Quantifying multi-scale advective hyporheic exchange through mass transfer

Mixing between water from free surface flow and subsurface pore-water in the sediment bed is known as hyporheic exchange. At local scales, flushing through porous sediments purifies streamborne water and biogeochemical cycling of oxygen and nutrients supports organisms in the benthic region of the sediment bed. At larger scales, hyporheic exchange provides the connectivity between streams, adjacent aquifers and floodplain networks, providing a critical link for the distribution and retention of nutrients and pollutants throughout the environment. This thesis proposes a framework to upscale advective hyporheic exchange processes using mass transfer coefficients, assuming a thin-film model and is presented in four key contributions: (1) an explicit derivation of the mass transfer coefficient for generic downwelling-upwelling cells based on the geometric addition of regional fluxes with periodic flushing of the hyporheic zone; (2) an examination of steady-state and turbulent mass transfer, demonstrating that steady and unsteady processes can each be represented by a mass transfer coefficient; (3) experiments indicate downwelling and upwelling cells are associated with steady and unsteady mass transfer coefficients; and (4) the analogy between the thin-film model and Ohm’s Law is used to shown that resistors representing steady and unsteady mass transfer are best aligned in parallel, suggesting mass transfer coefficients are additive. This provides a framework to model hyporheic exchange at different spatial and temporal scales, in which no contributing scale is rate limiting. It is anticipated that this will enhance existing predictive hyporheic exchange models, making them more applicable to the study and management of natural water resources at any scale