Fingerprinting TCE in a Bedrock Aquifer Using Compound‐Specific Isotope Analysis

Abstract

A dual isotope approach based on compound-specific isotope analysis (CSIA) of carbon (C) and chlorine (Cl) was used to identify sources of persistent trichloroethylene (TCE) that caused the shut-down in 1994 of a municipal well in an extensive fractured dolostone aquifer beneath Guelph, Ontario. Several nearby industrial properties have known subsurface TCE contamination; however, only one has created a comprehensive monitoring network in the bedrock. The impacted municipal well and many monitoring wells were sampled for volatile organic compounds (VOCs), inorganic parameters, and CSIA. A wide range in isotope values was observed at the study site. The TCE varies between -35.6‰ and -21.8‰ and from 1.6‰ to 3.2‰ for δ(13) C and δ(37) Cl, respectively. In case of cis-1,2-dichloroethene, the isotope values range between -36.3‰ and -18.9‰ and from 2.4‰ to 4.7‰ for δ(13) C and δ(37) Cl, respectively. The dual isotope approach represented by a plot of δ(13) C vs. δ(37) Cl shows the municipal well samples grouped in a domain clearly separate from all other samples from the property with the comprehensive well network. The CSIA results collected under non-pumping and short-term pumping conditions thus indicate that this particular property, which has been studied intensively for several years, is not a substantial contributor of the TCE presently in the municipal well under non-pumping conditions. This case study demonstrates that CSIA signatures would have been useful much earlier in the quest to examine sources of the TCE in the municipal well if bedrock monitoring wells had been located at several depths beneath each of the potential TCE-contributing properties. Moreover, the CSIA results show that microbial reductive dechlorination of TCE occurs in some parts of the bedrock aquifer. At this site, the use of CSIA for C and Cl in combination with analyses of VOC and redox parameters proved to be important due to the complexity introduced by biodegradation in the complex fractured rock aquifer. It is highly recommended to revisit the study when the municipal well is back into full operation.

Compound‐Specific Isotope Analyses to Assess TCE Biodegradation in a Fractured Dolomitic Aquifer

“…The heaviest δ 13 C TCE and δ 13 C cis ‐DCE values were sampled from wells 15S9 and 17S7 (on the edge of the plume), respectively, with the lightest δ 13 C TCE and δ 13 C cis ‐DCE values from wells 3S8 and RWS7 (along/near the central axis of the plume), respectively (Figure A, Table ). This is consistent with expected fractionation for dechlorination (e.g., Abe et al ; Lojkasek‐Lima et al ; Cretnik et al ; Liu et al ). Combined with TCE and cis ‐DCE carbon isotopic compositions that were lightest in samples where TCE comprised a majority of the total molar mass of ethenes, and heaviest in samples where TCE was a minor component, this indicates dechlorination is occurring on the plume fringe, but not in the center.…”

“…In general, for individual wells, δ 37 Cl cis-DCE values were heaviest relative to the standard, followed by: δ 37 Cl TCE , δ 13 C cis-DCE , and then δ 13 C TCE . These results are similar to results at two previous field studies (Lojkasek-Lima et al 2012;Kaown et al 2014). The exception to this trend was well RWS7, where δ 13 C TCE was similar to or slightly higher than δ 13 C cis-DCE .…”

“…Chlorine isotope fractionation of cis ‐DCE from TCE varied between 1.2 and 4.2‰ (mean: 2.6‰). The chlorine isotope enrichment of cis ‐DCE relative to TCE in individual samples is slightly larger than observed in one previous field study (−0.8 to 2.2‰, mean: 1.3‰; Lojkasek‐Lima et al ), but more similar to those observed in a second (1 to 3‰, Kaown et al ). These data indicate that TCE degradation is continuing beyond cis ‐DCE in parts of the aquifer, with the highest degradation occurring along the edges of the plume where TCE concentrations are lower.…”

“…The large range of TCE chlorine isotopic values observed in this study could be interpreted as an indication of a mixture of TCE sourced from a number of different manufacturers or TCE derived from PCE degradation as well as manufactured TCE (e.g., van Warmerdam et al ; Shouakar‐Stash et al ; Lojkasek‐Lima et al ); however, that does not seem to be the case at this site (Figure ). Although such a conclusion would be supported by the history of the site as a waste‐disposal center, the largest variation in δ 37 Cl isotopic composition occurs in samples with a very small molar fraction of TCE relative to other ethenes, indicating another process is affecting the isotopic composition (Figure ).…”

“…Therefore, management and remediation of persistent DNAPLs, such as TCE, is of critical importance (e.g., Pankow and Cherry 1996). Over the past two decades, considerable advances have been achieved in understanding the fundamental processes controlling the fate of DNAPLs spilled or released in heterogeneous unconsolidated geologic materials, but relatively less is known about behavior in fractured bedrock environments (e.g., Parker et al 1994Parker et al , 1996Parker et al , 1997Kueper and McWhorter 1996;Dickson and Thomson 2003;Sterling et al 2005;Lenczewski et al 2006;McLaren et al 2012;Lojkasek-Lima et al 2012). In spite of these efforts, it is widely recognized that few, if any, sites contaminated by DNAPLs have been fully remediated (e.g., Illman and Alvarez 2009).…”