Quantification of Sequential Chlorinated Ethene Degradation by Use of a Reactive Transport Model Incorporating Isotope Fractionation

Abstract

Compound-specific isotope analysis (CSIA) enables quantification of biodegradation by use of the Rayleigh equation. The Rayleigh equation fails, however, to describe the sequential degradation of chlorinated aliphatic hydrocarbons (CAHs) involving various intermediates that are controlled by simultaneous degradation and production. This paper shows how isotope fractionation during sequential degradation can be simulated in a 1D reactive transport code (PHREEQC-2). 12 C and 13 C isotopes of each CAH were simulated as separate species, and the ratio of the rate constants of the heavy to light isotope equaled the kinetic isotope fractionation factor for each degradation step. The developed multistep isotope fractionation reactive transport model (IF-RTM) adequately simulated reductive dechlorination of tetrachloroethene (PCE) to ethene in a microcosm experiment. Transport scenarios were performed to evaluate the effect of sorption and of different degradation rate constant ratios among CAH species on the downgradient isotope evolution. The power of the model to quantify degradation is illustrated for situations where mixed sources degrade and for situations where daughter products are removed by oxidative processes. Finally, the model was used to interpret the occurrence of reductive dechlorination at a field site. The developed methodology can easily be incorporated in 3D solute transport models to enable quantification of sequential CAH degradation in the field by CSIA.

Numerical simulation of isotope fractionation in steady-state bioreactive transport controlled by transverse mixing

“…To obtain the degradation rates for the individual light and heavy isotopologues, considering competition of both isotopologues for the same reactive sites, r deg is multiplied by the fraction of the light isotopologue ( 12 c S /c S tot ) and by the fraction of the heavy isotopologue and the isotope fractionation factor ( 13 c S /c S tot ⋅α B ), respectively Van Breukelen et al, 2005).…”

“…If one or more of these assumptions do not hold, the Rayleigh equation is no more strictly valid. Several studies have pointed out the limitations of the Rayleigh equation when applied to groundwater systems and have proposed methodologies to extend its validity to open systems (e.g., Fischer et al, 2007;Mak et al, 2006;Thullner et al, 2012;Van Breukelen, 2007) or alternative approaches to describe isotope fractionation in the presence of complex reaction networks (e.g., Van Breukelen et al, 2005). Recent experimental studies have shown that, besides (bio) chemical reactions, also physical mass transfer processes can significantly affect the isotopic composition of contaminants in groundwater systems.…”

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

“…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]. However, assuming that the carbon pool is conservative during anaerobic biodegradation (supposing DCBs → MCB → benzene) [32,33] and that aerobic processes are not inducing significant fractionations, particularly for CBs [23][24][25], δ 13 C sum values should retain their signature along the flowpath, regardless of the occurrence of degradation.…”

Impacts of microbial community composition on isotope fractionation during reductive dechlorination of tetrachloroethylene

“…In addition to the conventional methods focusing on identification and quantification of parent substrates and dehalogenated products, compound specific isotope analysis (CSIA) has been increasingly applied to monitor transformation of chlorinated contaminants (Hunkeler et al 1999;Sherwood Lollar et al 1999;Bloom et al 2000;Elsner et al 2004;Aeppli et al 2009;Morrill et al 2009;Schmidt et al 2010). Isotope ratio analysis of in situ contaminants involves the use of enrichment factors, generally obtained from lab-scale studies, to quantitatively estimate the fraction of contaminants transformed at a site (Elsner et al 2005;Van Breukelen et al 2005;VanStone et al 2005;Nijenhuis et al 2007). In order to avoid significant uncertainties in this process, it is important to choose enrichment factors appropriate for specific field sites and to understand the factors that influence this parameter.…”