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A novel non-invasive imaging technique was developed to investigate the biodegradation processes occurring at the fringe of a simulated contaminant plume. In conditions where the supply of electron acceptors is limited, biodegradation may be expected to be focused at the plume fringe and transverse dispersion will play a very important role. The technique permitted investigation of the mixing process of dissolved oxygen with the plume in a two dimensional porous media at laboratory scale. A small transparent aquifer (160 mm length, 130 mm width and 3 mm thickness) was made up of quartz plates and quartz sand (60-100 micrometers). Potassium acetate was used as a substrate and was continuously injected into the aquifer to form a plume. Ruthenium (II)-dichlorotris(1,10-phenanthroline) (Ru(phen) sub 3 Cl sub 2 ), a water soluble fluorescent dye which can be quenched by dissolved oxygen, was used as …