Modeling the Impacts of Hyporheic Zone Heterogeneity on Mass Exchange and Solute Transport in East Fork Poplar Creek, Tennessee, USA

Hyporheic residence time, solute transport, biogeochemical reactivity, and surface-groundwater fluid flow and mixing all influence chemical cycling and ecosystem functioning of streams. Most hyporheic zone (HZ) exchange investigations have examined the coupling of fluid flow and transport in either surface water or groundwater, but there is a need for further investigation of HZ processes including the coupling of surface water and groundwater models. For example, the commonly used transient storage model (e.g., OTIS code) considers transport in the stream with a 2-domain mass transfer (i.e., 1^st order rate-limited mass transfer) to account for porous media fluid flow and solute transport processes in the HZ and groundwater. This does not provide a mechanistic accounting for porous media processes that are critical to understanding impacts of hydraulic conductivity and gradient heterogeneity. To explicitly simulate the coupled processes of flow and solute transport in surface water and groundwater we coupled the watershed models with the groundwater models. Solute transfer through HZ was then based on the head gradient and hydraulic conductivity heterogeneities. The spatially heterogeneous distribution of HZ hydraulic conductivity in the model is being supported by field measurements of this property in East Fork Poplar Creek, Tennessee (EFPC). Several in-stream tracer tests in EFPC are being modeled with the OTIS code and the coupled model to compare HZ exchange and solute transport results. In particular, the effect of spatially heterogeneous sediment properties (e.g., hydraulic conductivity) on solute transport predictions is being evaluated. Results are providing insight on the consideration of porous media heterogeneity when simulating the flow and transport connectivity and exchange across the HZ.