Multi-isotope (carbon and chlorine) analysis for fingerprinting and site characterization at a fractured bedrock aquifer contaminated by chlorinated ethenes

Abstract

The use of compound specific multi-isotope approach (C and Cl) in the characterization of a chlorinated ethenes contaminated fractured aquifer allows the identification of several sources and contaminant plumes, as well as the occurrence of biodegradation and mixing processes. The study site is located in Spain with contamination resulting in groundwater concentrations of up to 50mg/L of trichloroethene (TCE), the most abundant chlorinated ethene, and 7 mg/L of tetrachloroethene (PCE). The potential sources of contamination including abandoned barrels, an underground tank, and a disposal lagoon, showed a wide range in δ(13)C values from -15.6 to -40.5‰ for TCE and from -18.5 to -32.4‰ for PCE, allowing the use of isotope fingerprinting for tracing of the origin and migration of these contaminants in the aquifer. In contrast, there is no difference between the δ(37)Cl values for TCE in the contaminant sources, ranging from +0.53 to +0.66‰. Variations of δ(37)Cl and δ(13)C in the different contaminant plumes were used to investigate the role of biodegradation in groundwater. Moreover, the isotopic data were incorporated into a reactive transport model for determination of whether the isotope pattern observed downstream from the tank's source could be explained by the simultaneous effect of mixing and biodegradation. The results demonstrate that a multi-isotope approach is a valuable tool for characterization of complex sites such as fractured bedrock aquifer contaminated by multiple sources, providing important information which can be used by consultants and site managers to prioritize and design more successful remediation strategies.

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

“…These data suggest a lower extent of degradation for both compounds downstream. CMs moving downstream in the pit source's influence area might inhibit chlorinated ethenes biodegradation (Bagley et al, 2000), explaining the results previously reported by Palau et al (2014).…”

“…The CT content decreased by nearly one order of magnitude between Mar-10 and Nov-14 and DCM and CS 2 were always b0.2 μM (Table A3). Table 1 CT, CF and DCM carbon isotopic composition (δ 13 C, ‰) of the three Òdena site's pollution sources previously identified by Palau et al (2014) are shown. Samples from the abandoned barrels, the tank and the pit were taken from 2002 to 2005. δ 13 C ranges of commercial CMs from different suppliers (see Table A2 in SI for details) are also shown.…”

“…To overcome these limitations, compound specific isotope analysis (CSIA) is increasingly used for source apportionment and in situ assessment of chlorinated ethenes and ethanes MNA (Wiegert et al, 2012(Wiegert et al, , 2013Kuder et al, 2013;Badin et al, 2014Badin et al, , 2016Kaown et al, 2014;Palau et al, 2014Palau et al, , 2016Audí-Miró et al, 2015). Moreover, dual isotopic studies (2D-CSIA) with carbon and chlorine isotope analyses have allowed more precise identification of pollutants' origin and fate as dual isotope slopes (Λ) reflect ongoing degradation mechanisms and can be compared with characteristic slopes from laboratory studied reactions (Hunkeler et al, 2009).…”

“…However, an aged source of contamination would have undergone fractionation processes in the unsaturated zone distinctly or not to those occurred in the saturated zone Hunkeler, 2012, 2013) and, thus, it would hamper reliable degradation processes discrimination. Therefore, if the original pure phase is not available, the study and comparison of isotope values from wells respect to the ones located in source areas (Wiegert et al, 2012) or to the known range of commercial solvents values (Holt et al, 1997;Jendrzejewski et al, 2001;Shouakar-Stash et al, 2003) is strongly advisable (Imfeld et al, 2008;Palau et al, 2014). To our knowledge, there are only few field CSIA studies dedicated to CMs.…”

Compound-Specific Isotope Analysis (CSIA) Application for Source Apportionment and Natural Attenuation Assessment of Chlorinated Benzenes

“…To overcome the high degree of uncertainty when using the molar fraction for SA purposes, an analysis was conducted on the basis of δ 13 C sum . As several contaminants were detected at the site and potential biodegradation can be hypothesized from the molar fraction distribution (Figure 2), the approach presented made use of the isotope balance δ 13 C sum rather than considering the isotopic signature of specific compounds (Table 1) [13,30]. Yet, if contaminants undergo a preferential transport along the plumes and/or when biodegradation processes are taking place, the δ 13 C sum interpretation is not an easy task [31].…”