Groundwater - Surface Water Interactions in the Discrete Fracture Networks of Bedrock Rivers

Kennedy, Celia Sylvia Cassis
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University of Guelph

Bedrock rivers exist where surface water flows along an exposed riverbed aquifer, but little is known about their physical and chemical properties. Groundwater and surface water are linked at the streambed interface, leading to shared sustainability issues. The sharing of common pathways into and out of the streambed fracture networks provides opportunity for the exchange of thermal, chemical and biological constituents, affecting water quality and ecosystem health. Alluvial rivers exhibit granular beds and their flow patterns are well understood. Much of our water-resource management decisions are based on alluvial river conceptual models using an equivalent porous media (EPM) approach. Since bedrock rivers are more challenging to instrument, their complex flow patterns have not been addressed in the discrete fracture network (DFN) context, thus, there is a gap in the literature. This is the first study of a bedrock river yielding a field-based conceptual model of the spatio-temporal variability of groundwater fluxes and head differentials between groundwater and surface water in the upper 0.30 m of an intact dolostone streambed. A field site along the Eramosa River, in Guelph, ON, Canada, was developed, where the longitudinally-stepped profile of a bedrock riffle-pool sequence exists within a channel meander. The new field site was heavily instrumented with an innovative monitoring system designed for use along vertical and bedding plane riverbed fractures. Thus, a three-fold contribution has been made, to advance our understanding of bedrock river flow systems, including: the design of tools for measuring hydraulic parameters, the development of a field site to test them, and the spatio-temporal conceptualization of groundwater – surface water exchanges along an intact bedrock river channel and glaciofluvial plain. Groundwater flow measurements ranged from -0.4 – 55 mL/min, with uncertainties of 13 – 40%. Fluxes of 0.12 – 0.99 m/day and average linear groundwater velocities of 7 – 985 m/day were estimated from flow. Relative head differentials, measured under suction between groundwater and surface water, ranged from 0.001 – 0.023 m +/- 0.001, and vertical hydraulic gradients ranged from 0.02 – 0.46. Groundwater velocities in a bedrock river were observed to be influenced by: (1) proximity to a vertical fracture, (2) topographic relief or elevation, (3) channel geometry and (4) regional boundary conditions.

groundwater, surface water, interactions, bedrock river, discrete fracture network, seepage, inclined coreholes, riverbed piezometer, streamflow gauge, channel bathymetry, streambed temperature, Eramosa River, Eramosa Formation, Goat Island Formation, Gasport Formation, Arkell Springs