The development of reactive barrier systems to remove metals from groundwater requires an improved understanding of the behavior of reaction fronts and the coupling between fluid flow, solute transport, reaction, and hydraulic changes. We report the results of experiments in which a dilute Fe(ClO4)3 solution was pumped through columns packed with mixtures of quartz sand and crushed calcite. The deposition of ferric oxyhydroxide peaked at, but was not restricted to, the visible precipitate front, where the neutralization of acidity generated by hydrolysis was completed. Even beyond this reaction front, calcite dissolution and ferric oxyhydroxide precipitation continued gradually. Upgradient from the front, where all calcite had been removed from the porous medium, ferric oxyhydroxide precipitation continued because of nucleation around older precipitate at a pH less than the influent pH. The velocity of the reaction front varied with the ratio of the initial volume of calcite to the pore water velocity, whereas the width of the zone of pronounced calcite dissolution varied as a function of the Damkohler number. These experiments illustrate how a model barrier can effectively and rapidly remove ferric iron from solution without fouling of the reactive surfaces. ¿ American Geophysical Union 1994