Identification of Abiotic Degradation Pathways of Chlorinated Ethenes by Compound-Specific Stable Isotope Analysis: A Proof-of-Concept Study

Dr. Tomasz Kuder | University of Oklahoma

ER-2623

Objective

At certain sites, abiotic degradation of chlorinated ethenes (CEs) may be an important attenuation mechanism. Unequivocal demonstration and quantitation of abiotic contaminant mass destruction remains difficult, typically due to the absence of pathway-specific degradation products or to poor mass balance of such products. This study centers on development of an alternative Compound-Specific Isotope Analysis (CSIA) methodology for identification of dominant degradation pathway(s). Two potential application areas are: (1) Differentiation of abiotic degradation on Fe(II)/Fe(III) mineral surfaces from aerobic biodegradation, in the absence of pathway-specific degradation products; and (2) Differentiation of biotic and abiotic reductive mechanisms at sites with permeable reactive barrier (PRB) activities or with related in-situ treatments. In the latter case, assessing the relative importance of the biotic and abiotic pathways may be challenging. The objective of this proof-of-concept study is to identify novel lines of evidence based on stable isotope ratios of the parent CEs and possibly also their degradation products, for differentiation between the effects of biotic and abiotic degradation pathways that may occur at contaminated field sites. A limitation in implementing CSIA in this study area is incomplete understanding of Cl and H isotope fractionation in the relevant reactions. The present study should provide the missing reference data from controlled degradation experiments as a prerequisite to future field implementation.

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Technical Approach

The data will be collected from laboratory experiments on degradation of trichloroethene (TCE) or tetrachloroethene (PCE). Abiotic degradation will be represented by reaction with zero-valent iron (ZVI). Anaerobic biodegradation will be represented by a mixed Dehalococcoides culture (BDI) and D. michiganensis, strain BB1. Aerobic biodegradation experiments will be conducted using three different organisms utilizing different mono- or dioxygenases. C+Cl+H isotope ratios will be determined for the parent TCE or PCE and for the degradation products in the reductive pathways. The data will be evaluated to determine potential lines of evidence for site assessment (such as 2D-CSIA trends differentiating relevant alternative degradation pathways). The obtained results will be used to model the effects of various combinations of competing degradation mechanisms on spatial and temporal concentration and CSIA patterns at representative field scenarios, for preliminary data visualization preliminary to actual field demonstration.

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Benefits

The CSIA approach was proven to be useful in the assessment of biodegradation of various volatile organic compounds (VOCs). Similar scope of CSIA applications can be envisioned for the assessment of abiotic degradation of CEs. Development of site assessment methodologies capable of discriminating between biotic and abiotic degradation and quantifying the degradation yields from different pathways would bring significant benefit to the Department of Defense through more efficient management of remediation effort. This proof-of-concept study aims to collect preliminary data as a prerequisite to a potential field demonstration of the CSIA methodology. This project should provide basic reference data on the isotope effects in the key degradation pathways. It will also identify the isotope-based lines of evidence with the best potential for discrimination between biotic and abiotic reaction pathways at field sites. (Anticipated Project Completion - 2018)

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Points of Contact

Principal Investigator

Dr. Tomasz Kuder

University of Oklahoma

Phone: 405-325-4453

Program Manager

Environmental Restoration

SERDP and ESTCP

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