Identification of abiotic and biotic reductive dechlorination in a chlorinated ethene plume after thermal source remediation by means of isotopic and molecular biology tools.
Thermal tetrachloroethene (PCE) remediation by steam injection in a sandy aquifer led to the release of dissolved organic carbon (DOC) from aquifer sediments resulting in more reduced redox conditions, accelerated PCE biodegradation, and changes in microbial populations. These changes were documented by comparing data collected prior to the remediation event and eight years later. Based on the premise that dual C-Cl isotope slopes reflect ongoing degradation pathways, the slopes associated with PCE and TCE suggest the predominance of biotic reductive dechlorination near the source area. PCE was the predominant chlorinated ethene near the source area prior to thermal treatment. After thermal treatment, cDCE became predominant. The biotic contribution to these changes was supported by the presence of Dehalococcoides sp. DNA (Dhc) and Dhc targeted rRNA close to the source area. In contrast, dual C-Cl isotope analysis together with the almost absent VC (13)C depletion in comparison to cDCE (13)C depletion suggested that cDCE was subject to abiotic degradation due to the presence of pyrite, possible surface-bound iron (II) or reduced iron sulphides in the downgradient part of the plume. This interpretation is supported by the relative lack of Dhc in the downgradient part of the plume. The results of this study show that thermal remediation can enhance the biodegradation of chlorinated ethenes, and that this effect can be traced to the mobilisation of DOC due to steam injection. This, in turn, results in more reduced redox conditions which favor active reductive dechlorination and/or may lead to a series of redox reactions which may consecutively trigger biotically induced abiotic degradation. Finally, this study illustrates the valuable complementary application of compound-specific isotopic analysis combined with molecular biology tools to evaluate which biogeochemical processes are taking place in an aquifer contaminated with chlorinated ethenes.
University of Neuchâtel, Centre for Hydrogeology & Geothermics (CHYN), Rue Emile Argand 11, CH 2000 Neuchâtel, Switzerland.
Alice
A
Badin
Technical University of Denmark (DTU), Department of Environmental Engineering, Miljøvej, DTU B113, DK 2800 Kgs. Lyngby, Denmark.
Mette M
MM
Broholm
Geological Survey of Denmark and Greenland (GEUS), Department of Geochemistry, Ø. Voldgade 10, 1350 København K, Denmark.
Carsten S
CS
Jacobsen
University of Neuchâtel, Centre for Hydrogeology & Geothermics (CHYN), Rue Emile Argand 11, CH 2000 Neuchâtel, Switzerland.
Jordi
J
Palau
SiREM, 130 Research Lane, Guelph, Ontario, N1G5G3, Canada.
Philip
P
Dennis
University of Neuchâtel, Centre for Hydrogeology & Geothermics (CHYN), Rue Emile Argand 11, CH 2000 Neuchâtel, Switzerland.
Daniel
D
Hunkeler
D001673:Biodegradation, Environmental; D002247:Carbon Isotopes / Q000032:analysis; D003718:Denmark; D052918:Environmental Restoration and Remediation / Q000379:methods*; D060587:Groundwater / Q000032:analysis / Q000737:chemistry* / Q000382:microbiology; D054879:Halogenation; D007501:Iron; D013440:Sulfides; D013750:Tetrachloroethylene / Q000032:analysis*; D014874:Water Pollutants, Chemical / Q000032:analysis*
D002247:Carbon Isotopes; D013440:Sulfides; D014874:Water Pollutants, Chemical; C011342:pyrite; D007501:Iron; D013750:Tetrachloroethylene
Chloroethenes; Molecular biology; Stable isotopes; Thermal treatment