A biodegradation model with consecutive fermentation and respiration processes, developed from microcosm experiments and simulated mathematically with microbial growth kinetics, has been implemented into a field-scale reactive transport model of a groundwater plume of phenolic contaminants. Simulation of the anaerobic plume core with H₂ and acetate as intermediate products of biodegradation allows the rates and parameter values for fermentation processes and individual respiratory terminal electron accepting processes (TEAPS) to be estimated using detailed, spatially discrete, hydrochemical field data. The modeling of field-scale plume development includes consideration of microbial acclimatization, substrate toxicity toward degradation, bioavailability of mineral oxides, and adsorption of biogenic Fe(II) species in the aquifer, identified from complementary laboratory process studies. The results suggest that plume core processes, particularly fermentation and Fe(III)-reduction, are more important for degradation than previously thought, possibly with a greater impact than plume fringe processes (aerobic respiration, denitrification, and SO₄-reduction). The accumulation of acetate as a fermentation product within the plume contributes significantly to the mass balance for carbon. These results demonstrate the value of quantifying fermentation products within organic contaminant plumes and strongly suggest that the conceptual model selected for reactive processes plays a dominant role in the quantitative assessment of risk reduction by naturally occurring biodegradation processes.