Oxidation of naturally occurring pyrite (FeS2) in certain low-lying clayey soils generates sulphuric acid, hence the term acid sulphate soils. A horizontal alkaline barrier was installed by radial grouting, for the purpose of remediating leachate from acid sulphate soils and preventing further oxidation. The current research relates to a large-scale field trial of this technique and the effect on the groundwater composition. In coastal Australia, a pyritic layer commonly exists in the soil at shallow depth that is at risk of oxidation, hence the main objective was to inject the barrier above the pyritic layer to (1) stop infiltration of oxygen to the pyritic layer and (2) neutralize any acidity stored in the soil. Two fine-grained alkaline materials, lime and fly ash, were assessed in this study. Lime was selected for its neutralizing capacity, and the fly ash was selected to accompany the lime to enhance the pozzolanic reactions. The optimum mix ratio of lime, fly ash and water to form an ideal slurry and the optimum depth and pressure of injection were experimentally determined. For the large-scale field trial, the slurry was injected into a systematic grid of 22 holes to form the reactive barrier. The groundwater composition was monitored in a network of observation holes across the study site to determine the effectiveness of the barrier. The average groundwater pH was 3.25 prior to installation of the barrier, and it rose to 4.6 after the barrier was installed. The influence of the barrier on the groundwater pH was greater in observation holes close to the barrier than in those further away. The concentrations of aluminium and iron decreased in the groundwater after the installation of the alkaline barrier. The ratio of Cl/SO4 in the groundwater increased after the barrier was installed, which confirmed that the barrier had successfully controlled the subsequent pyrite oxidation in the soil.