PNNL

Abstract

Solute transport in interconnected rivers and hyporheic zones is typically modeled through dual-domain models where first-order solute mass transfer between the two domains, Omega R and Omega HZ, is represented by a coefficient alpha. The transient storage model (TSM) is an example of such an approach. In practice, alpha is determined by fitting the tails of solute tracer breakthrough curves using a TSM. This approach has led to ambiguity regarding alpha's physical meaning and transferability. Here, we investigated the physical basis for alpha and tested it with virtual experiments through the fully coupled multiphysics model hyporheicFoam for the Omega R-Omega HZ system. hyporheicFoam explicitly simulated coupled flow and solute transport over a kilometer with centimeter-scale resolution. Model results were analyzed to calculate alpha following its theoretical definition directly. When this calculated alpha is used within a TSM, the TSM is able to reproduce the true solute transport, thereby showing that this alpha is physically meaningful and correct.