Abstract
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.
Highlights
Fractured sedimentary bedrock represents an important source of water for many communities around the world
This uncertainty in the driving mechanism of observed electrical changes below the riverbed hindered our ability to definitively define the vertical extent of a potential groundwater–surface water mixing zone, our geophysical dataset does suggest that a groundwater–surface water mixing in a bedrock environment may be more limited due to the lower effective porosity of rock and heterogeneous and anisotropic fracture distributions
Time-lapse resistivity measurements were performed across a 200 m reach of the Eramosa River during low and highstage periods
Summary
Fractured sedimentary bedrock represents an important source of water for many communities around the world. The Eramosa River – a major tributary of the Speed River within the Grand River watershed in Ontario, Canada – resides upon a regional bedrock aquifer of densely fractured dolostone with dissolution-enhanced conduits and karst features (e.g., Kunert et al, 1998; Kunert and Coniglio, 2002; Cole et al, 2009). This aquifer represents the sole source of drinking water for the region, the potential effects of increased groundwater pumping on the overlying bedrock river and surrounding ecosystems are not yet understood. The temperate southern Ontario climate subjects the river to a wide range of seasonal conditions, including high precipitation periods in spring and fall, hot and dry summers, and variable degrees of ground frost and surface water freeze-up during the winter months (Fig. 3a)
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