Identification of Degradation Pathways of Chlorohydrocarbons in Saturated Low-Permeability Sediments Using Compound-Specific Isotope Analysis

Abstract

This study aims to investigate whether compound-specific carbon isotope analysis (CSIA) can be used to differentiate the degradation pathways of chlorohydrocarbons in saturated low-permeability sediments. For that purpose, a site was selected, where a complex mixture of chlorohydrocarbons contaminated an aquifer-aquitard system. Almost 50 years after contaminant releases, high-resolution concentration, CSIA, and microbial profiles were determined. The CSIA profiles showed that in the aquitard cis-dichloroethene (cDCE), first considered as a degradation product of trichloroethene (TCE), is produced by the dichloroelimination of 1,1,2,2-tetrachloroethane (TeCA). In contrast, TeCA degrades to TCE via dehydrohalogenation in the aquifer, indicating that the aquifer-aquitard interface separates two different degradation pathways for TeCA. Moreover, the CSIA profiles showed that chloroform (CF) is degraded to dichloromethane (DCM) via hydrogenolysis in the aquitard and, to a minor degree, produced by the degradation of carbon tetrachloride (CT). Several microorganisms capable of degrading chlorohydrocarbons were detected in the aquitard, suggesting that aquitard degradation is microbially mediated. Furthermore, numerical simulations reproduced the aquitard concentration and CSIA profiles well, which allowed the determination of degradation rates for each transformation pathway. This improves the prediction of contaminant fate in the aquitard and potential magnitude of impacts on the adjacent aquifer due to back-diffusion.

Use of dual element isotope analysis and microcosm studies to determine the origin and potential anaerobic biodegradation of dichloromethane in two multi-contaminated aquifers

“…and Acetobacterium sp. could be somehow involved in the degradation of CF as well, as described elsewhere (Egli et al, 1990(Egli et al, , 1988Gälli and McCarty, 1989;Wanner et al, 2018).…”

“…Also, the anaerobic degradation of CF by Acetobacterium sp. comprises both a reductive branch leading to DCM and an oxidative pathway leading to CO2 (Egli et al, 1990(Egli et al, , 1988Wanner et al, 2018). DCM can be also produced co-metabolically by, for example, Methanosarcina sp.…”

A New Method for Determining Compound Specific Carbon Isotope of Chlorinated Solvents in Porewater

“…CSIA of CSs in porewater allows for the determination of the initial isotopic composition of the spilled contaminant and the presence or absence of degradation processes in the sediments (Passeport et al 2016). This methodology has been successfully applied in saturated low‐permeability sediments to differentiate and quantify the degradation pathways of CSs (Wanner et al 2016, 2018a) and to quantify the rate of degradation during enhanced reductive dehalogenation (Damgaard et al 2013). Likewise, this technique is complementary to the CSIA of CSs in groundwater, and, until now, only the studies by Wanner et al (2016, 2018a) have coupled the CSIA of porewater and groundwater.…”

“…This methodology has been successfully applied in saturated low‐permeability sediments to differentiate and quantify the degradation pathways of CSs (Wanner et al 2016, 2018a) and to quantify the rate of degradation during enhanced reductive dehalogenation (Damgaard et al 2013). Likewise, this technique is complementary to the CSIA of CSs in groundwater, and, until now, only the studies by Wanner et al (2016, 2018a) have coupled the CSIA of porewater and groundwater. Wanner et al (2018a) studied low‐permeability sediments to predict the potential magnitude of the impacts of back diffusion on adjacent aquifers.…”

“…Likewise, this technique is complementary to the CSIA of CSs in groundwater, and, until now, only the studies by Wanner et al (2016, 2018a) have coupled the CSIA of porewater and groundwater. Wanner et al (2018a) studied low‐permeability sediments to predict the potential magnitude of the impacts of back diffusion on adjacent aquifers. For the analysis of CSs present in porewater, the extraction of the CSs and their subsequent concentrations in water (Damgaard et al 2012, 2013; Wanner et al 2018a) and methanol (Wanner et al 2016) were determined using the purge‐and‐trap analytical technique.…”