Direct experimental evidence of non-first order degradation kinetics and sorption-induced isotopic fractionation in a mesoscale aquifer: 13C/12C Analysis of a transient toluene pulse

Shiran Qiu, Dominik Eckert, Olaf A. Cirpka, Marko Huenniger, Peter Knappett, Piotr Maloszewski, Rainer U. Meckenstock, Christian Griebler, Martin Elsner

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Abstract

The injection of a mixed toluene and D2O (conservative tracer) pulse into a pristine mesoscale aquifer enabled a first direct experimental comparison of contaminant-specific isotopic fractionation from sorption versus biodegradation and transverse dispersion on a relevant scale. Water samples were taken from two vertically resolved sampling ports at 4.2 m distance. Analysis of deuterium and toluene concentrations allowed quantifying the extent of sorption (R = 1.25) and biodegradation (37% and 44% of initial toluene at the two sampling ports). Sorption and biodegradation were found to directly affect toluene 13C/12C breakthrough curves. In particular, isotope trends demonstrated that biodegradation underwent Michaelis-Menten kinetics rather than first-order kinetics. Carbon isotope enrichment factors obtained from an optimized reactive transport model (Eckert et al., this issue) including a possible isotope fractionation of transverse dispersion were εequsorption = -0.31 ‰, ε kintransverse-dispersion = -0.82 ‰, and εkinbiodegradation = -2.15 ‰. Extrapolation of our results to the scenario of a continuous injection predicted that (i) the bias in isotope fractionation from sorption, but not transverse dispersion, may be avoided when the plume reaches steady-state; and (ii) the relevance from both processes is expected to decrease at longer flow distances when isotope fractionation of degradation increasingly dominates.

Original languageEnglish
Pages (from-to)6892-6899
Number of pages8
JournalEnvironmental Science and Technology
Volume47
Issue number13
DOIs
StatePublished - 2 Jul 2013
Externally publishedYes

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