The A-01 National Pollutant Discharge Elimination System (NPDES) outfall at the Savannah River Site receives process wastewater discharges and stormwater runoff from the Savannah River National Laboratory. Routine monitoring indicated that copper concentrations were regularly higher than discharge permit limit, and water routinely failed toxicity tests. These conditions necessitated treatment of nearly 1 million gal of water/day (3.78 million L of water/day) plus storm runoff. Washington Savannah River Company personnel explored options to bring process and runoff waters into compliance with the permit conditions, including source reduction, engineering solutions, and biological solutions. A conceptual design for a constructed wetland treatment system was developed, and the full-scale system was constructed and began operation in 2000. The overall objective of our research was to better understand the mechanisms of operation of the A-01 wetland treatment system to provide better input to the design of future systems. The system is a vegetated surface flow wetland with a hydraulic retention time of approximately 48 hr. Copper, mercury, and lead removal efficiencies are very high, all in excess of 80% removal from water passing through the wetland system. Zinc removal is 60%, and nickel is generally unaffected. Dissolved organic carbon in the water column is increased by the system and reduces the toxicity of the effluent. Concentrations of metals in the A-01 wetland treatment system sediments generally decrease with depth and along the flow path through the wetland. Sequential extraction results indicate that most metals are tightly bound to wetland sediments.