Are the mass transfer parameters in an in-stream transient storage model physically meaningful?

Abstract:

Hyporheic zones and in-stream stagnant zones delay the transport of solute in rivers. To model this impact on bulk (or integrated) solute transport, transient storage models (TSMs, or mobile-immobile domain models to be more general) attempt to represent the bulk transport process with delays by connecting a one-dimensional channel with well-mixed storage zones. Mass transfer between the channel and these storage zones are parameterized as first-order exchange with the mass transfer coefficient alpha (α); the extent of the storage zone is expressed as its flow-perpendicular cross-sectional area As which is typically normalized by or related with the channel cross-sectional area A. However, the physical meaningfulness of parameters used in these TSMs, such as the popular implementation in the USGS code OTIS (for one-dimensional transport with inflow and storage) is still unclear and seldom scrutinized with well-constrained experiments. We conducted a robust assessment by using the fully coupled novel model, hyporheicFoam, to understand the flow and reactive transport in multiple zones. In this study, we use OTIS-P (a modified version of OTIS) in conjunction with the novel hyporheicFoam to study the physical meaning of the TSM parameters alpha and As. The integral flow and solute transport across the multiple zones will be solved by hyporheicFoam for a one-kilometer long system. That is hyporheicFoam simulations will provide everything about the channel-transient (hyporheic zone) storage system, i.e. alpha and As. Using the hyporheicFoam, we simulated the BTCs at different positions along the channel. In conjunction with the hyporheicFoam generated data, OTIS-P has been used to analyze the transport process and to quantify alpha and As, plus other parameters. The virtual experiments considered cases with different sediment permeability, which should impact alpha. Preliminary results show that for reasonably small sediment hydraulic conductivity, the hyporheic flow does significantly affect the solute transport, especially on the long tail and the TSM parameters. Next, we expect to provide a robust assessment of whether the TSM parameters are indeed physically meaningfully or if they should be viewed simply as fitting parameters that help with prediction with nonetheless unclear physical underlying them.