title: Modelling biodegradation of hydrocarbons in aquifers: when is the use of the instantaneous reaction approximation justified?
abstract: In-situ bio-remediation is a viable cleanup alternative for aquifers contaminated by hydrocarbons such as BTEX. Transport models of varying complexity and capabilities are used to quantify their degradation. A model that has gained wide acceptance in applications is BIOPLUME II, which assumes that oxygen-limited biodegradation takes place as an instantaneous reaction. In this work we have employed theoretical analysis, using non-dimensional variables, and numerical modelling to establish a quantitative criterion demarcating the range of validity of the instantaneous reaction approximation against biodegradation kinetics. Oxygen was the limiting species and sorption was ignored. This criterion relates <Da>(o), the Dahmköhler number at oxygen depletion, to O(o)*, the ratio of initial to input oxygen concentration, <Da>(o) > or = 0.7(O(o)*)(2) + 0.1O(o)* + 1.8. The derived <Da>(o) reflects the intrinsic characteristics of the physical transport and of the biochemical reaction, including the effect of biomass density. Relative availability of oxygen and hydrocarbons exerts a small influence on results. Theory, verified and refined via numerical simulations, showed that significant deviations of instantaneous reactions from kinetics are to be expected in the space-time region s<L(d), t<T(d) ('near source' and 'initial period'). Under the assumptions considered, numerical simulations also verified the wide applicability of the computationally efficient, stoichiometry-based (algebraic) BIOPLUME concept. Kinetic modelling is required only in active (engineered) bio-remediation cases, with high velocities (e.g., near pumped wells), and for short distances from the source.
authors.affiliation: National Observatory of Athens, Institute For Environmental Research and Sustainable Development, I Metaxa and V Pavlou, GR-152 36 Palaia Penteli, Athens, Greece. akoussis@env.meteo.noa.gr
authors.forename: Antonis D
authors.initials: AD
authors.lastname: Koussis
authors.forename: Stelios
authors.initials: S
authors.lastname: Pesmajoglou
authors.forename: Dimitra
authors.initials: D
authors.lastname: Syriopoulou
mesh_terms: D001673:Biodegradation, Environmental; D006838:Hydrocarbons / Q000378:metabolism*; D007700:Kinetics; D008962:Models, Theoretical; D010100:Oxygen; D012989:Soil Pollutants / Q000378:metabolism*; D014874:Water Pollutants, Chemical / Q000378:metabolism*
chemical_list: D006838:Hydrocarbons; D012989:Soil Pollutants; D014874:Water Pollutants, Chemical; D010100:Oxygen