This project will address major challenges associated with quantitative assessment of abiotic natural attenuation (NA), an important element of passive management of contaminated groundwater sites. The objective of this proof-of-concept scope project is to develop a novel oxidation-reduction potential (ORP) kit for laboratory and field measurements of redox potentials (i.e., thermodynamics) of reactive aquifer solids, which can be further related to the rates (i.e., kinetics) of abiotic NA mediated by these solid phases.
The fundamental principle of the ORP kit is built upon well-established science that electron shuttles accelerate electron transfer involved in redox processes. Here, electron shuttles serve to mediate electron transfer between reactive aquifer solids and an electrode, thereby improving the electrode sensitivity to the reactive aquifer solids, resulting in more accurate potentiometric measurements. The designed ORP kit will consist of a mixture of electron shuttles to cover a wide range of redox potentials relevant to abiotic NA. Laboratory tests will be conducted to refine and optimize the composition and protocol of the ORP kit using reference materials, including model reactive minerals and the site materials that have been well characterized in the prior work. The kit will then be applied to aquifer materials collected from other sites where abiotic NA has been shown or suggested to be relevant, to generate additional ORP measurement data points. Correlations between redox potentials and abiotic contaminant degradation rates established by the reference materials can then be used to estimate abiotic NA rates mediated by aquifer materials at any given site.
This research will lead to a first-of-its-kind practical tool and method that is capable of achieving quantitative assessment of the redox states of aquifer solids. The kit is designed to aid conventional ORP measurement by an ORP combo electrode, offering a novel but easy-to-use monitoring approach that can be incorporated into routine field monitoring programs. More accurate, reliable, and reproducible ORP measurement of reactive solid minerals can be used (i) to screen the potential of abiotic NA during remedy evaluation; (ii) to establish site-specific correlations for abiotic degradation rates during treatability studies; and (iii) to better inform the conceptual site model (CSM) whether abiotic NA is sustained during long-term monitoring. Finally, the ease and low cost to collect the ORP data makes it feasible to establish a portfolio-wide dataset across the Department of Defense (DoD) sites that can support site managers to form a data-driven and well informed decision during evaluation and monitoring of transition strategies involving NA, resulting in significant cost savings to DoD and facilitating the regulatory acceptance of NA where appropriate. (Anticipated Project Completion - 2023)