Abiotic attenuation is a key restoration process at many complex sites and, as a consequence, accurate measures of reaction rates are important for site decision-making. The ideal approach for assessing abiotic attenuation rates would be rapid, in situ measurements using the contaminant of concern. Unfortunately, in situ reaction rates for chlorinated solvents like trichloroethene (TCE) are too slow to be directly measured in situ, even under enhanced remediation conditions. Due to this, the standard approach for estimating these rates is laboratory batch tests. However, the timeframe for those experiments is typically years, limiting their use for decision-making and raising issues regarding stability of the core materials during those tests. A more rapid laboratory approach is to use structurally-similar surrogate compounds, whose attenuation behavior is similar to TCE across a broad range of abiotic mineral reactions, but whose rates are 100-1000 times faster than TCE. Another approach is to use redox-active dyes as probes, because their reaction rates are rapid enough to be used for in situ measurements.
The overarching goal of this project was to develop new tools for measuring, and ultimately predicting, in situ abiotic reduction rates of groundwater contaminants of concern (CoCs). The approach to accomplish this was to collect well-preserved field cores using the cryogenic core collection technique and then test those cores in the laboratory to assess abiotic reduction rates.
Two independent but complimentary measurement approaches were used. The first involved a reactivity probe, in this case carbon tetrachloride (CT), to measure abiotic reduction rates in batch experiments. The second involved careful measurement of reduction potential using a platinum electrode and electron shuttle molecules that facilitated electrical contact between the electrode and the surfaces of aquifer solids in slurried samples. The reactivity and reduction potential measurement for each field sample were then plotted against each other to demonstrate a correlation that could be used to predict reaction rate. This approach is represented schematically in the figure below, where the photo on the right shows the “rolling tube reactors” used for the CT reactivity measurements and the photo on the left represents the process of measuring reduction potential using a combination electrode and an electron shuttle. As part of this work, the project team also demonstrated that the predictive power of this approach can be somewhat enhanced by including pH in the correlation equation.
The processes of freezing and storage have the potential to alter the reactivity of the soils. To examine this, the project team used materials taken from laboratory columns operated under iron and sulfate reducing conditions and compared their reactivity after being frozen and stored for 60 days against samples that had never been frozen. The data showed, in general, that there was no significant loss of reactivity as the result of freezing and storage.
This project has resulted in an approach where core samples from field sites can be quickly assessed to determine the rate at which abiotic reduction is likely to occur in situ. This assessment can be accomplished either by direct measurement of reactivity in batch tests, or by careful measurement of reduction potential and the use of a correlation equation.
The next steps in the development of this approach should be: 1) its expanded use at field sites; 2) comparison to other methods for estimating abiotic reactivity; 3) expanding the range of reactivity probes to include other CoCs; and 4) characterization of the relationship between reduction products and the mineral phases that control their formation.
Fan, D., M. Bradley, A. Hinkle, R.L. Johnson, and P.G. Tratnyek. 2016. Chemical Reactivity Probes for Assessing Abiotic Natural Attenuation by Reducing Iron Minerals. Environmental Science & Technology, 50:1868−1876.
Kiaalhosseini, S., R.L. Johnson, R.C. Rogers, M. Irriani-Renno, M.R. Olson, M. Lyverse, and T.C. Sale. 2016. Cryogenic Core Collection (C3) from Unconsolidated Subsurface Media. Groundwater Monitoring and Remediation, 36:41-49. doi:10.1111/gwmr.12186
Kocur, C.M.D., D. Fan, P.G. Tratnyek, and R.L. Johnson 2020. Predicting Abiotic Reduction Rates using Cryogenically Collected Soil Cores and Mediated Reduction Potential Measurements. Environmental Science & Technology Letters, 7(1):20–26.
Richards, P.M., Y. Liang, R.L. Johnson, and T.E. Mattes. 2019. Cryogenic Soil Coring Reveals Coexistence of Aerobic and Anaerobic Vinyl Chloride Degrading Bacteria in a Chlorinated Ethene Contaminated Aquifer. Water Research, 157:281-291.