Assessment of Degradation Pathways in an Aquifer with Mixed Chlorinated Hydrocarbon Contamination Using Stable Isotope Analysis

Abstract

The demonstration of monitored natural attenuation (MNA) of chlorinated hydrocarbons in groundwater is typically conducted through the evaluation of concentration trends and parent−daughter product relationships along prevailing groundwater flow paths. Unfortunately, at sites contaminated by mixtures of chlorinated ethenes, ethanes, and methanes, the evaluation of MNA by using solely concentration data and parent−daughter relationships can result in erroneous conclusions regarding the degradation mechanisms that are truly active at the site, since many of the daughter products can be derived from multiple parent compounds. Stable carbon isotope analysis was used, in conjunction with concentration data, to clarify and confirm the active degradation pathways at a former waste solvent disposal site where at least 14 different chlorinated hydrocarbons have been detected in the groundwater. The isotope data indicate that TCE, initially believed to be present as a disposed product and/or a PCE dechlorination intermediate, is attributable to dehydrochlorination of 1,1,2,2-PCA. The isotope data further support that vinyl chloride and ethene in the site groundwater result from dichloroelimination of 1,1,2-trichlorethane and 1,2-dichloroethane, respectively, rather than from reductive dechlorination of the chlorinated ethenes PCE, TCE, or 1,2-DCE. The isotope data confirm that the chlorinated ethanes and chlorinated methanes are undergoing significant intrinsic degradation, whereas degradation of the chlorinated ethenes may be limited. In addition to the classical trend of enriched isotope values of the parent compounds with increasing distance associated to biodegradation, shifts of isotope ratios of degradation byproduct in the opposite direction due to mixing of isotopically light byproducts of biodegradation with compounds from the source are shown to be of high diagnostic value. These data underline the value of stable isotope analysis in confirming transformation processes at sites with complex mixtures of chlorinated compounds.

Stable carbon isotope analysis to distinguish biotic and abiotic degradation of 1,1,1-trichloroethane in groundwater sediments

“…For 1,2-DCA, it was demonstrated that two aerobic biodegradation pathways lead to significantly different carbon isotope fractionation Aravena, 2000, Hirschorn et al, 2004). Furthermore, Hunkeler et al (2005) and Hirschorn et al (2007) demonstrated the use of stable carbon isotope ratios to differentiate chlorinated ethene (TCE, DCE and VC) production by reductive dechlorination of PCE from production by dehydrochlorination (b-dehaloelimination) of 1,1,2,2-tetrachloroethane, 1,1,2-trichloroethane and 1,2-dichloroethane, and to differentiate ethane production by reductive dechlorination of VC from dehydrochlorination of 1,2-DCA. Lojkasek-Lima et al (2012) used CSIA data to verify that biotic degradation of TCE as well as abiotic degradation by Fe(0) occurred at a Fe(0) reactive wall in a TCE plume.…”

Identification of chlorinated solvents degradation zones in clay till by high resolution chemical, microbial and compound specific isotope analysis

“…Biotic and abiotic degradations of chlorinated ethenes and ethanes have been documented by compound specific isotope analysis (CSIA) in several studies (for chlorinated ethenes e.g. Courbet et al, 2011;Hunkeler et al, 2005;Lollar et al, 2001 and for chlorinated ethanes e.g. Lollar et al, 2010).…”

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).…”