Quantifying Bedform-Scale Hyporheic Exchange in Bedrock Rivers: The Eramosa Bedrock River Field Site

The effects of river channel characteristics on hydrostatic and hydrodynamic pressure variations that drive hyporheic exchange have been the focus of numerous studies over the past decade. The hydrogeological settings of such studies have exclusively considered alluvial rivers, where the riverbed contains layers of unconsolidated fluvial sediment. Departing from convention, we seek to determine whether hyporheic exchange occurs within fractured bedrock rivers which lack unconsolidated fluvial sediment overburden. Here we expect advective pathways are dominated by discrete, planar fractures with very low porosity, which lead to less potential mixing with groundwater, very rapid velocities and low residence times. Our study focuses on a 61m riffle-pool-riffle sequence of the Eramosa River, near Guelph, Ontario, which flows along a reach of fractured sedimentary bedrock.

To simulate hyporheic exchange we have developed a discrete fracture network (DFN) finite element groundwater model of the riffle-pool-riffle sequence. We defined our fracture network based on field investigations to determine representative distributions of fracture frequency, length, orientation, and hydraulic aperture. Realizations of the DFN via Monte Carlo simulation were compared to an equivalent porous media (EPM) model, assuming homogeneous anisotropic conditions throughout the domain. For both DFN and EPM models, we quantify hyporheic exchange fluxes by releasing a conservative solute tracer in areas of downwelling and calculate cumulative mass distributions in areas of upwelling.

Results indicate that hyporheic exchange in a bedrock river setting is sensitive to the coincidence of hydrodynamic pressure variations at the riverbed and the presence of connected fracture pathways across the riffle-pool-riffle sequence. However, further work is required in identifying the exact role of fracture network heterogeneity in defining the lateral to vertical extent of groundwater flow paths and hyporheic exchange residence time distributions.