Improved interpretation of groundwater-surface water interactions along a stream reach using 3D high-resolution combined DC resistivity and induced polarization (DC-IP) geoelectrical imaging

Common approaches for characterizing streambed architecture, and its influence on groundwater-surface water (GW-SW) exchanges, are generally limited by their invasiveness and low spatial sampling density, which is a particular issue in streambeds that typically have high spatial heterogeneity. Combined DC resistivity and induced polarization (DC-IP) imaging can provide rapid, non-invasive and continuous information on streambed lithology; however, its full potential remains unrealized, leading to its underutilization for streambed in-vestigations. The objective of this study is to demonstrate the value of DC-IP imaging, in both 3D and high -resolution, for characterizing streambed architecture and interpretating GW-SW exchange patterns. The study focused on a 50 m long stream reach located in Kintore, Ontario, Canada. Traditional methods - streambed temperature mapping, vertical head gradient measurements, streambed porewater quality, and sediment cores - were used to qualitatively identify spatial GW-SW exchanges. Underwater 3D DC-IP surveying was then con-ducted across the stream reach to obtain high-resolution distributions of resistivity and chargeability. Resistivity first identified three distinct zones along the stream reach: Zone 1 (0-12 m) and Zone 3 (38-50 m) exhibits high resistivity (>100 ohm-m), while Zone 2 (12-38 m) exhibits relatively low resistivity (<40 ohm-m). Chargeability highly complements resistivity by confirming that the shallow streambed contains only non-clayey materials; therefore, the more resistive Zones 1 and 3 is attributed to more permeable coarse sand and gravel, while the more conductive Zone 2 is attributed to less permeable finer sand, and consequently, increased porewater EC due to longer residence times and hyporheic exchanges. This geoelectrical interpretation is well-supported by in-formation from traditional methods (e.g., higher temperature and hydraulic gradients correspond to the more permeable Zone 1 and Zone 3). This study demonstrates the unrealized value of spatially continuous, high -resolution DC-IP information for mapping streambed architecture and its control on GW-SW exchange patterns.

Automated summary

This study demonstrates the value of DC-IP imaging, in both 3D and high-resolution, for characterizing streambed architecture and interpreting groundwater-surface water exchange patterns. It shows that DC-IP imaging can provide rapid, non-invasive, and continuous information on streambed lithology and its control on GW-SW exchanges, which has been underutilized in previous investigations.