Different organic-based permeable reactive barrier (PRB) systems were evaluated for their effectiveness in treating heavy metals, arsenic and acidity in groundwater. The data suggest that although arsenic was being mobilized from aquifer sediments downgradient of the PRB, the amounts being mobilized were gradually decreasing over time. A low iron concentration would be expected to limit arsenic removal by limiting coprecipitation reactions with iron sulfides, iron carbonates, and/or iron (oxy)hydroxides. Iron acidity was greater than 25-fold while aluminum acidity was approximately twofold greater than at the Louisiana site. Sinks for sulfate entering the PRB include formation of metal sulfides, elemental sulfur, precipitation as gypsum, and formation of organically bound sulfur. The findings of the Louisiana PRB study indicated that a cow manure-limestone-based PRB system effectively removed heavy metals, arsenic, and acidity from acidic sulfate-containing waters; although a significant dissolved iron content entering the PRB may have been a prerequisite for removal of arsenic.