Mineral surfaces play an important role in the attenuation of contaminants in the subsurface. As a consequence, understanding the reduction potential of those surfaces is important for predicting the rates at which contaminants will be degraded in groundwater. Conventional measurement of oxidation-reduction potential (ORP) using a platinum electrode reflects aqueous solution chemistry, but does not fully reflect redox processes at mineral surfaces.
To improve the assessment of reduction potential, Dr. Paul Tratnyek and his team at the Oregon Health & Science University are developing redox-sensitive indicators as chemical redox probes (CRPs) under SERDP project ER-2308. To be effective, a CRP must come in direct contact with the mineral surfaces. This can be accomplished using “push-pull” tests or, more precisely, pull-push-pull tests in which: 1) groundwater is carefully brought to the surface and mixed with one or more CRPs, 2) re-injected into the subsurface, 3) allowed to remain in the subsurface for periods of minutes to hours depending on site conditions, 4) extracted back to ground surface, 5) analyzed by UV-visible or fluorescence spectroscopy to determine the extent of reduction of the dye, and 6) the fraction of the dye that is reduced is used in the Nernst equation to calculate a reduction potential that includes the effects of the mineral surfaces. This process is shown in the accompanying video.
In general, redox potentials determined on aquifer material using CRPs are 100 or more millivolts more negative (more reducing) than the ORPs measured on pore water in the conventional fashion. Preliminary results also indicated that the CRP determined potentials are more predictive of contaminant reduction rates.
The new tools developed under this project will directly benefit the planning and implementation of remedies for chlorinated solvent contaminated groundwater. They will be applicable early in the site characterization process to help determine what remediation approach is best suited to the site. They also will be useful in a "before and after" mode to assess the effectiveness of augmentation or stimulation. Finally, they will help to determine when remediation activities at sites are not performing as anticipated and can provide an improved scientific basis for terminating remediation activities, if necessary.