Headspace Analysis: A New Application for Isotopic Characterization of Dissolved Organic Contaminants

Petroleum products and industrial solvents are among the most ubiquitous contaminants of soil and groundwater and the source of several common and hazardous volatile organic chemicals (VOCs). Volatilization is a key determinant of the fate of VOCs in the subsurface environment, impacting contaminant partitioning between the aqueous, gaseous, and nonaqueous liquid phases. This study uses stable carbon isotope analysis to investigate the isotopic effects involved in volatilization of trichloroethylene (TCE) and toluene from both free product (or pure phase) and aqueous solutions. Results indicate that, during volatilization from the aqueous phase and from free product, the isotopic composition of TCE and toluene remains unchanged within reproducibility limits. These results have two important implications for contaminant hydrogeology. First, they suggest that carbon isotopic signatures may be useful in tracking contaminant transport between the vapor, aqueous, and NAPL phases since they remain conservative during phase changes. Second, they demonstrate the utility of headspace extraction (sampling of the vapor phase or headspace above an aqueous solution) as a preparatory technique for isotopic analysis of dissolved VOCs. Headspace isotopic analysis provides a straightforward and rapid technique for δ13C analysis of dissolved organic contaminants at concentrations as low as hundreds of ppb.