Quantification of biodegradation for o-xylene and naphthalene using first order decay models, Michaelis–Menten kinetics and stable carbon isotopes Philipp Blum a, ⁎, Daniel Hunkeler b , Matthias Weede c , Christof Beyer a,d , Peter Grathwohl a , Barbara Morasch b,e a University of Tübingen, Center for Applied Geoscience (ZAG), Sigwartstrasse 10, 72076 Tübingen, Germany b University of Neuchâtel, Centre for Hydrogeology, Rue Emile-Argand 11, 2009 Neuchâtel, Switzerland c URS Deutschland GmbH, Sutelstrasse 2, 30659 Hannover, Germany d University of Kiel, Institute of Geosciences, Ludewig-Meyn-Str.10, 24118 Kiel, Germany e Environmental Chemistry Laboratory (LCE), EPFL - ENAC - ISTE, Station 2, 1015 Lausanne, Switzerland article info abstract Article history: Received 8 February 2008 Received in revised form 11 November 2008 Accepted 30 November 2008 Available online 14 December 2008 At a former wood preservation plant severely contaminated with coal tar oil, in situ bulk attenuation and biodegradation rate constants for several monoaromatic (BTEX) and polyaromatic hydrocarbons (PAH) were determined using (1) classical first order decay models, (2) Michaelis–Menten degradation kinetics (MM), and (3) stable carbon isotopes, for o- xylene and naphthalene. The first order bulk attenuation rate constant for o-xylene was calculated to be 0.0025 d - 1 and a novel stable isotope-based first order model, which also accounted for the respective redox conditions, resulted in a slightly smaller biodegradation rate constant of 0.0019 d - 1 . Based on MM-kinetics, the o-xylene concentration decreased with a maximum rate of k max =0.1 μg/L/d. The bulk attenuation rate constant of naphthalene retrieved from the classical first order decay model was 0.0038 d - 1 . The stable isotope-based biodegradation rate constant of 0.0027 d - 1 was smaller in the reduced zone, while residual naphthalene in the oxic part of the plume further downgradient was degraded at a higher rate of 0.0038 d - 1 . With MM-kinetics a maximum degradation rate of k max = 12 μg/L/d was determined. Although best fits were obtained by MM-kinetics, we consider the carbon stable isotope-based approach more appropriate as it is specific for biodegradation (not overall attenuation) and at the same time accounts for the dominant electron-accepting process. For o-xylene a field based isotope enrichment factor ε field of - 1.4 could be determined using the Rayleigh model, which closely matched values from laboratory studies of o-xylene degradation under sulfate-reducing conditions. © 2008 Elsevier B.V. All rights reserved. Keywords: Natural attenuation Redox zones Contaminant Plume Compound-specific isotope analysis (CSIA) Rate constants Polyaromatic hydrocarbons (PAH) BTEX 1. Introduction Active remediation techniques such as pump-and-treat or excavation are economically not feasible for the large number of soil and groundwater contaminations existing worldwide. Monitored natural attenuation (MNA) as a passive remedia- tion strategy is considered a worthwhile alternative. Numer- ous studies have shown that natural attenuation (NA) processes (e.g. dilution, sorption, dispersion, (bio-) degrada- tion) have the potential to manage groundwater contamina- tion at many sites (e.g. Rice et al., 1995; Mace et al., 1997; Grathwohl et al., 2000), which becomes evident in the observation that many dissolved contaminant plumes are not expanding any more. In recent years, short and steady state contaminant plumes are becoming increasingly toler- ated and MNA is implemented as a remediation strategy in the United States and in Europe when no receptor is at risk (NRC, 1999; Rügner et al., 2006). (Bio-)Degradation deserves special attention, because, out of all NA processes in ground- water, it is the only one substantially effective in removing contaminants from the environment for many compound Journal of Contaminant Hydrology 105 (2009) 118–130 ⁎ Corresponding author. Tel.: +49 7071 29 73170; fax: +49 70715059. E-mail address: philipp.blum@uni-tuebingen.de (P. Blum). 0169-7722/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jconhyd.2008.11.009 Contents lists available at ScienceDirect Journal of Contaminant Hydrology journal homepage: www.elsevier.com/locate/jconhyd