The U.S. EPA Office of Research and Development’s National Risk Management Research Laboratory (NRMRL) and Region 8 have begun evaluating the performance of a pilot-scale permeable reactive barrier (PRB) to treat arsenic-contaminated ground water at the East Helena Superfund site near Helena, MT. High ground-water flow rates coupled with high arsenic concentrations required the barrier design to involve wider dimensions in the path of ground-water flow than most PRBs currently in operation. Barrier construction also was challenged by the presence of some boulders in the subsurface, requiring use of large excavation equipment. Preliminary results indicate that arsenic concentrations as high as 20 mg/L in ground water entering the PRB are reduced to concentrations below 10 µg/L within the barrier. Concentration reductions downgradient of the PRB are anticipated after construction impacts on the treatment system subside and the ambient ground-water flow system is re-established.