A laboratory continuous flow-through sand reactor was used to study qualitatively and quantitatively the biogeochemical processes resulting from an input of an easily degradable organic substance (lactate) into a model aquifer. The primary occurring redox processes are mediated by microorganisms and can be described by the classical sequence of inorganic redox reactions in aquatic systems. In the steady state situation, the nitrate and sulfate reduction follow a pseudo first-order kinetics, with respective rate constants of (2.7 +/- 0.2) . 10(-1) min-1 and (2.3 +/- 0.3) . 10(-2) min-1. These rate constants are within the same range of other laboratory and field studies when taking the population densitv of microorganisms into account.