Conceptualizing Spatially Dynamic Groundwater-Surface Water Interactions along a Fractured Sedimentary Bedrock Riverbed through Electrical Resistivity and Induced Conductivity Measurements

The Eramosa River located within the Grand River Watershed in Ontario, Canada, resides upon a densely fractured dolostone aquifer with abundant dissolution-enhanced channel features. While the bedrock aquifer represents a major component of the total water supply for the surrounding region, potential effects of increased groundwater extraction on surface water and surrounding ecosystems are not yet fully understood. In general, little is known about the nature of the interaction between surface water and groundwater along densely fractured sedimentary bedrock riverbeds. For instance, the timing and magnitude of groundwater discharge to surface water and surface water recharge in a fractured carbonate rock environment is expected to differ significantly from its alluvial channel analogs due to the presence of highly transmissive, planar and strongly orientated discrete fracture features that control flow. Non-invasive geophysical methods represent an opportunity to evaluate shallow groundwater dynamics with minimal environmental risk to biota and, ultimately enhance the interpretation of direct hydrogeologic data.

Preliminary spatial electrical conductivity measurements collected along a 200 m reach of the river, suggests a seasonally dynamic hyporheic response over a riffle-pool sequence. Based on these results, time-lapse electrical resistivity measurements are being collected every 2-3 weeks along two ~50 m transects perpendicular to the river: the first transect considers changes across an intact bedrock section with exposed vertical and horizontal fracturing along the riverbed, while the second is positioned across a weathered bedrock section with a thin layer of course gravel and sand alluvium. The objective of this study is to collect electrical resistivity snapshots across the riverbed (6-8 m depths) to non-invasively quantify temporal and spatial changes in groundwater-surface water mixing from low to high-stage periods through multiple seasonal cycles. These geophysical measurements are being supported by multilevel vertical hydraulic head profiles, groundwater discharge measurements, continuous aqueous electrical conductivity of the surface water and groundwater, and time-lapse groundwater temperature profiles along lined boreholes.