The role of hyporheic fluxes in regional groundwater modelling

The effect of hyporheic fluxes on deep groundwater flow field was investigated in a numerical modelling framework over a spectrum of spatial scales ranging from local bed forms to landscape structures in a Swedish boreal catchment. The groundwater modelling was conducted for the whole catchment in which the site-specific landscape morphology and geological heterogeneity were accounted for. Deep groundwater discharge was quantified through conducting particle tracing analysis for 10,000 inert particles (grid of 100 × 100) released from a flat horizontal surface located 500 meter below the minimum topographical elevation. Further, the streambed scale modelling was performed independently by applying an exact spectral solution to the hyporheic fluxes in streambeds based on fluctuations of the streambed topography. Monte Carlo simulations were used in the streambed scale modelling to cover uncertainties in hydrostatic and dynamic head contributions, as well as topographic fluctuations. Through superpositioning of the two model results, we found that the magnitude of deep groundwater vertical velocity at the stream-water interface was generally lower than the hyporheic exchange velocity at the streambed interface. Finally, the deep groundwater particles’ travel time and the fragmentation of groundwater upwelling zones used as the main metrics to evaluate the impact of hyporheic fluxes on deep groundwater flow field. The results showed that the regional groundwater travel time distribution near the streambed surface was influenced by hyporheic fluxes, an impact that was  substantial for the particles with longer travel times. The size of coherent groundwater upwelling zone at the streambed interface was also affected by hyporheic fluxes. Almost half the superimposed cases were found to be more fragmented due to the presence of hyporheic flow field, which shifted the cumulative distribution function for upwelling regions towards smaller areas. This study, highlights the role of hyporheic fluxes in groundwater modelling, which controls the streambed sediment ecosystem as well as fate and transport of contaminations between aquifer and streams.