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 δ 13 C analysis of dissolved organic contaminants at concentrations as low as hundreds of ppb.

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To accurately interpret isotopic data obtained for volatile organic compounds (VOCs) dissolved in groundwater systems, the isotopic effects of subsurface processes must be understood. Previous work has demonstrated that volatilization and dissolution of BTEX and chlorinated ethene compounds are not significantly isotopically fractionating. This study characterized the carbon isotopic effects of equilibrium sorption of perchloroethylene, trichloroethylene, benzene, and toluene to both graphite and activated carbon directly in batch experiments over a range of 10-90% sorption. Results demonstrate that, over this range, equilibrium sorption of these VOCs to graphite and activated carbon does not result in significant carbon isotopic fractionation within the +/-0.5% accuracy and reproducibility associated with compound-specific isotope analysis. This implies that the isotopic values of dissolved VOCs will not be significantly affected by equilibrium sorption in the subsurface. Therefore, isotopic analysis has potential to be used in the field to differentiate between mass losses due to isotopically fractionating processes such as biodegradation versus mass loss due to nondegradative processes.

The Monkstown zero-valent iron permeable reactive barrier (ZVI PRB), Europe's oldest commercially-installed ZVI PRB, had been treating trichloroethene (TCE) contaminated groundwater for about 10 years on the Nortel Network site in Northern Ireland when cores from the reactive zone were collected in December, 2006. Groundwater data from 2001-2006 indicated that TCE is still being remediated to below detection limits as the contaminated groundwater flows through the PRB. Ca and Fe carbonates, crystalline and amorphous Fe sulfides, and Fe (hydr)oxides have precipitated in the granular ZVI material in the PRB. The greatest variety of minerals is associated with a approximately 1-2 cm thick, slightly cemented crust on top (up-gradient influent entrance) of the ZVI section of the PRB and also with the discontinuous cemented ZVI material ( approximately 23 cm thick) directly below it. The greatest presence of microbial communities also occurred in the up-gradient influent portion of the PRB compared to its down-gradient effluent section, with the latter possibly due to less favorable conditions (i.e., high pH, low oxygen) for microbial growth. The ZVI filings in the down-gradient effluent section of the PRB have a projected life span of >10 years compared with ZVI filings from the continuous to discontinuous cemented up-gradient ZVI section (upper approximately 25 cm) of the PRB, which may have a life span of only approximately 2-5 more years. Supporting Information from applied, multi-tracer testing indicated that restricted groundwater flow is occurring in the upper approximately 25 cm of the ZVI section and preferential pathways have also formed in this PRB over its 10 years of operation.

Evaluating the impact that airborne contamination associated with Athabasca oil sands (AOS) mining operations has on the surrounding boreal forest ecosystem requires a rigorous approach to source discrimination. This study presents a century-long historical record of source apportionment of polycyclic aromatic hydrocarbons (PAHs) in dated sediments from two headwater lakes located approximately 40 and 55 km east from the main area of open pit mining activities. Concentrations of the 16 Environmental Protection Agency (EPA) priority PAHs in addition to retene, dibenzothiophene (DBT), and six alkylated groups were measured, and both PAH molecular diagnostic ratios and carbon isotopic signatures (δ(13)C) of individual PAHs were used to differentiate natural from anthropogenic inputs. Although concentrations of PAHs in these lakes were low and below the Canadian Council of Ministers of the Environment (CCME) guidelines, diagnostic ratios pointed to an increasingly larger input of petroleum-derived (i.e., petrogenic) PAHs over the past 30 years concomitant with δ(13)C values progressively shifting to the value of unprocessed AOS bitumen. This petrogenic source is attributed to the deposition of bitumen in dust particles associated with wind erosion from open pit mines.

Equilibrium headspace analysis of toluene for δ2H isotopic composition by continuous flow compound specific isotope mass spectrometry was determined to have an accuracy and reproducibility of ±5‰. Using this analytical approach, the hydrogen isotope fractionation produced by anaerobic biodegradation of toluene was evaluated in laboratory experiments using a mixed methanogenic consortium. A large, reproducible 2H-enrichment in the residual toluene of greater than 60‰ was observed at greater than 95% degradation, reflecting the preferential biodegradation of molecules containing the light (1H) isotope. Recent studies evaluating the magnitude of carbon isotope fractionation produced during biodegradation of aromatic hydrocarbons have documented heavy isotope (13C) enrichment in the residual contaminant approximately an order of magnitude smaller than those reported here for 2H. The very large isotopic enrichment in 2H suggests that under anaerobic conditions compound specific hydrogen isotope analysis may provide a more reliable means of validating intrinsic bioremediation of aromatic hydrocarbons than stable carbon isotope analysis. Combined application of stable carbon and hydrogen isotope analysis in an anaerobic groundwater has the potential to provide two important diagnostic tools. Relatively insensitive to biodegradation by mixed consortia, stable carbon isotope values may provide information about different sources of contaminant, while hydrogen isotope values provide an assessment of the degree of attenuation due to biodegradation.

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Carbon isotope fractionation produced by anaerobic biodegradation of toluene was evaluated in laboratory experiments under both methanogenic and sulfate-reducing conditions. A small (∼2‰) but highly reproducible 13 Cenrichment in the residual toluene at advanced stages of microbial transformation was observed in both cultures. The maximum isotopic enrichment observed in the residual toluene was +2.0‰ and +2.4‰ for the methanogenic and sulfate-reducing cultures, respectively, corresponding to isotopic enrichment factors ( ) of -0.5 and -0.8. Because the accuracy and reproducibility associated with gas chromatograph-combustion-isotope ratio mass spectrometry (GC/C/IRMS) is (0.5‰, delineating which of these two terminal electron-accepting processes (TEAP) is responsible for the biodegradation of toluene at field sites will not be possible. However, the potential does exist to use compound-specific isotope analysis (CSIA), in conjunction with other methodologies, as a means of validating advanced stages of intrinsic bioremediation in anaerobic systems. Caution is urged that relating this small (∼2‰) fractionation to biodegradation at complex field sites will prove a challenge.