Non-invasive monitoring of a reactive soil transition zone during water table fluctuations using spectral induced polarization (SIP) and electrodic potential (EP)

Transition zones separating the unsaturated and saturated domains in soils are a hotspot for biogeochemical activity. They are challenging to study because they are dynamic, requiring high resolution temporal and spatial data acquisition methods to capture the biogeochemical processes across the water table. Non-invasive geophysical techniques, such as spectral induced polarization (SIP) and electrodic potential (EP), offer comparatively inexpensive monitoring approaches that yield data on changes in soil electrical properties, driven by reactive processes at high spatial and temporal resolutions. We investigated SIP and EP signal variations in artificial soil-filled columns, experiencing periodic water table fluctuations in order to: (1) assess the effectiveness of SIP and EP in monitoring a complex soil transition zone, and (2) couple the measured geophysical signals to changes in physical, chemical and microbial properties. SIP responses showed a clear dependence on the depth-distribution of microbial biomass. Dynamic imaginary conductivity (σ'') responses were only detected in the water table fluctuation zone and, in contrast to real conductivity (σ') data, did not exhibit a direct soil moisture driven dependence. We attribute the observed dynamics in σ'' to microbially driven reactions. An EP anomaly arose concurrent to the production of SO₄^2- as a result of oxygenation at depth during drainage of the columns. Our findings show that continuous SIP and EP signals, in conjunction with periodic measurements of geochemical indicators, can help determine the location and temporal variability of biogeochemical activity and be used to monitor targeted reaction zones and pathways in complex soil environments.