Removal of contaminants from groundwater using permeable reactive barriers (PRBs) is a cost-effective and popular engineering solution used throughout the world. Dissolved pollutants in groundwater are removed through geochemical processes that make PRBs effective for different types of contaminants. In achieving this, it is vital to determine the optimum width of the PRB to allow adequate residence time within the barrier and to establish its longevity. For this purpose, both field monitoring and geochemical modelling were conducted for a trial PRB located in the Shoalhaven Floodplain, south of Wollongong in Australia. In this study, the optimum PRB width is evaluated numerically, based on the neutralization effectiveness, i.e., when acidic groundwater travels through the alkaline PRB. A model developed previously has been extended considering the residence time, reaction kinetics, mineral precipitation-induced reduction in porosity and hydraulic conductivity, influent concentrations of the contaminants, and groundwater flow velocity. Longevity of the PRB is determined with respect to groundwater flow rates and amount of reactive material consumed.