Hydrogen Isotope Exchange between Trichloroethene and Water under Mild Environmental Conditions─Implications for the Use of Hydrogen CSIA in Contaminated Site Assessment

Abstract

Trichloroethylene (TCE) is a key legacy environmental contaminant and a major target for contaminant remediation projects. Hydrogen compound-specific isotope analysis (CSIA) of halocarbons is a relatively recent technique, predominantly used as a line of evidence for apportionment of TCE sources. To date, no published data addressed the potential for hydrogen exchange between TCE and hydroxide in a water solution under typical groundwater pH conditions. In this study, we determined the pH- and temperature-dependent rates of exchange in controlled experiments to constrain the scope of applications of hydrogen CSIA at field sites contaminated by TCE. Relatively high rates of the exchange were associated with above-neutral pH typical of carbonate aquifers (half-lives of years to decades) and slower, but not negligible, rates were associated with sub-neutral pH. Hydrogen exchange alters the hydrogen isotope composition of TCE in solution, masking initial source signatures or degradation signatures over environmentally relevant time scales. In effect, the TCE–water exchange restricts the classical applications of hydrogen CSIA data to a subset of distinctly acidic environments. On the other hand, as the rates of the exchange can be predicted for specific combinations of pH and temperature, the process could be potentially utilized in age-dating of TCE contamination.