Articles | Volume 21, issue 6
Hydrol. Earth Syst. Sci., 21, 3105–3123, 2017
Hydrol. Earth Syst. Sci., 21, 3105–3123, 2017

Research article 29 Jun 2017

Research article | 29 Jun 2017

Electrical resistivity dynamics beneath a fractured sedimentary bedrock riverbed in response to temperature and groundwater–surface water exchange

Colby M. Steelman, Celia S. Kennedy, Donovan C. Capes, and Beth L. Parker Colby M. Steelman et al.
  • G360 Institute for Groundwater Research, College of Engineering and Physical Sciences, University of Guelph, Guelph, Ontario, Canada

Abstract. Bedrock rivers occur where surface water flows along an exposed rock surface. Fractured sedimentary bedrock can exhibit variable groundwater residence times, anisotropic flow paths, and heterogeneity, along with diffusive exchange between fractures and rock matrix. These properties of the rock will affect thermal transients in the riverbed and groundwater–surface water exchange. In this study, surface electrical methods were used as a non-invasive technique to assess the scale and temporal variability of riverbed temperature and groundwater–surface water interaction beneath a sedimentary bedrock riverbed. Conditions were monitored at a semi-daily to semi-weekly interval over a full annual period that included a seasonal freeze–thaw cycle. Surface electromagnetic induction (EMI) and electrical resistivity tomography (ERT) methods captured conditions beneath the riverbed along a pool–riffle sequence of the Eramosa River in Canada. Geophysical datasets were accompanied by continuous measurements of aqueous specific conductance, temperature, and river stage. Time-lapse vertical temperature trolling within a lined borehole adjacent to the river revealed active groundwater flow zones along fracture networks within the upper 10 m of rock. EMI measurements collected during cooler high-flow and warmer low-flow periods identified a spatiotemporal riverbed response that was largely dependent upon riverbed morphology and seasonal groundwater temperature. Time-lapse ERT profiles across the pool and riffle sequence identified seasonal transients within the upper 2 and 3 m of rock, respectively, with spatial variations controlled by riverbed morphology (pool versus riffle) and dominant surficial rock properties (competent versus weathered rock rubble surface). While the pool and riffle both exhibited a dynamic resistivity through seasonal cooling and warming cycles, conditions beneath the pool were more variable, largely due to the formation of river ice during the winter season. We show that surface electrical resistivity methods have the capacity to detect and resolve electrical resistivity transience beneath a fractured bedrock riverbed in response to porewater temperature and specific conductance fluctuations over a complete annual cycle.

Short summary
The Eramosa River flows along a fractured sedimentary bedrock aquifer with large subsurface channel features. This study examines the potential for groundwater–surface water exchange beneath the fractured bedrock riverbed and the impacts of seasonal and intraseasonal flow system transience on the geoelectrical properties of the rock. Our results will have implications to the conceptual understanding of groundwater–surface water interaction within fractured bedrock river environments.