Lawrence Berkeley National Laboratory

We have evaluated the usefulness of the self-potential (SP) geophysical method to track the onset and location of microbial sulfate-reduction in saturated sediments during organic carbon amendment. Following stimulation of sulfate-reducing bacteria (SRB) by addition of lactate, anomalous voltages exceeding 600 mV correlated in space and time with the accumulation of dissolved sulfide. Abiotic experiments in which the sulfide concentration at the measurement electrode was systematically varied showed a positive correlation between the magnitude of the SP anomaly and differences in the half-cell potential associated with the measurement and reference electrodes. Thus, we infer that the SP anomalies resulted from electrochemical differences that developed between sulfide-rich regions and areas having higher oxidation potential. In neither experiment did generation of an SP anomaly require the presence of an in situ electronic conductor, as is required by other models. These findings emphasize the importance of incorporation of electrochemical effects at electrode surfaces in interpretation of SP data from geophysical studies. We conclude that SP measurements provide a minimally invasive means for monitoring stimulated sulfate-reduction within saturated sediments.