Legacy spills of chlorinated ethenes (CE) remain one of the key environmental challenges, at Department of Defense facilities and elsewhere, in the U.S. and worldwide. At certain sites, abiotic degradation of CEs may be an important attenuation mechanism. Unequivocal demonstration and quantitation of abiotic chemical mass destruction remains difficult, typically, due to the absence of pathway-specific degradation products or due to poor mass balance of such products. To fully benefit from abiotic degradation in chemical remediation work, the process must be documentable. This project explored the utility of multi-element compound-specific isotope analysis (CSIA) to identify evidence of abiotic degradation in scenarios with concurrent abiotic and biological degradation. The objective was to identify isotope parameters that would be diagnostic of abiotic pathways including in the presence of degradation yield from concurrent biodegradation of the same CEs. 

Degradation experiments were conducted, and published data were evaluated to collect information of multi-element (carbon, chloride, hydrogen) isotope effects in reaction pathways that can plausibly be expected to compete in two specific field scenarios: 1) aerobic environments conducive to abiotic degradation on iron mineral surfaces and to aerobic biodegradation; and 2) overlapping biotic and abiotic reductive dechlorination.

Data were collected from microcosm experiments, with zero-valent iron (ZVI) degradation serving as a model for abiotic reactions. Isotope effect data were collected from ZVI experiments, and from several aerobic and anaerobic biodegradation experiments. Parent CEs (tetrachloroethene or trichloroethene [TCE]) and their dechlorination products (down to cis-1,2-dichloroethene) were analyzed. The data were evaluated by pair-wise comparison of the specific reactions, to identify the characteristic of the isotope record that are most informative in pathway discrimination.

Three highlights from this project are:

1. Hydrogen CSIA characterization of dechlorination products from biotic and abiotic reductive dechlorination appears to yield highly specific information that clearly delineated biological and abiotic product yields based on their hydrogen isotope signatures. The approach is readily deployable in field site assessments to constrain the relative significance of biotic and abiotic reduction of CEs.
2. Carbon-chlorine CSIA can readily discriminate between aerobic biodegradation TCE and reductive dechlorination and possibly from Fenton-like reactions (more research is necessary to ascertain the latter). The results should be robust for systems with biodegradation dominated by one specific type of oxygenases, but mixed methane and toluene oxygenases can be confounding.
3. A study of CE susceptibility to hydrogen exchange with water was performed for quality control purposes. The results from that study have broader significance for any application of hydrogen CSIA in the assessment of chlorinated solvents. One significant conclusion is that under certain conditions, hydrogen isotope signatures of TCE are subject to relatively rapid overprinting by the exchange and this should be borne in mind when planning hydrogen-CSIA activities.

More specific answers from CSIA should translate to better quality of contaminated site assessment and to cost savings though more effective site remediation. While this demonstration shows promise for better characterization of sites with abiotic degradation processes, routine field deployment would benefit from real-world examples of successful applications to overcome reluctance of the remediation community.