Time-lapse 3D electrical resistivity tomography for detecting preferential flow paths in a forestry hillslope using a brilliant blue tracer

Abstract

Understanding subsurface water movement, particularly preferential water flow, is critical for water resource management and predicting contaminant transport. However, traditional invasive methods like dye staining experiments are destructive and only provide static information. Consequently, this thesis investigates the effectiveness of 3D time-lapse electrical resistivity tomography (ERT) for monitoring infiltration dynamics on a forested hillslope in Lower Austria. A Brilliant Blue saline tracer was applied to a 0.8 × 0.8 m plot during a two-hour irrigation event. ERT measurements were collected at 7-minute intervals and processed using various inversion strategies to resolve resistivity changes over time. The resistivity data were then used to delineate preferential flow paths and estimate pore water velocities.The ERT monitoring successfully captured the decrease in resistivity, directly corresponding to the infiltration of the tracer.The wetting front migrated rapidly to a depth of 0.25 m within the first 45 minutes before slowing to a maximum of 0.28 m at the end of the irrigation. Pore water velocities, derived from resistivity breakthrough curves, varied from 0.0002 cm/min in the soil matrix to up to 6 cm/min in localized preferential flow paths. Comparison of the ERT inversion results with the post-excavation Brilliant Blue staining patterns confirmed that the centers of conductive anomalies align with the staining patterns, although discrete macropores and root channels could not beresolved due to the limited resolution of the method.These results demonstrate that 3D time-lapse ERT is a robust, non-invasive tool for quantifying the spatio-temporal dynamics of preferential flow, though its resolution remains constrained by the electrode array geometry and the inherent smoothing of the inversion process.