Ionic contribution to the self-potential signals associated with a redox front

Abstract

In contaminant plumes or in the case of ore bodies, a source current density is produced at depth in response to the presence of a gradient of the redox potential. Two charge carriers can exist in such a medium: electrons and ions. Two contributions to the source current density are associated with these charge carriers (i) the gradient of the chemical potential of the ionic species and (ii) the gradient of the chemical potential of the electrons (i.e., the gradient of the redox potential). We ran a set of experiments in which a geobattery is generated using electrolysis reactions of a pore water solution containing iron. A DC power supply is used to impose a difference of electrical potential of 3 V between a working platinum electrode (anode) and an auxiliary platinum electrode (cathode). Both electrodes inserted into a tank filled with a well-calibrated sand infiltrated by a (0.01 mol L(-1) KCl+0.0035 mol L(-)(1) FeSO(4)) solution. After the direct current is turned off, we follow the pH, the redox potential, and the self-potential at several time intervals. The self-potential anomalies amount to a few tens of millivolts after the current is turned off and decreases over time. After several days, all the redox-active compounds produced initially by the electrolysis reactions are consumed through chemical reactions and the self-potential anomalies fall to zero. The resulting self-potential anomalies are shown to be much weaker than the self-potential anomalies observed in the presence of an electronic conductor in the laboratory or in the field. In the presence of a biotic or an abiotic electronic conductor, the self-potential anomalies can amount to a few hundred millivolts. These observations point out indirectly the potential role of bacteria forming biofilms in the transfer of electrons through sharp redox potential gradient in contaminant plumes that are rich in organic matter.

Self-potential investigations of a gravel bar in a restored river corridor

“…Despite the complications mentioned above, the SP method continues to receive considerable interest in hydrogeology as SP data are sensitive to contaminant transport (e.g., Maineult et al, 2004Maineult et al, , 2005Revil et al, 2009), redox processes (e.g., Linde and Revil, 2007), flow in saturated (e.g., Maineult et al, 2008;Bolève et al, 2009;Jardani et al, 2007) and unsaturated porous media (e.g., Thony et al, 1997;Doussan et al, 2002;Linde et al, 2007a), flow in fractures (e.g., Wishart et al, 2006), the water table elevation (e.g., Fournier, 1989;Revil et al, 2003;Rizzo et al, 2004), or the thickness of the vadose zone (Aubert and Yéné Atangana, 1996), etc. For example, the SP method might potentially be used to estimate water fluxes in the vadose zone (Thony et al, 1997).…”

A microbial fuel cell in contaminated ground delineated by electrical self‐potential and normalized induced polarization data

“…Redox driven electrochemical gradients have produced self‐potential signals associated with ore deposits [ Sato and Mooney , 1960; Bigalke and Grabner , 1997] which coined the term ‘geobattery’ and more recently with contaminant plumes [ Naudet et al , 2004, Arora et al , 2007; Minsley et al , 2007]. Laboratory work has helped to define the relationship between self‐potential and redox potential [ Maineult et al , 2006; Castermant et al , 2008; Revil et al , 2009] where the self‐potential should be constrained by the range of Eh and should require the presence of an electronic conductor. In addition to these traditionally recognized self‐potential sources, microbially mediated coupled ion‐electron flows have recently emerged as a subset of the electrochemical mechanism for natural subsurface current flow [ Revil et al , 2010].…”

Characterizing the Self‐Potential Response to Concentration Gradients in Heterogeneous Subsurface Environments

“…The methods presented here have relevance to any subsurface environment where SP monitoring is conducted and where there is evidence that concentration gradients exist. Therefore, our rigorous and reliable method has broad application to support parameterization of geoelectric models and to understand electrical signals observed during SP monitoring of, amongst other things, enhanced oil and gas recovery (Gulamali et al, ; Jackson, Gulamali, et al, ), contaminant plume migration in aquifers (Arora et al, ; Naudet, ; Revil et al, ), tracer tests (Ikard et al, ; Jougnot et al, ), and seawater intrusion (Graham et al, ; MacAllister et al, ).…”

“…In that context, self‐potential (SP) monitoring has been shown to have a plethora of applications for fluid monitoring in subsurface environments. For example, SP monitoring has been applied to a variety of hydrological applications, including pumping tests (Jackson et al, ; Maineult et al, ; Rizzo et al, ), flow in a buried paleochannel (Revil et al, ), in a sinkhole (Jardani et al, ), subglacial flows (Kulessa, ), and monitoring of contaminant plumes (Arora et al, ; Naudet, ; Revil et al, ). SP can also be used to monitor enhanced oil and gas recovery (Gulamali et al, ; Jackson et al, ) and to characterize oil and gas reservoirs, including to better understand reservoir wettability (Alroudhan et al, ; Alroudhan et al, ; Jackson et al, ).…”

“…For example, MacAllister et al () and Graham et al () demonstrate that borehole measurements of SP can provide spatial and temporal warning of an intruding saltwater‐freshwater interface in coastal aquifers. Such concentration gradients also exist in oil reservoirs during water flooding for enhanced oil recovery (Gulamali et al, ; Jackson, Gulamali, et al, ) and in contaminated aquifers (Revil et al, ).…”