Geophysical Imaging of Hyporheic Mixing Dynamics Within Compound Bar Deposits

The interactions between surface water and groundwater (hyporheic exchange) influence water quality and control numerous physical, chemical, and biological processes. Despite its importance, hyporheic exchange and the associated dynamics of solute mixing are often difficult to characterize due to spatial (e.g., sedimentary heterogeneity) and temporal (e.g., river stage fluctuation) variabilities. This study applied geophysical techniques (electromagnetic induction (EMI) and electrical resistivity tomography (ERT)), grain size analysis, loss on ignition (LOI), and x-ray fluorescence (XRF) to examine the hyporheic zone within a compound bar (CB) deposit with complex sedimentary architectures within a gravel dominated river system in SW Ohio. EMI was used to analyze the entirety of the CB which depicted sedimentary heterogeneity with variability in bulk electrical conductivity. A zone of relatively high electrical conductivity was determined to be fine grained cross-bar channel fill. Such channel fills play an important role in hyporheic exchange dynamics and the resulting biogeochemical processes due to their significant differences in physical and geochemical properties compared to different sediments within the CB deposit. A grid of electrodes for time-lapse ERT survey was placed transecting the area of high electrical conductivity found in EMI. Results from ERT also depict distinct zonation within the CB, and analysis of changes in electrical resistivity over the survey period allow for the detection of preferential flow paths through sediments with high hydraulic conductivities (e.g., sandy gravel). Characteristics of sediment samples observed in LOI and XRF (e.g., organic matter content) also allow for the inference of locations favorable for enhanced geochemical and microbial activity within the CB. While geophysical methods have been previously employed in the hyporheic zone there is novelty to this study’s approach. EMI has not been extensively used in the characterization the hyporheic zone, nor has it been used to inform locations for an ERT survey. Few studies have addressed the role that CB deposits play in the hyporheic zone. Results from this thesis will provide fundamental insights into hyporheic exchange and effective techniques applied in this study could have implications for improving future study of hyporheic exchange and developing numerical models through representing most important features of sedimentary architecture.