title: 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.
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 (13)C/(12)C 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 ε(equ)(sorption) = -0.31 ‰, ε(kin)(transverse-dispersion) = -0.82 ‰, and ε(kin)(biodegradation) = -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.
authors.affiliation: Institute of Groundwater Ecology, Helmholtz Zentrum München, Ingolstadter Landstraße 1, 85764 Neuherberg, Germany.
authors.forename: Shiran
authors.initials: S
authors.lastname: Qiu
authors.forename: Dominik
authors.initials: D
authors.lastname: Eckert
authors.forename: Olaf A
authors.initials: OA
authors.lastname: Cirpka
authors.forename: Marko
authors.initials: M
authors.lastname: Huenniger
authors.forename: Peter
authors.initials: P
authors.lastname: Knappett
authors.forename: Piotr
authors.initials: P
authors.lastname: Maloszewski
authors.forename: Rainer U
authors.initials: RU
authors.lastname: Meckenstock
authors.forename: Christian
authors.initials: C
authors.lastname: Griebler
authors.forename: Martin
authors.initials: M
authors.lastname: Elsner
mesh_terms: D000327:Adsorption; D001419:Bacteria / Q000378:metabolism; D001673:Biodegradation, Environmental; D002247:Carbon Isotopes / Q000032:analysis; D060587:Groundwater; D007700:Kinetics; D014050:Toluene / Q000737:chemistry* / Q000378:metabolism*; D014874:Water Pollutants, Chemical / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D002247:Carbon Isotopes; D014874:Water Pollutants, Chemical; D014050:Toluene