Linking Transition-Zone Geophysical Signals to Governing Reactive Processes in the Capillary Fringe

Abstract:

Hydro(bio)geophysical measurement techniques, such as Spectral Induced Polarization (SIP), provide high resolution information of soil electrical properties that are a function of key biogeochemical processes (i.e. biomineralization, reductive dissolution, microbial growth, etc.). The challenge, in using SIP for the accurate characterization and quantification of subsurface geochemistry, lies in the a priori linkage of measured electrical responses to changes in biogeochemical processes. We conducted a state-of-the-art soil column experiment with fully integrated monitoring of hydro-bio-geophysical variables, where an artificial soil mixture was subjected to monthly drainage and imbibition cycles. Periodic SIP and self-potential (SP) measurements were performed on the system to decouple hydro-geochemical drivers governing transient soil electrical responses. Solid-phase and aqueous geochemical results from the 10-month column experiment, showed depletion of organic carbon, sulphur accumulation and enhanced microbial activity within the water-table fluctuation zone. Transient SIP imaginary conductivity (σ'') profiles showed a distinct dynamic behaviour within the transition zone. Peaks in σ'' were inversely related to the degree of saturation, occurring within the zone of highest microbial activity and corresponded to periods of oxygenation, enhanced CO₂ production and sulphur oxidation. Pulses in headspace CO₂ fluxes were observed at the onset of water table drawdown, with decreasing peak magnitude following each subsequent wet-dry cycle, indicating the consumption of the finite labile carbon pool. An unsaturated flow and reactive transport model was developed to incorporate all dominant processes, highlighted from our conceptual understanding of experimental results, in order to fit experimental results of aqueous geochemical measurements, gas fluxes and solid phase geochemical profiles. Ongoing model calibration will shed light on the temporal dynamics of C, S and Fe geochemistry and allow for the decoupling of specific geochemical drivers behind the measured SIP and SP signals.