Theme: 10. Characterization, Fate and Transport

Topic Area: 10a. Advanced Investigation Tools and Techniques

Formatted Title:

Hydrogen Isotope Exchange between Trichloroethene and Water: A Disadvantage for the Use of Hydrogen CSIA for TCE Source Apportionment and an Opportunity for Developing Contaminant Dating Applications

Background/Objectives:

Hydrogen compound-specific isotope analysis (CSIA) is a mainstream isotope tool applied in contaminant assessment, both for contaminant source apportionment and for characterizing degradation processes. It is well recognized that hydrogen CSIA applications must necessarily be restricted to analysis of hydrogen that is not subject to exchange with water, if the results are to be representative of the contaminant source and/or degradation. Hydrogen atoms readily exchange with water in polar functional groups (e.g., alcohol, carboxylic acids), but carbon-bound hydrogen atoms are generally thought to be recalcitrant with respect to this process. In contrast, we have recently demonstrated that carbon-bound hydrogen exchange with water is relatively fast for aqueous trichloroethene (TCE) at above-neutral pH and/or at temperatures corresponding to groundwater in warm climates. Under these conditions, the hydrogen isotope composition of TCE would be significantly altered by the exchange within the lifespan of a contaminant plume (Kuder and Ojeda, ACS EST Water 2023, 3, 712−719). In this presentation, we will discuss the implications of these recent findings with the focus on the assessment of the sources and fate of TCE. Where the exchange complicates the legacy applications of using hydrogen isotope data as a line of evidence for apportionment of TCE sources, it opens an interesting and novel avenue of contamination age dating applications.

Approach/Activities:

In the first part of the presentation, we will summarize the results from batch equilibration experiments which permitted determination of TCE-water hydrogen exchange rates. In the second part of the presentation, we will discuss how this evidence limits interpretations when using hydrogen isotope data as a line of evidence to apportion the sources of TCE (TCE solvent versus TCE product of reductive dechlorination). Finally, we will present examples of historical field data sets produced by the OU Laboratory to illustrate the convergence of the hydrogen isotope composition of TCE with that of ambient water over the timescales of residence in groundwater time.

Results/Lessons Learned:

The results from laboratory experiments demonstrate hydrogen exchange with half-lives well below 10 years projected for certain environments, such as for alkaline waters in carbonate aquifers. While the rates of the exchange are significantly slower in acidic groundwater and at lower temperatures, the traditional focus on using hydrogen CSIA data for TCE source apportionment appears poorly warranted. On the other hand, hydrogen data have the potential to constrain the time of residence of TCE in the environment, in that the detection of isotope signatures trending towards equilibrium with local water would indicate certain minimum residence times which can be calculated using known rate constraints of the exchange reaction. Conversely, detection of highly 2H-enriched TCE typical of manufactured solvent in waters conducive to rapid exchange would be a strong indication of recently dissolved TCE and a likely presence of an active DNAPL source. The same principle of data interpretation can be extrapolated to other compounds susceptible to the exchange.

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Ann Ojeda