Monitoring redox sensitive conditions at the groundwater interface using electrical resistivity and self-potential

Commonly used redox potential sensors provide only point measurements and are error prone. On the other hand, destructive sampling can potentially cause contact with air which will influence the redox state. Hence, assessing redox conditions in soil and groundwater is challenging. A conductive metal bar connecting the aerobic and anaerobic zones has shown to give a Self-Potential (SP) anomaly due to an electron flow created by the electrochemical differences between the two zones (Revil et al, 2015). Since anaerobic degradation causes the release iron and manganese ions (assuming oxides are available), the electrical conductivity of the water phase is increased and therefore electrical resistivity (ER) imaging should be an applicable method. Also the release of methane gas (if iron and manganese oxides are unavailable), will reduce the water saturation, another ER sensitive change. In this work we assess whether a combination of 3D ER and surface SP time-lapse measurements can map the development of redox zonation in partially water filled laboratory sand tanks during localised degradation of an organic contaminant (propylene glycol). Pore water was sampled to map degradation and water chemistry.

When iron and manganese oxides were available, degradation reduced resistivity, because of cation release in pore water. While increased resistivity, most likely from methane production which reduced water saturation, was measured when iron and manganese oxides were unavailable. A vertically installed metal pipe connecting the PG degrading zone with the near surface oxygenated zone higher up in the profile caused an SP anomaly, indicating flow of electron surplus from the anaerobic zone through the metal pipe. This detailed laboratory study in a static water system demonstrates that the combination of time lapse ER and SP are powerful tools for mapping the locations of redox affected degradation processes.

Reference. Revil, A., Fernandez, P., Mao, D., French, H., Bloem, E. and Binley, A. (2015). Self-potential monitoring of the enhanced biodegradation of an organic contaminant using a bioelectrochemical cell. The Leading Edge, 34(2), pp.198-202.