Quantification of biodegradation for <i>o</i>-xylene and naphthalene using first order decay models, Michaelis–Menten kinetics and stable carbon isotopes

At a former wood preservation plant severely contaminated with coal tar oil, <i>in situ</i> 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 <i>o</i>-xylene and naphthalene. The first order bulk attenuation rate constant for <i>o</i>-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 <i>o</i>-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 <i>o</i>-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 <i>o</i>-xylene degradation under sulfate-reducing conditions.