We determined in situ reductive transformation rates of tetrachloroethene (PCE) in a contaminated aquifer by combining compound-specific carbon stable isotope analysis (CSIA) of the contaminants with tracer-based ((3)H-(3)He) groundwater dating. With increasing distance from the source, PCE was gradually transformed to trichloroethene (TCE), cis-dichloroethene (cDCE), and vinyl chloride (VC). Using the in situ determined carbon isotopic enrichment factor of -3.3 +/- 1.2 per thousand allowed for quantification of the PCE-to-TCE transformation based on isotopic (delta(13)C) shifts. By combining these estimates of the extent of PCE transformation with measured groundwater residence times (between 16 and 36 years) we calculated half-lives of 2.8 +/- 0.8 years (k = 0.27 +/- 0.09 yr(-1)) for the PCE-to-TCE transformation. Carbon isotope mass balances including the chloroethenes PCE, TCE, cDCE, and VC (delta(13)C(Sigma(CEs))) enabled an assessment of complete PCE dechlorination to nonchlorinated products. Shifts of delta(13)C(Sigma(CEs)) at the fringe of the plume of more than 25 per thousand pointed to dechlorination beyond VC of up to 55 +/- 17% of the chloroethene mass. Calculated rates for this multistep dechlorination were highly variable throughout the aquifer (k = 0.4 +/- 0.4 yr(-1)), suggesting that PCE reduction to nonchlorinated products occurred only in locally restricted zones of the investigated site.

Trinitrotoluene (TNT) and dinitrotoluene (DNT) originating from 50 years of explosives production have heavily contaminated two stacked aquifers in the vicinity of Lisboa, Portugal. To assess if these poly-nitroaromatic compounds (P-NACs) are being degraded in the subsurface, tracer-based groundwater dating techniques combined with compound-specific isotope analyses (CSIA) were applied. The groundwater residence times were distinctly different in the two aerobic aquifers, as determined by the tritium ((3)H)-(3)He method. In the contaminated zones, the upper aquifer exhibited groundwater ages of 25 years, whereas the lower (presumably confined) aquifer contained hardly any tritium which indicates water ages >55 years. P-NACs-containing waste waters are known to have leaked into the upper, unconfined aquifer. However, P-NACs were present in both aquifers in high concentrations (up to 33000 microg L(-1) TNT), which implies a hydraulic connection, although tritium concentrations and chemical data suggest two separated aquifers. Based on the (3)H-(3)He groundwater dating and the presence of very high P-NAC concentrations, the contamination of the lower aquifer must have happened during the early stage of the explosive production, i.e. >50 years ago. Despite this 'old' contamination, TNT and DNT have not been transformed until to date as is demonstrated by the negligible changes in their carbon isotopic signatures (delta(13)C). Thus, P-NACs are very recalcitrant to degradation at the investigated site. If the aquifers remain aerobic, TNT and DNT are expected to persist in the subsurface for many decades to centuries. The presented approach of assessing time scales of natural attenuation at the field scale by the combination of CSIA and (3)H-(3)He water dating has the potential to be applied to any other groundwater contaminants, such as chlorinated hydrocarbons, gasoline components, heterocyclic carbenes, or polyaromatic hydrocarbons.

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We developed a method for the vacuum extraction (VacEx) of volatile organic compounds (VOCs) from water samples for ultratrace determinations of carbon isotopic signatures. Our method permits compound-specific stable carbon isotope analysis (CSIA) at VOC concentrations as low as 0.03-1.34 microg/L. VacEx was developed to extract and preconcentrate VOCs for subsequent carbon-CSIA by the standard technique purge-and-trap (P&T) coupled to an isotope-ratio mass spectrometer (IRMS). Even without complete extraction, the delta(13)C signatures of VOCs determined by VacEx-P&T-IRMS were in good agreement (deviation <1 per thousand) with signatures determined by P&T-IRMS. This indicates that VacEx does not cause isotopic discrimination. Limits of quantification (LOQs) for delta(13)C analysis were: 0.03-0.06 microg/L for benzene, toluene, o-xylene, m-p-xylene and ethylbenzene, 0.09 microg/L for methyl tert-butyl ether (MTBE), and 0.18-0.27 microg/L for trans-DCE, cis-DCE, TCE and PCE. These are the lowest LOQs reported to date for continuous-flow isotope-ratio determinations using a commercially available and automated system. To our knowledge, analytical protocols adopted from noble gas analysis in water were applied for the first time to determine the isotope composition of organic contaminants. We applied VacEx in a field study to illustrate how the determination of VOC isotopic signatures at very low concentrations opens new avenues in the in situ assessment of these priority groundwater pollutants.