Compound-specific carbon and chlorine isotope analysis of organic contaminants in soil and groundwater
Authors:   , Zimmermann Jeremy 
Published: December 2023

Chlorinated aliphatic hydrocarbons and aromatic petroleum hydrocarbons belong to the group of volatile organic compounds (VOC) and are common subsurface contaminants. They are naturally attenuated by dilution, sorption and degradation. Classical approaches in contaminant hydrogeology are often limited in their potential for monitoring attenuation processes, as they require that initial contaminants and degradation products be adequately captured by the sampling and analysis methods. Compound specific stable isotope analysis (CSIA) has emerged in recent decades as a versatile tool for tracking these processes. CSIA does not rely on concentration measurements. Degradation causes an isotopic enrichment in the initial contaminants and an isotopic depletion in degradation products. In addition to carbon isotope ratios, additional isotope system such as chlorine can be monitored for mechanistic insights. While carbon CSIA has established itself in the routine assessment of contaminated sites, studies incorporating chlorine isotopes have only recently become more common. The aim of this work was to demonstrate the potential of compound-specific carbon and chlorine isotope analysis. It focuses on analytical aspects, isotope fractionation for different processes, and field applications. A review provides an overview of chlorine isotope fractionation of chlorinated contaminants in the subsurface by different processes and presents analytical techniques in chlorine isotope analysis. A summary of successful field applications illustrates the potential of using chlorine isotope data. Approaches in modelling chlorine isotope fractionation are also discussed. An analytical method was developed for lowering method detection limits of chlorine isotope ratio measurements to the sub-μg L−1 range for tetrachloroethene (PCE) and trichloroethene (TCE) in aqueous solution using headspace solid-phase microextraction (HS-SPME) coupled to gas chromatography (GC) quadrupole mass spectrometry. The novel SPME Arrow system featuring an increased sorptive phase volume was used. SPME parameters were optimized with a design of experiments approach to achieve maximum intensity. Mass spectrometric parameters and chlorine isotope ratio calculation schemes were also investigated and optimized. Any chlorine isotope fractionation was accounted for through a two-point calibration with externally referenced standards. Precise δ37Cl measurements (standard deviations below 0.5‰) were achieved for concentrations of 0.4 μg L−1 (PCE) to 0.8 μg L−1 (TCE). Another analytical method was developed to extend the applicability of carbon CSIA to complex volatile organic compound mixtures in soil samples. Soil extraction methods were compared and a heart-cutting two-dimensional GC-GC isotope-ratio mass spectrometry (IRMS) method developed. Good chromatographic separation for the target compounds in difficult matrices and high accuracy (trueness and precision) for carbon isotopic analysis were achieved. The use of purge and trap concentration allowed high volumes of extraction solvents to be analyzed, hereby lowering method detection limits of carbon CSIA in soil. In a laboratory study, isotope fractionation during abiotic degradation of chlorinated ethenes was investigated. As opposed to many biodegradation processes, abiotic degradation is likely to yield non-chlorinated, generally non-toxic degradation products. However, this is difficult to demonstrate in the field. Dual carbon-chlorine isotope slopes are useful to investigate abiotic degradation in the field mediated by naturally occurring reactive iron phases. The degradation of chlorinated ethenes cis-1,2-dichloroethene (DCE), TCE and PCE by iron(II)sulfide (FeS) was investigated. FeS was found to be reactive with TCE and PCE, but not with cis-1,2-DCE. Carbon-chlorine isotope enrichment slopes ∆δ13C/∆δ37Cl were 11.0 ± 0.8 for PCE and 4.1 ± 0.2 for TCE. For PCE, the obtained slope is sufficiently distinct from those measured for biodegradation and can thus be applied to distinguish degradation processes in the field. In a field study, the effect of source isolation on contaminant longevity was assessed at a site contaminated by a complex mixture of benzene, toluene, ethylbenzene and xylenes (BTEX) and chlorinated ethanes, ethenes and methanes. Through soil and groundwater concentration profiles over time and space, the importance of forward diffusion into low permeability zones, degradation, and back-diffusion of initial contaminants and degradation products after source isolation was highlighted. Compound-specific carbon- and chlorine isotope analysis of groundwater and soil extracts showed that chloroform is degraded to dichloromethane through hydrogenolysis, while 1,1,2,2-tetrachloroethane is degraded to trichloroethene and cis-1,2-dichloroethene. The study demonstrated the potential of applying compound-specific isotope analysis not only to groundwater, but also to soil. Soil is shown to be an important matrix that often controls the long-term fate of contaminants. In conclusion, the improvements in analytical methods and understanding of isotopic fractionation processes presented in this thesis offer an important contribution to the field of compound-specific isotope analysis, and shall further advance its routine application in contaminated site management. The approaches developed and discussed herein may be adapted to other organic compounds and applied in future investigations that aim to track the environmental fate of organic contaminants.