INFLUENCE OF STREAMBED HETEROGENEITY ON GROUNDWATER-STREAM INTERACTIONS: INSIGHTS FROM COMBINED GEOPHYSICAL, TEMPERATURE AND GEOLOGICAL MEASUREMENTS

Groundwater-surface water interactions are highly spatial and temporally heterogeneous. Understanding this heterogeneity is needed to evaluate the transfer of water, nutrients and contaminants between GW and SW systems. Effective characterization of the GW-SW interface is challenging as traditional techniques are typically invasive, labor-intensive and suffer from low sampling density. The DC resistivity and, more recently, induced polarization (IP) geophysical methods are being increasingly used in environmental studies as they provide continuous and non-invasive imaging of subsurface properties. However their application to GW-SW studies is limited. The objective of this study was to evaluate the linkages between streambed heterogeneity and GW-SW interactions using combined DC-IP surveys supplemented by temperature, hydraulic gradient, and geological measurements.

The study site was a 50 m long headwater stream reach located in an agricultural area of the Thames watershed, Ontario, Canada. A high resolution DC-IP survey was performed along a 50 m x 2 m surface grid through the stream reach for high resolution characterization of the streambed subsurface heterogeneity. These survey results were compared with high resolution streambed temperature measurements and vertical hydraulic gradient measurements used to identify spatial GW-SW exchange patterns across the reach. Further, long-term vertical temperature transducer arrays were installed along the reach to quantify temporal variability in the GW-SW exchange fluxes.

The 3D electrical resistivity and chargeability images determined from DC and IP, respectively, reveal the structural heterogeneity of the streambed. Higher resistivity, indicating more permeable, sandy material, coincided with areas with strong groundwater upwelling, as determined by streambed temperature maps and vertical hydraulic gradient measurements. Lower resistivity and higher chargeability, which indicates less permeable, clayey material, was associated with weaker downwelling-upwelling patterns This study highlights the utility of using high-resolution DC-IP surveys for quantifying geological controls on groundwater-surface water interactions, which is useful for informing design and interpretation of GW-SW monitoring strategies and data.