Recent work on remediation of oxidized chemicals with permeable reactive barriers (PRBs) containing iron metal (Fe0) has shown that remediation performance is strongly affected by the layers of precipitates that form on the iron surface over time, and that the quantity and composition of these precipitates vary greatly depending on the composition of the groundwater. Despite the enormous success of Fe0 PRBs, the identity and significance of this surface precipitate is still unclear. This research proposes to develop a series of experimental protocols to characterize the composition of the surface precipitates forming in the Fe0 PRBs and evaluate how variations in solution chemistry are linked to changes in the precipitate coatings, which then enhance or inhibit PRB performance.