HYDROLOGICAL PROCESSES Hydrol. Process. 22, 4905–4921 (2008) Published online 10 September 2008 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/hyp.7113 A kinetic approach for simulating redox-controlled fringe and core biodegradation processes in groundwater: model development and application to a landﬁll site in Piedmont, Italy Massimo Rolle, 1 * T. Prabhakar Clement, 2 Rajandrea Sethi 1 and Antonio Di Molfetta 1 1 Dipartimento del Territorio, dell’Ambiente e delle Geotecnologie, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129, Torino, Italy 2 Department of Civil Engineering, 212 Harbert Engineering Center, Auburn University, AL 36849-5337, USA Abstract: A three-dimensional model for predicting redox controlled, multi-species reactive transport processes in groundwater systems is presented. The model equations were fully integrated within a MODFLOW-family reactive transport code, RT3D. The model can simulate organic compound biodegradation coupled to different terminal electron acceptor processes. A computational approach, which uses the spatial and temporal distribution of the rates of different redox reactions, is proposed to map redox zones. The method allows one to quantify and visualize the biological degradation reactions occurring in three distinct patterns involving fringe, pseudo-core and core processes. The capabilities of the numerical model are demonstrated using two hypothetical examples: a batch problem and a simpliﬁed two-dimensional reactive transport problem. The model is then applied to an unconﬁned aquifer underlying a leaking landﬁll located near the city of Turin, in Piedmont (Italy). At this site, high organic load from the landﬁll leachate activates different biogeochemical processes, including aerobic degradation, denitriﬁcation, manganese reduction, iron reduction, sulfate reduction and methanogenesis. The model was able to describe and quantify these complex biogeochemical processes. The proposed model offers a rational framework for simulating coupled reactive transport processes occurring beneath a landﬁll site. Copyright  2008 John Wiley & Sons, Ltd. KEY WORDS redox zonation; TEAPs; reaction rates; reactive transport modelling; landﬁll; leachate Received 31 October 2007; Accepted 18 June 2008 INTRODUCTION The release of organic pollutants into the subsurface can activate multiple biogeochemical reaction processes such as sorption, ion exchange, precipitation/dissolution and redox reactions. The most important biochemical pro- cesses that control the fate and transport of pollutants in groundwater are the microbially mediated redox reac- tions. Microorganisms can degrade an organic pollutant through different types of terminal electron acceptor pro- cesses (TEAPs) such as aerobic respiration, denitriﬁca- tion, Mn(IV) reduction, Fe(III) reduction, sulfate reduc- tion and methanogenesis. These redox processes occur in sequential order, which is controlled by the level of free energy generated during the electron acceptor consump- tion reaction (Table I). However, the spatial distribution of these reactions would not only depend on the free energy level, but also on the availability of appropri- ate microbes, organic substrates, nutrients and electron acceptors, and on the kinetics of the degradation reac- tions. * Correspondence to: Massimo Rolle, University of Tuebingen, Centre for Applied Geoscience, Sigwartsrtasse 10, D-72076, Tuebingen, Germany. E-mail: massimo.rolle@uni-tuebingen.de A succession of redox zones (as shown in Figure 1) is commonly observed in many sites contaminated by oxi- dizable organic compounds (Baedecker and Back, 1979; Chapelle et al., 1995, 2002; Christensen et al., 2001; Cozzarelli et al., 2001; Brun et al., 2002; Van Breukelen et al., 2004). Unique biogeochemical conditions available within these zones may selectively favour biodegradation of the contaminant and make a signiﬁcant contribution to the overall rate of degradation. At the plume fringe, pro- cesses involving soluble electron donors and acceptors are typically active (Cirpka et al., 1999; Lerner et al., 2000; Maier and Grathwohl, 2006). These reactions are primarily limited by the diffusive ﬂux driven by trans- verse dispersion, where soluble electron acceptors such as O 2 , NO 3  and SO 4 2 diffuse inwards, from the sur- rounding groundwater, and are made available for bac- teria to support the degradation process. On the other hand, at the plume core, various anaerobic degradation processes such as iron reduction, manganese reduction, and methanogenesis are active (Christensen et al., 2001; Cozzarelli et al., 2001; Chapelle et al., 2002; Schreiber et al., 2004). The efﬁciency and distribution pattern of these biodegradation reactions are often determined by the availability of the electron donors or acceptors that are originally present or are released from the solid aquifer matrix. Copyright  2008 John Wiley & Sons, Ltd.