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
Kyle Robinson, C. E. Robinson, James W. Roy, Meghan A. Vissers, Angelos Almpanis, Uwe Schneidewind, Christopher Power
Abstract
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 investigations. 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 conducted 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 information 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.