Isotopic and Geochemical Assessment of in Situ Biodegradation of Chlorinated Hydrocarbons

Abstract

Currently there is no in situ method to detect and quantify complete mineralization of chlorinated hydrocarbons (CHCs) to CO 2 . Combined isotopic measurements in conjunction with traditional chemical techniques were used to assess in situ biodegradation of trichloroethylene (TCE) and carbon tetrachloride (CT). Vadose zone CHC, ethene, ethane, methane, O 2 , and CO 2 concentrations were analyzed using gas chromatography over 114 days at the Savannah River Site. δ 13 C of CHC and δ 13 C and 14 C of vadose zone CO 2 , sediment organic matter, and groundwater dissolved inorganic carbon (DIC) were measured. Intermediate metabolites of TCE and CT accounted for e10% of total CHCs. δ 13 C of cis-1,2-dichloroethylene (DCE) was always heavier than TCE indicating substantial DCE biodegradation. 14 C-CO 2 values ranged from 84 to 128 percent modern carbon (pMC), suggesting that plant root-respired CO 2 was dominant. 14 C-CO 2 values decreased over time (up to 12 pMC), and contaminated groundwater 14 C-DIC (76 pMC) was substantially depleted relative to the control (121 pMC). 14 C provided a direct measure of complete CHC mineralization in vadose zone and groundwater in situ and may improve remediation strategies.

Carbon isotope fractionation during aerobic biodegradation of n-alkanes and aromatic compounds in unsaturated sand

“…Usually intrinsic biodegradation by indigenous microbial populations is the key process for contaminant elimination and methods are required to demonstrate its efficacy. In groundwater studies, compound-specific isotope analysis (CSIA) has been used increasingly to demonstrate in situ biodegradation of various types of organic contaminants (Kelley et al, 1997;Sturchio et al, 1998;Hunkeler et al, 1999;Sherwood Lollar et al, 2001;Kolhatkar et al, 2002;Mancini et al, 2002;Meckenstock et al, 2002;Song et al, 2002;Kirtland et al, 2003;Richnow et al, 2003a,b;Chu et al, 2004;Griebler et al, 2004;Steinbach et al, 2004;Hunkeler et al, 2005;Morrill et al, 2005). In contrast, only a few studies have investigated the use of CSIA in the unsaturated zone (Stehmeier et al, 1999;Kirtland et al, 2005).…”

Measuring Carbon-based Contaminant Mineralization Using Combined CO₂ Flux and Radiocarbon Analyses

“…It is thus possible to estimate the proportion of the entire CO 2 pool (respired carbon) attributable to the contaminant. Using solely this proportion, fossil-hydrocarbon or industrial chemical oxidation at field sites has been confirmed [7][8][9][10][11][12][13] . This proportion of contaminant derived CO 2 can then be coupled with total CO 2 mineralization rate (all CO 2 collected per unit time and volume) to determine intrinsic contaminant mineralization rate.…”

“…Because very distinct isotopic signatures are available in carbon-based contaminants, carbon isotopes have been recently applied to understanding contaminant attenuation processes at field sites [5][6][7][8][9][10][11][12][13] . Stable carbon isotopes can be used to determine if a source is attenuating based on Rayleigh distillation kinetics (c.f.…”

“…The technique described here relies on the observation that any CO 2 generated from a fossil derived contaminant will be devoid of 14 C while CO 2 generated from microorganisms degrading NOM will contain easily-measurable amounts of 14 C. Measuring 14 CO 2 also allows one to directly link full contaminant degradation (i.e., mineralization) to a harmless end product. 14 CO 2 analysis has been used to follow fossil fuel-derived contaminant degradation products [7][8][9][10][11][12][13] . This is due to the analytical resolution between end members (fossil and contemporary) which is roughly 1,100 parts per thousand (‰).…”

Unravelling long-term source removal effects and chlorinated methanes natural attenuation processes by C and Cl stable isotopic patterns at a complex field site

“…To our knowledge, there are only few field CSIA studies dedicated to CMs. These studies are mainly based on carbon isotope measurements to confirm CT degradation in the non-saturated zone (Kirtland et al, 2003), to characterize the very depleted δ 13 C of CT and CF industrially produced from methane (Nijenhuis et al, 2013), to confirm CF degradation, along with concentration data, in a multi-contaminant polluted aquifer (Hunkeler et al, 2005) and/or to evaluate CF remediation treatment by alkaline hydrolysis in Òdena site (NE Spain) . New methods for chlorine isotope analyses of CMs and reference Λ values have only became available recently (Heckel et al, 2017a(Heckel et al, , 2017bTorrentó et al, 2017;Rodríguez-Fernández et al, 2018a, 2018b.…”