Tracking in situ biodegradation of 1,2-dichloroethenes in a model wetland.

The spatial and temporal biogeochemical development of a model wetland loaded with cis- and trans-1,2-dichloroethene contaminated groundwater was characterized over 430 days by hydrogeochemical and compound-specific isotope analyses (CSIA). The hydrogeochemistry dramatically changed over time from oxic to strongly reducing conditions as emphasized by increasing concentrations of ferrous iron, sulfide, and methane since day 225. delta(13)C values for trans- and cis-DCE substantially changed over the flow path and correlated over time with DCE removal. The carbon enrichment factor values (epsilon) retrieved from the wetland became progressively larger over the investigation period, ranging from -1.7 +/- 0.3% per hundred to -32.6 +/- 2.2% per hundred. This indicated that less fractionating DCE oxidation was progressively replaced by reductive dechlorination, associated with a more pronounced isotopic effect and further confirmed by the detection of vinyl chloride and ethene since day 250. This study demonstrates the linkage between hydrogeochemical variability and intrinsic degradation processes and highlights the potential of CSIA to trace the temporal and spatial changes of the dominant degradation mechanism of DCE in natural or engineered systems.