The 488-D Ash Basin (488-DAB) is an unlined, earthen landfill containing approximately 1 million t of dry ash and coal reject material at the U.S. Department of Energy's Savannah River Site, South Carolina. The pyritic nature of the coal rejects has resulted in the formation of acidic drainage, which has contributed to groundwater deterioration and threatened biota in adjacent wetlands. Establishment of a vegetation cover to both deplete oxygen through biological means and optimize evapotranspiration has been established as a remedial alternative for reducing acidic drainage generation in the 488-DAB. To determine the potential benefits of a cover, a series of characterization studies were conducted prior to field deployment to gain a better understanding of the metal attenuation processes and to use water quality and substrate data to evaluate the potential effectiveness of this remedial approach. The characterization study indicated that metal attenuation was primarily controlled by fluctuating redox and pH gradients associated with alternating saturated and unsaturated conditions in the basin. Based on this information, a vegetative cover could reduce the production of acid leachate over time, pending that oxygen transport to the subsurface is limited.