tests validate the approach.

Abstract

We present a hybrid approach to groundwater transport modeling, "CTRW-on-a-streamline", that allows continuous-time random walk (CTRW) particle tracking on large-scale, explicitly-delineated heterogeneous groundwater velocity fields. The combination of a non-Fickian transport model (in this case, the CTRW) with general heterogeneous velocity fields represents an advance of the current state of the art, in which non-Fickian transport models or heterogeneous velocity fields are employed, but generally not both. We present a general method for doing this particle tracking that fully separates the model parameters characterizing macroscopic flow, subscale advective heterogeneity, and mobile-immobile mass transfer, such that each can be directly specified a priori from available data. The method is formalized and connections to classic CTRW and subordination approaches are made. Numerical corroboration is presented.