INFORMATION LOSS IN ELECTRICAL INVERSIONS: NUMERICAL STUDY OF SOLUTE TRACER TESTS IN THE HYPORHEIC ZONE

Hyporheic zones retain solutes and influence stream water quality, a process that has traditionally been studied with solute tracer injections. Over the last decade, a growing number of injections have been paired with time-lapse resistivity imaging to map the arrival and flushing of solute within the hyporheic zone. Inversion results like pixel breakthrough curves significantly extend our understanding of hyporheic processes but are also influenced by inversion artifacts such as smoothing. Here, we use a synthetic modeling approach to explore questions about the detection capabilities of electrical resistivity imaging for hyporheic zones. For a synthetic channel cross-section, we create a coupled fluid flow, solute transport, and electrical flow model to simulate solute tracer movement and voltage readings during a time-lapse electrical resistivity survey. We then invert the synthetic resistivity data to obtain electrical resistivity images for comparison with known solute distributions. We explore the influence of different solute plateau concentrations, injection periods, and channel geometries. Preliminary results show that inversions detect a significant fraction of salt remaining in the hyporheic zone for several hours after the injection has ended, and the detection window can be extended by increasing the solute concentration of the injection plateau. During the early part of the detection period, the area of the hyporheic zone that contains solute is overestimated due to smoothing in the inversion, but the area rapidly diminishes and becomes underestimated as solute spreads within the hyporheic zone and concentrations decline below detection in the inversion. Our results can be used to guide future field experiments and the interpretation of inversions.